compat: stop mangling refusal messages with HTML escapes

The server's 426 body reached the user as "needs \u003e= 0.2.0, \u003c 1.0.0":
Go escapes <, > and & by default for JSON destined for a page, which this is
not. Disabled at the encoder. The client now parses the error field rather than
pattern-matching it, so it survives whatever a future encoder decides to escape.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CLAUDE.md

@@ -32,10 +32,25 @@ Keep prober result IDs aligned with the measurement-schema test-type registry.
 
 ## Versioning
 
-Prefer **patch** bumps (`server-v0.3.1`) for additive/incremental work; reserve **minor** bumps
+Both artifacts are **SemVer**. Prefer **patch** bumps (`server-v0.3.1`) for additive/incremental
-for real milestones. Don't burn through minor versions. Tags are namespaced: `server-v*` for the
+work; reserve **minor** bumps for real milestones. Don't burn through minor versions. Tags are
-Go server, `v*` for the app. Pushing a `server-v*` tag runs CI → binaries + Gitea release +
+namespaced: `server-v*` for the Go server, `v*` for the app. Pushing a `server-v*` tag runs CI →
-registry image; the server on fmr can `--self-update` from those releases.
+binaries + Gitea release + registry image; the server on fmr can `--self-update` from those
+releases.
+
+The app's version lives once, as `appVersionName` in `app/build.gradle.kts`; **`versionCode` is
+derived from it** (`major*1e6 + minor*1e4 + patch*10`). Never set it by hand — a second number a
+human has to remember to bump eventually disagrees with the first.
+
+**Versions are load-bearing** (probe-protocol.md §8): the server refuses apps outside its window
+with `426`, and the app refuses servers outside its own. Two axes, kept separate:
+- `protocol_version` — *can* they talk. The correctness axis; below 1.0.0 the **minor** is the
+  breaking axis.
+- release-version window — *may* they, per policy. `[min, max)`, bounds at breaking boundaries so
+  a patch never strands a fleet. Client bounds: `Compat.kt`. Server: `ECHOLOT_MIN/MAX_APP_VERSION`.
+
+Raise a minimum only when older peers are actively harmful, and say why in the constant's comment.
+`GET /v1/profile` must stay ungated — it is how a refused client learns what it needs.
 
 ## Layout
 
@@ -52,6 +67,29 @@ echolot-prober/                the capability prober (self-contained Gradle buil
     ui/ProberScreen.kt         result cards colored by verdict
 ```
 
+## Client modules (echolot-app/)
+
+Pure Kotlin/JVM where possible, so the interesting logic is unit-testable without a device and can
+be exercised against the live server from the PC:
+
+- `core-protocol` — control plane (pinned TLS) + ELT1 UDP data plane. **One `ProbeSession` per
+  server session, for its whole lifetime**: a second one restarts sequence numbers, the server's
+  anti-replay window discards every packet, and granted sends then target the closed socket.
+- `core-measurement` — the schema types. `core-engine` — composes probes into documents.
+- `core-privacy` — the §8 anonymizer (`full` / `balanced` / `strict`). Field classification lives
+  in one table (`Classification.kt`); keep it there rather than annotating models.
+- `core-archive` — on-device run storage + retention. `enabled` is separate from the three
+  ceilings: all-zeros means "no limits", not "keep nothing".
+
+**The local archive keeps the unredacted document; anonymization happens per upload, on the way
+out.** Never redact what is stored locally.
+
+### Live testing without a device
+`echolot-app/scripts/test-fmr.sh [gradle-task] [test-filter]` mints an enrollment token over SSH,
+enrolls, computes the SPKI pin from the served cert and runs a `Live*Test` against fmr. This covers
+the whole server-facing vertical (granted sends, downstream MTU, uploads) with no phone involved —
+use it before asking the user to test on hardware.
+
 ## Conventions
 
 - Every probe returns a `ProbeResult` — never throws to the caller (MainActivity wraps anyway).
@@ -107,6 +145,12 @@ First build downloads AGP/Compose/Shizuku from Google Maven + Maven Central.
   Shizuku, **toggle Wireless debugging off/on** — Shizuku keeps running (separate process), a fresh
   port + mDNS record appear, and the beacon/connector recover. Plan the Shizuku-tier dev loop
   around this (or USB, if ever available).
+- **Empty-jar race with the IDE.** VSCodium's Java/Kotlin extension runs its own Gradle daemon on
+  the same project; when it overlaps a CLI build, a module's `build/libs/*.jar` can end up
+  containing only a manifest, and Gradle then considers `jar` up-to-date. Dependent modules fail
+  with "Unresolved reference" on symbols that plainly exist. Fix: `rm -f <module>/build/libs/*.jar`
+  and re-run the `jar` task. Suspect this whenever a reference resolves in one module but not in
+  its consumer.
 - As of the AGP 9.2.0 / Gradle 9.6.0 / Kotlin 2.2.10 bump, JDK 17+ (including 25) works —
   AGP 9 requires Gradle 9.1.0+ and Kotlin 2.2.10+ as its minimum KGP version. On machines with
   Android Studio, its `jbr` directory works as `JAVA_HOME`.

docs/build-status.md

@@ -543,3 +543,81 @@ Best available behavior, now implemented: the banner still opens Shizuku, but th
 exact steps there ("Pairing", then "Start"), and a second tap target opens **Developer options**
 (`Settings.ACTION_APPLICATION_DEVELOPMENT_SETTINGS` — public and exported) since Wireless
 debugging must be enabled first for Shizuku's wireless start to work at all.
+
+### Downstream measurements: asymmetric grants, DF-mode big_send (server-v0.4.0 … v0.4.2, 2026-08-01)
+The client can measure a round trip and the largest packet it can *send*. It cannot measure the
+largest packet it can *receive*, or downstream-only loss — those need the server to push, which is
+exactly what §3.4 gates behind an asymmetric grant. Implemented and verified live from the PC:
+
+- **`session.Grant`** — created per action, bound at creation to the session's *observed*
+  data-plane source (no grant without a verified destination), clamped to server limits, with a
+  byte budget, an average-rate ceiling and an expiry. Unit-tested for each of those refusals.
+- **`downtrain`** — N packets of size S every I µs; the client derives downstream loss,
+  reordering and inter-arrival spacing.
+- **`big_send`** — one datagram per requested size. **DF is on by default**, so the largest size
+  that arrives *is* the downstream path MTU. Without DF the kernel fragments and the result only
+  says whether fragments get through — a different fact, and the reason the schema has both
+  `mtu.pmtud_down` and `mtu.frag_delivery`. Sizes above the server's own egress MTU (from the
+  startup self-test) are refused up front and reported as `max_df_bytes`, so an absence caused by
+  our kernel is never read as a limit of the client's path.
+
+Live from the PC against fmr: downstream path MTU **1500** (1472 payload, DF), fragmented delivery
+up to **4000**, downstream train **100/100, 0 % loss, 0 reordered**, inter-arrival 3.3 ms for a
+3000 µs send interval.
+
+#### Two bugs this shook out, both invisible in a single-homed lab
+1. **Granted sends went out from the wrong local address** (fixed in server-v0.4.2). fmr binds two
+   IPv4 addresses; `connFor` returned whichever socket of the right family came first in the bind
+   list. A train for a session established on `.150` left from `.151` and every packet was dropped
+   by the client's NAT, which has no mapping for that pair. tcpdump showed all 50 leaving, the
+   client saw none — reported as *100 % downstream loss*, a confident measurement of something
+   that never happened. Sessions now record which of our own bound addresses received their
+   traffic and granted sends go back through that socket; `connfor_test.go` pins both that and the
+   family fallback.
+2. **A second `ProbeSession` on one server session is silently dead.** Sequence numbers restart at
+   zero client-side while the server's anti-replay window keeps counting, so every packet is
+   discarded as a replay — and because the server then never records the new source, the grant
+   still targets the closed socket. `ServerMeasurement` now uses one ProbeSession for the whole
+   run; `ProbeSession`'s doc comment states the constraint.
+
+### Run archive, anonymizer and uploads (2026-08-01)
+Three pieces, deliberately separate:
+
+- **`core-archive`** — one JSON file per run plus an index entry, in a plain directory the user can
+  inspect or delete with a file manager. Retention (max runs / max age / max total bytes) is
+  enforced on every save rather than by a sweeper. `enabled` is a separate flag from the three
+  ceilings because "no limits" and "keep nothing" are opposite intentions; collapsing them onto
+  all-zeros is how a user who turns the caps off ends up with an empty history. 13 tests.
+- **`core-privacy`** — the schema §8 anonymizer, three levels. `full` (your own server) changes
+  nothing; `balanced` pseudonymizes SSIDs/hostnames, keeps the OUI half of a MAC and the /16 of a
+  public IP, keeps RFC1918 verbatim (it describes topology, not a person), and *drops* neighbour
+  inventories (SSDP/ARP/scan results) rather than mangling them; `strict` keeps only metrics,
+  statuses and finding codes. Pseudonyms are consistent within a document and — by default — not
+  across documents, so an upload endpoint cannot link a device's runs; a stable salt is opt-in for
+  people diffing their own history. Classification is one readable table, not annotations spread
+  across modules. 14 tests, each pinning a property someone's privacy depends on.
+- **Server-side upload policy** — `off | anonymous | account`, plus max size, retention days, max
+  runs per device, and the *least* anonymization accepted. The profile advertises all of it so the
+  app presents the choice honestly instead of discovering the rules by being rejected. `account`
+  refuses today rather than falling back to anonymous: picking the strict setting before OIDC
+  lands must not silently mean the loose one.
+
+**The archive holds the unredacted document; redaction happens on the way out, per upload.** The
+local archive is the user's own data on their own device, and redacting it would destroy exactly
+the detail that makes a week-old run worth keeping.
+
+App-side: settings screen (archive limits, privacy level with a plain-language description of what
+each keeps, auto-upload off by default, server URL/pin/credential), history screen showing whether
+each run left the device, and a **preview of the exact bytes an upload would send** — an anonymizer
+the user cannot inspect is only a promise.
+
+Live round trip against fmr: uploaded a run, listed it, fetched it back and asserted the SSID, the
+SSDP neighbour name and the free-text note are absent from what the server stores while the
+finding code and the metrics survive, then deleted it.
+
+### Still open
+- `mtu.pmtud_up` (DF + errqueue), `frag_send`, `throughput`, TRAIN_REPORT retrieval.
+- Enrollment UI in the app (server URL/pin/credential are typed in by hand today).
+- Accounts/OIDC on the server, which is what `uploads=account` is waiting for.
+- Nothing in this entry has been exercised on a phone yet — all of it was verified from the PC
+  against the live server. On-device verification is the next step.

docs/probe-protocol.md

@@ -192,14 +192,77 @@ Note: exact RDATA constants to be frozen in the implementation's `dns_reference.
 
 Same daemon, separate listener (default localhost-only): health + self-test (are both IPs live, is the canary zone delegated correctly, is UDP reachable from outside — tested via a public echolot "mirror" if configured), enrollment token management (create/expire/scope), device list + revocation, retention settings, QR rendering (client-side JS). Out of scope for this spec beyond the endpoints above.
 
-## 8. Cross-references to the measurement schema
+## 8. Version compatibility
+
+Both artifacts are versioned with **SemVer**: the Go server (`server-vX.Y.Z` tags) and the Android
+app (`versionName`; `versionCode` is derived from it, never maintained separately). Two independent
+things are checked, and conflating them is the mistake this section exists to prevent.
+
+### 8.1 Protocol version — *can* these builds talk?
+
+`protocol_version` is the version of **this document**. It is advertised in the profile
+(`compat.protocol_version`) and is the correctness axis: a peer in a different breaking series
+cannot be talked to, whatever its release version says. Below `1.0.0` the **minor** is the breaking
+axis (SemVer §4); at and above it, the major is. A patch bump of the protocol never splits a fleet.
+
+### 8.2 Release-version window — *should* they, per policy?
+
+Each side declares the range of peer release versions it will work with, as `[min, max)` —
+**minimum inclusive, maximum exclusive**, because the useful bound is always "the version that
+broke it" and writing that literally is unambiguous. An empty maximum means unbounded.
+
+The server advertises its window and enforces it:
+
+```jsonc
+"compat": {
+  "protocol_version": "1.0.0",
+  "schema_version": "1.0.0",
+  "app_min": "0.2.0",
+  "app_max": "1.0.0"     // exclusive; "" = no upper bound
+}
+```
+
+Operators override it with `ECHOLOT_MIN_APP_VERSION` / `ECHOLOT_MAX_APP_VERSION` (or
+`--min-app-version` / `--max-app-version`). A malformed bound is **fatal at startup**, not ignored:
+a typo must not silently disable a restriction the operator meant to set.
+
+The app sends its version on every control-plane request:
+
+```
+X-Echolot-App-Version: 0.2.0
+```
+
+and carries its own bounds for the server (`MIN_SERVER` / `MAX_SERVER` in `Compat.kt`). It checks
+the profile in **both** directions — is the server in our range, and are we in the server's — so a
+mismatch is reported before a run starts rather than discovered halfway through one.
+
+### 8.3 Rules
+
+1. **`GET /v1/profile` is never gated.** It is where a refused client learns which version it needs.
+   Gating it leaves the user with a network error instead of an answer, defeating the check.
+2. **Refusal is `426 Upgrade Required`**, with a body naming both versions and the accepted window:
+   ```json
+   { "error": "app 0.1.0 is older than this build supports (needs >= 0.2.0, < 1.0.0). Update the app.",
+     "app_version": "0.1.0", "accepts_app": ">= 0.2.0, < 1.0.0",
+     "server_version": "0.5.0", "protocol_version": "1.0.0" }
+   ```
+3. **An unparseable or absent version is `unknown`, and is allowed.** Development builds report
+   `dev`, and a client too old to send the header cannot be identified anyway. The check exists to
+   turn confusing failures into clear ones; refusing what it cannot identify does the opposite.
+4. **Bounds move at breaking boundaries, not at releases.** Shipping a patch must never require
+   editing a range. A minimum is raised only when older peers are actually harmful — e.g. the app
+   requires server `>= 0.4.2` because earlier multi-homed servers sent granted traffic from an
+   address the session never used, which the client measured as 100 % downstream loss. A
+   confidently wrong measurement is worse than a refused one.
+
+## 9. Cross-references to the measurement schema
 
 - Observation-block fields (§3.3) appear as `*_seen_by_server` columns in `train` evidence (schema §6.2).
 - `TIMESYNC` (§3.2) produces the `time.server_offset` test; without it, cross-clock fields must not be compared (schema §6.2 note).
 - Capability strings (§2.3) are copied verbatim into `server_sessions[].capabilities` (schema §5); tests skipped for missing capability get `status: "unsupported"`.
 - `session_id` maps to `server_sessions[].session_id`; `action_id`s appear in test `params`.
 
-## 9. Open items
+## 10. Open items
 
 1. Whether TRAIN_REPORT should also stream during long trains (partial reports every N packets) for live UI feedback — leaning yes, same type with a `flags` bit.
 2. Throughput methodology (fixed streams vs BBR-style ramp) — decide with `perf.*` test design.

echolot-app/app/build.gradle.kts

@@ -8,6 +8,20 @@ plugins {
     alias(libs.plugins.kotlin.serialization)
 }
 
+// The app's version is SemVer and lives here, once. versionCode is derived from it rather than
+// maintained alongside: Play/F-Droid need a monotonically increasing integer, but a second number
+// that a human has to remember to bump is a number that eventually disagrees with the first — and
+// the version is now load-bearing, since the server decides whether to serve us by it.
+//
+// major*1_000_000 + minor*10_000 + patch*10 leaves room for 9 patch-level rebuilds (the trailing
+// digit) without disturbing the mapping, and stays inside the 2_100_000_000 ceiling until major 2100.
+val appVersionName = "0.2.0"
+
+fun versionCodeOf(semver: String): Int {
+    val (major, minor, patch) = semver.substringBefore('-').split(".").map(String::toInt)
+    return major * 1_000_000 + minor * 10_000 + patch * 10
+}
+
 android {
     namespace = "app.echo_lot.app"
     compileSdk = 36
@@ -16,12 +30,15 @@ android {
         applicationId = "app.echo_lot.app"
         minSdk = 26
         targetSdk = 36
-        versionCode = 2
+        versionCode = versionCodeOf(appVersionName)
-        versionName = "0.2.0"
+        versionName = appVersionName
         // Automation: `adb shell am start -n app.echo_lot.app/.MainActivity --ez autorun true`
         // runs a measurement immediately and POSTs the report here (dev collection endpoint).
         buildConfigField("String", "REPORT_UPLOAD_URL", "\"http://89.185.109.150:443/report\"")
         buildConfigField("String", "REPORT_UPLOAD_SECRET", "\"D4OmG5gGJsElqVVbtYIZbR\"")
+        // The bare SemVer, without the debug build's "-dev" suffix stripped away by the server's
+        // parser anyway — sent to servers so they can apply their compatibility window.
+        buildConfigField("String", "APP_SEMVER", "\"$appVersionName\"")
     }
     buildTypes {
         release { isMinifyEnabled = false }

echolot-app/app/src/main/kotlin/app/echo_lot/app/MainActivity.kt

@@ -88,6 +88,8 @@ class MainActivity : ComponentActivity() {
                                 lifecycleScope.launch { preview = vm.uploadPreview(r.id) }
                             }
                         },
+                        onCheckServer = vm::checkServer,
+                        serverStatus = vm.state.archiveStatus,
                         onBack = { screen = Screen.RUN },
                       )
                       Screen.HISTORY -> HistoryScreen(

echolot-app/app/src/main/kotlin/app/echo_lot/app/RunStore.kt

@@ -10,8 +10,10 @@ import app.echo_lot.measurement.MeasurementDocument
 import app.echo_lot.privacy.Anonymizer
 import app.echo_lot.privacy.PrivacyLevel
 import app.echo_lot.privacy.Salt
+import app.echo_lot.protocol.Compat
 import app.echo_lot.protocol.ControlClient
 import app.echo_lot.protocol.UploadRefused
+import app.echo_lot.protocol.VersionRefused
 import kotlinx.serialization.json.Json
 import kotlinx.serialization.json.JsonObject
 import kotlinx.serialization.json.jsonObject
@@ -66,10 +68,46 @@ class RunStore(context: Context, private val settings: Settings) {
         data class Sent(val serverName: String, val detail: String) : UploadOutcome
         /** The operator's policy says no. Not retryable, and not the user's fault. */
         data class Refused(val reason: String) : UploadOutcome
+        /**
+         * The two builds do not go together. Kept apart from [Refused] and [Failed] because the
+         * remedy is different and specific — install a particular version — and a message that
+         * says so is worth more than one that says "upload failed".
+         */
+        data class Incompatible(val reason: String) : UploadOutcome
         data class Failed(val detail: String) : UploadOutcome
         data object NotConfigured : UploadOutcome
     }
 
+    private fun client() = ControlClient(
+        settings.serverUrl, setOf(settings.serverPin), BuildConfig.APP_SEMVER,
+    )
+
+    /**
+     * Checks the configured server without uploading anything: reachable, pinned, compatible, and
+     * willing to accept runs. Lets the user find out in settings rather than from a failed run.
+     */
+    fun checkServer(): String {
+        if (!settings.serverConfigured) return "Fill in the server URL, pin and credential first."
+        return try {
+            val profile = client().profile(settings.serverCredential)
+            val compat = Compat.check(profile, BuildConfig.APP_SEMVER)
+            val head = "${profile.name} · server ${profile.serverVersion} · " +
+                "protocol ${profile.compat.protocolVersion.ifBlank { "unstated" }}"
+            when {
+                compat.message != null -> head + "\n" + compat.message
+                else -> {
+                    val uploads = profile.uploads.refusalReason()
+                        ?: "uploads accepted (min anonymization: ${profile.uploads.minAnonymization})"
+                    head + "\nCompatible. " + uploads
+                }
+            }
+        } catch (e: VersionRefused) {
+            "This server will not serve this app: ${e.message}"
+        } catch (t: Throwable) {
+            "Could not reach the server: ${t.message ?: t.javaClass.simpleName}"
+        }
+    }
+
     /**
      * Uploads one archived run to the configured server, redacting first.
      *
@@ -81,8 +119,14 @@ class RunStore(context: Context, private val settings: Settings) {
         if (!settings.serverConfigured) return UploadOutcome.NotConfigured
         val docJson = read(runId) ?: return UploadOutcome.Failed("run $runId is not in the archive")
         return try {
-            val client = ControlClient(settings.serverUrl, setOf(settings.serverPin))
+            val client = client()
             val profile = client.profile(settings.serverCredential)
+
+            // Compatibility before policy: an incompatible server may well advertise an upload
+            // policy it would never actually apply to us.
+            val compat = Compat.check(profile, BuildConfig.APP_SEMVER)
+            if (!compat.usable) return UploadOutcome.Incompatible(compat.message ?: "incompatible versions")
+
             profile.uploads.refusalReason()?.let { return UploadOutcome.Refused(it) }
 
             val level = PrivacyLevel.max(
@@ -93,6 +137,8 @@ class RunStore(context: Context, private val settings: Settings) {
             val reply = client.uploadRun(settings.serverCredential, body)
             archive.markUploaded(runId, profile.name)
             UploadOutcome.Sent(profile.name, "as $level, ${body.toByteArray().size} bytes: ${reply.take(120)}")
+        } catch (e: VersionRefused) {
+            UploadOutcome.Incompatible(e.message ?: "the server refused this app's version")
         } catch (e: UploadRefused) {
             UploadOutcome.Refused(e.message ?: "refused by the server")
         } catch (t: Throwable) {

echolot-app/app/src/main/kotlin/app/echo_lot/app/RunViewModel.kt

@@ -144,6 +144,7 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
     private fun describe(o: RunStore.UploadOutcome): String = when (o) {
         is RunStore.UploadOutcome.Sent -> "uploaded to ${o.serverName} (${o.detail})"
         is RunStore.UploadOutcome.Refused -> "server refused the upload: ${o.reason}"
+        is RunStore.UploadOutcome.Incompatible -> "version mismatch: ${o.reason}"
         is RunStore.UploadOutcome.Failed -> "upload failed: ${o.detail}"
         RunStore.UploadOutcome.NotConfigured -> "no server configured, so nothing was uploaded"
     }
@@ -195,6 +196,14 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
 
     fun archivedBytes(): Long = store.totalBytes()
 
+    /** Settings-screen action: report what the configured server is and whether we can use it. */
+    fun checkServer() {
+        viewModelScope.launch {
+            state = state.copy(archiveStatus = "checking server …")
+            state = state.copy(archiveStatus = withContext(Dispatchers.IO) { store.checkServer() })
+        }
+    }
+
     /** Re-applies retention after the user changes the limits. */
     fun applyRetention() {
         viewModelScope.launch {

echolot-app/app/src/main/kotlin/app/echo_lot/app/SettingsScreen.kt

@@ -46,6 +46,8 @@ fun SettingsScreen(
     onApplyRetention: () -> Unit,
     onDeleteAll: () -> Unit,
     onPreviewUpload: () -> Unit,
+    onCheckServer: () -> Unit,
+    serverStatus: String?,
     onBack: () -> Unit,
 ) {
     // SharedPreferences is not observable, so mirror each value into Compose state and write
@@ -166,12 +168,26 @@ fun SettingsScreen(
                     textStyle = MaterialTheme.typography.bodySmall.copy(fontFamily = FontFamily.Monospace),
                     modifier = Modifier.fillMaxWidth(),
                 )
+                Row(verticalAlignment = Alignment.CenterVertically) {
+                    Button(onClick = onCheckServer) { Text("Check server") }
                     Text(
-                    if (settings.serverConfigured) "Server configured."
+                        "  " + if (settings.serverConfigured) "Configured."
                         else "Uploads stay off until all three fields are set.",
                         style = MaterialTheme.typography.bodySmall,
                     )
                 }
+                // Version compatibility is checked here rather than discovered mid-run: a server
+                // that will refuse this build should say so before a measurement is wasted.
+                serverStatus?.let {
+                    Text(it, style = MaterialTheme.typography.bodySmall)
+                }
+                Text(
+                    "This app is ${BuildConfig.APP_SEMVER} and speaks probe protocol " +
+                        "${app.echo_lot.protocol.Compat.PROTOCOL_VERSION}. It works with servers " +
+                        "${app.echo_lot.protocol.Compat.serverRange}.",
+                    style = MaterialTheme.typography.bodySmall,
+                )
+            }
         }
         Spacer(Modifier.height(24.dp))
     }

echolot-app/core-engine/src/main/kotlin/app/echo_lot/engine/DownstreamMeasurement.kt

@@ -0,0 +1,344 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package app.echo_lot.engine
+
+import app.echo_lot.measurement.*
+import app.echo_lot.protocol.ControlClient
+import app.echo_lot.protocol.ProbeSession
+import app.echo_lot.protocol.Wire
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonObject
+import kotlinx.serialization.json.encodeToJsonElement
+
+/**
+ * The measurements only the far end can make: what the *downstream* path does to traffic the
+ * client never asked for packet-by-packet.
+ *
+ * A client alone can measure a round trip, and it can find the largest packet it can *send*. It
+ * cannot find the largest packet it can *receive*, or whether the network drops downstream
+ * packets independently of upstream ones — those need a server willing to push, which is why the
+ * protocol gates them behind an asymmetric grant (probe-protocol.md §3.4).
+ *
+ * Three separate facts come out, and keeping them separate is the point:
+ *   - `mtu.pmtud_down` — the largest datagram that arrives *unfragmented*. This is the number
+ *     that matters for anything setting DF, and it is only meaningful because the server sets DF.
+ *   - `mtu.frag_delivery` — whether larger datagrams arrive once the network is allowed to
+ *     fragment them. A path can be fine for one and broken for the other; conflating them is how
+ *     you get "MTU is 4000" on a link that drops every DF packet over 1400.
+ *   - `train.udp_downstream` — loss, reordering and arrival spacing in the download direction.
+ */
+class DownstreamMeasurement(private val ids: IdSource) {
+
+    private val json = Json { encodeDefaults = true; explicitNulls = true }
+
+    /** How long to wait for a granted burst after the server accepts the action. */
+    private val collectWindowMs = 4_000L
+
+    /**
+     * Runs all three against an already-primed session.
+     *
+     * [session] must already have sent at least one ECHO: the grant is bound to the source the
+     * server has actually observed, so an unprimed session gets a 409 rather than a grant. That
+     * is the anti-amplification rule doing its job, not an error to work around.
+     */
+    fun run(
+        credential: String,
+        sessionId: String,
+        control: ControlClient,
+        probe: ProbeSession,
+        sessionRef: String,
+        sizes: List<Int> = DEFAULT_SIZES,
+        trainCount: Int = 100,
+        trainSizeBytes: Int = 300,
+        trainIntervalUs: Int = 3_000,
+    ): Pair<List<Test>, List<Finding>> {
+        val tests = ArrayList<Test>()
+        val findings = ArrayList<Finding>()
+
+        val df = bigSend(credential, sessionId, control, probe, sessionRef, sizes, df = true)
+        val frag = bigSend(credential, sessionId, control, probe, sessionRef, sizes, df = false)
+        val train = downTrain(
+            credential, sessionId, control, probe, sessionRef, trainCount, trainSizeBytes, trainIntervalUs,
+        )
+
+        tests.add(df.test); tests.add(frag.test); tests.add(train.test)
+
+        // A downstream MTU below the classic 1500-byte Ethernet payload is worth saying out loud:
+        // it is the usual cause of "small requests work, large responses hang".
+        val pathMtu = df.largestDelivered
+        if (pathMtu != null && pathMtu > 0) {
+            val ipMtu = pathMtu + IP_UDP_OVERHEAD4
+            if (ipMtu < 1500) {
+                findings.add(
+                    finding(
+                        "mtu.reduced_downstream", Category.MTU, Severity.LOW, df.test.id,
+                        "Downstream path MTU is $ipMtu bytes, below 1500",
+                        "The largest datagram that reached this device without fragmenting was " +
+                            "$pathMtu bytes of payload ($ipMtu on the wire). Tunnels (PPPoE, VPN, " +
+                            "IPv6-in-IPv4) commonly do this; it is only a fault when something " +
+                            "on the path also blocks the ICMP messages that let senders discover it.",
+                    ),
+                )
+            }
+            // The dangerous combination: unfragmented large packets vanish AND fragments do too,
+            // so a sender that never gets told will retransmit into a black hole.
+            val fragLargest = frag.largestDelivered ?: 0
+            if (fragLargest <= pathMtu && sizes.any { it > pathMtu }) {
+                findings.add(
+                    finding(
+                        "mtu.downstream_blackhole", Category.MTU, Severity.MEDIUM, frag.test.id,
+                        "Datagrams above $pathMtu bytes are dropped downstream, fragmented or not",
+                        "Nothing larger than $pathMtu bytes arrived, even when the network was " +
+                            "free to fragment it. Traffic that relies on large responses will " +
+                            "stall rather than fail cleanly.",
+                    ),
+                )
+            }
+        }
+
+        if (train.received == 0) {
+            findings.add(
+                finding(
+                    "connectivity.downstream_blocked", Category.CONNECTIVITY, Severity.HIGH, train.test.id,
+                    "No server-initiated packets arrived",
+                    "The server sent ${train.sent} packets toward this device and none arrived, " +
+                        "while the round-trip echo worked. Something on the path forwards replies " +
+                        "but drops traffic the device did not individually solicit.",
+                ),
+            )
+        } else if (train.lossPct >= 5.0) {
+            findings.add(
+                finding(
+                    "connectivity.downstream_loss", Category.CONNECTIVITY, Severity.MEDIUM, train.test.id,
+                    "Downstream loss of ${round1(train.lossPct)}%",
+                    "${train.sent - train.received} of ${train.sent} packets sent toward this " +
+                        "device were lost. Downstream loss is invisible to a round-trip test, " +
+                        "which reports only that *something* was lost somewhere.",
+                ),
+            )
+        }
+        if (train.reordered > 0) {
+            findings.add(
+                finding(
+                    "connectivity.downstream_reorder", Category.CONNECTIVITY, Severity.LOW, train.test.id,
+                    "${train.reordered} downstream packet(s) arrived out of order",
+                    "Packets arrived in a different order than they were sent. Usually per-packet " +
+                        "load balancing across links; harmless for most traffic, not for all of it.",
+                ),
+            )
+        }
+        return tests to findings
+    }
+
+    // ---- big_send ---------------------------------------------------------------------
+
+    private class SizeResult(val test: Test, val largestDelivered: Int?)
+
+    private fun bigSend(
+        credential: String, sessionId: String, control: ControlClient, probe: ProbeSession,
+        sessionRef: String, sizes: List<Int>, df: Boolean,
+    ): SizeResult {
+        val testId = ids.uuid()
+        val started = ids.monoNs()
+        val requested = sizes.joinToString(",")
+
+        val reply = runCatching {
+            control.action(
+                credential, sessionId,
+                """{"action":"big_send","df":$df,"sizes_bytes":[$requested]}""",
+            )
+        }
+        if (reply.isFailure) {
+            return SizeResult(
+                Test(
+                    id = testId, type = if (df) TestType.MTU_PMTUD_DOWN else TestType.MTU_FRAG_DELIVERY,
+                    sessionRef = sessionRef, tier = Tier.APP,
+                    startedMonoNs = started, endedMonoNs = ids.monoNs(),
+                    status = TestStatus.UNSUPPORTED,
+                    error = TestError("action_refused", reply.exceptionOrNull()?.message ?: "big_send refused"),
+                ),
+                null,
+            )
+        }
+
+        // The server tells us which sizes it actually put on the wire. With DF it refuses
+        // anything above its own egress MTU, and treating those as "lost downstream" would
+        // blame the client's network for our own limit.
+        val accepted = parseIntArray(reply.getOrNull(), "sizes_bytes").ifEmpty { sizes }
+        val serverMaxDf = parseInt(reply.getOrNull(), "max_df_bytes")
+
+        val arrived = probe.collectGranted(collectWindowMs)
+            .filter { it.type == Wire.TYPE_BIG_SEND }
+            .map { it.sizeBytes }
+            .distinct()
+            .sorted()
+        val largest = arrived.maxOrNull()
+
+        val metrics = json.encodeToJsonElement(
+            BigSendMetrics(
+                requestedBytes = sizes,
+                sentBytes = accepted,
+                deliveredBytes = arrived,
+                largestDeliveredBytes = largest,
+                dontFragment = df,
+                serverMaxDfBytes = serverMaxDf,
+                // Only meaningful for the DF run; the IP-level MTU is the payload plus headers.
+                pathMtuBytes = if (df && largest != null) largest + IP_UDP_OVERHEAD4 else null,
+            ),
+        ) as JsonObject
+
+        val status = when {
+            arrived.isEmpty() -> TestStatus.FAILED
+            arrived.size < accepted.size -> TestStatus.PARTIAL
+            else -> TestStatus.OK
+        }
+        return SizeResult(
+            Test(
+                id = testId, type = if (df) TestType.MTU_PMTUD_DOWN else TestType.MTU_FRAG_DELIVERY,
+                sessionRef = sessionRef, tier = Tier.APP,
+                startedMonoNs = started, endedMonoNs = ids.monoNs(),
+                status = status, metrics = metrics,
+            ),
+            largest,
+        )
+    }
+
+    // ---- downtrain --------------------------------------------------------------------
+
+    private class TrainResult(
+        val test: Test, val sent: Int, val received: Int, val lossPct: Double, val reordered: Int,
+    )
+
+    private fun downTrain(
+        credential: String, sessionId: String, control: ControlClient, probe: ProbeSession,
+        sessionRef: String, count: Int, sizeBytes: Int, intervalUs: Int,
+    ): TrainResult {
+        val testId = ids.uuid()
+        val started = ids.monoNs()
+
+        val reply = runCatching {
+            control.action(
+                credential, sessionId,
+                """{"action":"downtrain","count":$count,"size_bytes":$sizeBytes,"interval_us":$intervalUs}""",
+            )
+        }
+        if (reply.isFailure) {
+            return TrainResult(
+                Test(
+                    id = testId, type = TestType.TRAIN_UDP_DOWNSTREAM, sessionRef = sessionRef,
+                    tier = Tier.APP, startedMonoNs = started, endedMonoNs = ids.monoNs(),
+                    status = TestStatus.UNSUPPORTED,
+                    error = TestError("action_refused", reply.exceptionOrNull()?.message ?: "downtrain refused"),
+                ),
+                0, 0, 0.0, 0,
+            )
+        }
+        val sent = parseInt(reply.getOrNull(), "count") ?: count
+
+        val got = probe.collectGranted(collectWindowMs).filter { it.type == Wire.TYPE_DOWNTRAIN_DATA }
+        val received = got.size
+        val lossPct = if (sent == 0) 0.0 else (sent - received) * 100.0 / sent
+
+        // Reordering: a packet whose sequence is below the highest already seen. Counting
+        // inversions rather than "not sorted" keeps one late packet from being reported as
+        // dozens of reorder events.
+        var highest = -1
+        var reordered = 0
+        for (p in got) {
+            if (p.seq < highest) reordered++ else highest = p.seq
+        }
+
+        // Columnar evidence per the schema: what arrived, when, and how big — so every metric
+        // above is recomputable by a reader who does not trust our arithmetic.
+        val evidence = TrainEvidence(
+            epochMonoNs = started,
+            seq = got.map { it.seq },
+            tTxNs = got.map { null },
+            tRxNs = got.map { it.tRxNs },
+            sizeBytes = got.map { it.sizeBytes },
+        ).toEvidence()
+
+        val interArrival = got.zipWithNext { a, b -> (b.tRxNs - a.tRxNs) / 1_000_000.0 }
+        val metrics = json.encodeToJsonElement(
+            DownTrainMetrics(
+                sent = sent, received = received, lossPct = round1(lossPct),
+                reorderedPackets = reordered,
+                sizeBytes = sizeBytes,
+                interArrivalMsAvg = interArrival.average().takeIf { interArrival.isNotEmpty() }?.let(::round1),
+                interArrivalMsMax = interArrival.maxOrNull()?.let(::round1),
+                sendIntervalUs = intervalUs,
+            ),
+        ) as JsonObject
+
+        val status = when {
+            received == 0 -> TestStatus.FAILED
+            received < sent -> TestStatus.PARTIAL
+            else -> TestStatus.OK
+        }
+        return TrainResult(
+            Test(
+                id = testId, type = TestType.TRAIN_UDP_DOWNSTREAM, sessionRef = sessionRef, tier = Tier.APP,
+                startedMonoNs = started, endedMonoNs = ids.monoNs(),
+                status = status, evidence = evidence, metrics = metrics,
+            ),
+            sent, received, lossPct, reordered,
+        )
+    }
+
+    // ---- helpers ----------------------------------------------------------------------
+
+    private fun finding(code: String, cat: Category, sev: Severity, testId: String, title: String, desc: String) =
+        Finding(
+            id = ids.uuid(), code = code, category = cat, severity = sev, confidence = Confidence.HIGH,
+            title = title, description = desc, evidenceRefs = listOf(EvidenceRef(testId)),
+        )
+
+    /** Minimal scalar extraction from the action reply; the shape is small and server-owned. */
+    private fun parseInt(body: String?, key: String): Int? =
+        body?.let { Regex("\"$key\"\\s*:\\s*(-?\\d+)").find(it)?.groupValues?.get(1)?.toIntOrNull() }
+
+    private fun parseIntArray(body: String?, key: String): List<Int> =
+        body?.let { b ->
+            Regex("\"$key\"\\s*:\\s*\\[([^\\]]*)\\]").find(b)?.groupValues?.get(1)
+                ?.split(",")?.mapNotNull { it.trim().toIntOrNull() }
+        } ?: emptyList()
+
+    private companion object {
+        /** IPv4 (20) + UDP (8). The v6 case is 48; reported per-family once v6 sessions land. */
+        const val IP_UDP_OVERHEAD4 = 28
+
+        /** Straddles the usual suspects: 1500 Ethernet, 1492 PPPoE, 1400-ish tunnels. */
+        val DEFAULT_SIZES = listOf(600, 1200, 1372, 1400, 1450, 1472, 1500, 2000, 4000)
+
+        fun round1(v: Double) = Math.round(v * 10.0) / 10.0
+    }
+}
+
+/** Metrics for mtu.pmtud_down / mtu.frag_delivery. */
+@Serializable
+data class BigSendMetrics(
+    @SerialName("requested_bytes") val requestedBytes: List<Int>,
+    @SerialName("sent_bytes") val sentBytes: List<Int>,
+    @SerialName("delivered_bytes") val deliveredBytes: List<Int>,
+    @SerialName("largest_delivered_bytes") val largestDeliveredBytes: Int? = null,
+    @SerialName("dont_fragment") val dontFragment: Boolean,
+    /** The server's own DF ceiling; sizes above it were never sent and are not path evidence. */
+    @SerialName("server_max_df_bytes") val serverMaxDfBytes: Int? = null,
+    @SerialName("path_mtu_bytes") val pathMtuBytes: Int? = null,
+)
+
+/** Metrics for train.udp_downstream. */
+@Serializable
+data class DownTrainMetrics(
+    val sent: Int,
+    val received: Int,
+    @SerialName("loss_pct") val lossPct: Double,
+    @SerialName("reordered_packets") val reorderedPackets: Int,
+    @SerialName("size_bytes") val sizeBytes: Int,
+    @SerialName("inter_arrival_ms_avg") val interArrivalMsAvg: Double? = null,
+    @SerialName("inter_arrival_ms_max") val interArrivalMsMax: Double? = null,
+    @SerialName("send_interval_us") val sendIntervalUs: Int,
+)

echolot-app/core-engine/src/main/kotlin/app/echo_lot/engine/ServerMeasurement.kt

@@ -36,6 +36,12 @@ class ServerMeasurement(
         val udpPort: Int,
         val echoCount: Int = 20,
         val echoPaddingBytes: Int = 64,
+        /**
+         * Whether to ask the server to push traffic back (downstream MTU and downstream train).
+         * Costs a few hundred kB of download and needs a server that advertises the grants, so
+         * it is a flag rather than an assumption.
+         */
+        val downstream: Boolean = true,
     )
 
     fun run(cfg: Config): MeasurementDocument {
@@ -57,11 +63,32 @@ class ServerMeasurement(
             target = SessionTarget(ip4 = cfg.udpHost, udpPort = cfg.udpPort),
         )
 
-        val (test, findings) = echoTrain(cfg, control, session, startMono)
+        val tests = ArrayList<Test>()
+        val allFindings = ArrayList<Finding>()
+
+        // One ProbeSession for the whole run. A second one would open a new socket and restart
+        // the sequence counter, which the server's anti-replay window correctly rejects — so the
+        // re-primed source is never recorded and every granted send goes to the old, closed port.
+        // Session identity lives on the server; the socket must live as long as it does.
+        ProbeSession(cfg.credential, session, cfg.udpHost, cfg.udpPort).use { ps ->
+            val (test, findings) = echoTrain(cfg, ps, startMono)
+            tests.add(test)
+            allFindings.addAll(findings)
+
+            // Downstream needs a session the server has already seen traffic from — the echo
+            // train just provided that — and a server that advertises the grants. Skipped
+            // quietly against an older server rather than reported as a failure of the network.
+            if (cfg.downstream && profile.supports("downtrain") && profile.supports("big-send")) {
+                val (dsTests, dsFindings) = DownstreamMeasurement(ids)
+                    .run(cfg.credential, session.sessionId, control, ps, sessionRef = "sess-1")
+                tests.addAll(dsTests)
+                allFindings.addAll(dsFindings)
+            }
+        }
 
         control.deleteSession(cfg.credential, session.sessionId)
 
-        val summary = Verdicts.derive(listOf(test), findings)
+        val summary = Verdicts.derive(tests, allFindings)
         return MeasurementDocument(
             run = Run(
                 id = runId, trigger = Trigger.MANUAL, startedAt = startWall, endedAt = ids.nowWall(),
@@ -70,14 +97,14 @@ class ServerMeasurement(
                 tiers = Tiers(app = true),
             ),
             serverSessions = listOf(serverSession),
-            tests = listOf(test),
+            tests = tests,
-            findings = findings,
+            findings = allFindings,
             summary = summary,
         )
     }
 
     private fun echoTrain(
-        cfg: Config, control: ControlClient, session: app.echo_lot.protocol.SessionResponse, startMono: Long,
+        cfg: Config, ps: ProbeSession, startMono: Long,
     ): Pair<Test, List<Finding>> {
         val testId = ids.uuid()
         val seqs = ArrayList<Int>()
@@ -87,7 +114,6 @@ class ServerMeasurement(
         val rtts = ArrayList<Double>()
         val observedPorts = LinkedHashSet<Int>()
 
-        ProbeSession(cfg.credential, session, cfg.udpHost, cfg.udpPort).use { ps ->
         for (i in 0 until cfg.echoCount) {
             val txMono = ids.monoNs() - startMono
             val r = ps.echo(cfg.echoPaddingBytes)
@@ -102,7 +128,6 @@ class ServerMeasurement(
                 tRx.add(null)
             }
         }
-        }
 
         val sent = cfg.echoCount
         val received = rtts.size

echolot-app/core-engine/src/test/kotlin/app/echo_lot/engine/LiveCompatTest.kt

@@ -0,0 +1,71 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package app.echo_lot.engine
+
+import app.echo_lot.protocol.Compat
+import app.echo_lot.protocol.ControlClient
+import app.echo_lot.protocol.VersionRefused
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNotNull
+import kotlin.test.assertTrue
+import kotlin.test.fail
+
+/**
+ * Checks the version gate against a LIVE server — the half that unit tests cannot reach, because
+ * the whole point is that two independently-built artifacts agree. Self-skips without
+ * ECHOLOT_LIVE_*.
+ */
+class LiveCompatTest {
+
+    private val url = System.getenv("ECHOLOT_LIVE_URL")
+    private val pin = System.getenv("ECHOLOT_LIVE_PIN")
+    private val cred = System.getenv("ECHOLOT_LIVE_CRED")
+
+    private fun clientAs(version: String) = ControlClient(url!!, setOf(pin!!), version)
+
+    @Test
+    fun theServerAdvertisesAndEnforcesItsWindow() {
+        if (url == null || pin == null || cred == null) {
+            println("LiveCompatTest skipped (no ECHOLOT_LIVE_* env)"); return
+        }
+
+        // The profile must state the window — without it the app cannot pre-empt a refusal.
+        val profile = clientAs("0.2.0").profile(cred)
+        println("server ${profile.serverVersion} protocol=${profile.compat.protocolVersion} " +
+            "accepts app [${profile.compat.appMin}, ${profile.compat.appMax})")
+        assertTrue(profile.compat.protocolVersion.isNotBlank(), "profile omits protocol_version")
+        assertTrue(profile.compat.appMin.isNotBlank(), "profile omits app_min")
+
+        // This build must be inside it, or every other live test here is meaningless.
+        val verdict = Compat.check(profile, "0.2.0")
+        assertEquals(Compat.Verdict.OK, verdict.verdict, verdict.message ?: "")
+
+        // The profile stays reachable for a version the server would otherwise refuse: that is
+        // how a refused client discovers what it needs.
+        val ancient = clientAs("0.1.0")
+        val stillReadable = ancient.profile(cred)
+        assertEquals(profile.serverVersion, stillReadable.serverVersion,
+            "the profile endpoint must never be gated on app version")
+
+        // And a gated endpoint refuses it, with a message naming the window.
+        try {
+            ancient.createSession(cred, System.getenv("ECHOLOT_LIVE_TARGET") ?: "fmr")
+            fail("server accepted a session from an out-of-window app")
+        } catch (e: VersionRefused) {
+            val msg = assertNotNull(e.message)
+            println("refused as expected: $msg")
+            assertTrue(msg.contains("0.1.0"), "refusal should name the offending version: $msg")
+            assertTrue(msg.contains(profile.compat.appMin), "refusal should name the window: $msg")
+        }
+
+        // Too new is refused the same way — the window is a range, not a floor.
+        try {
+            clientAs("99.0.0").createSession(cred, System.getenv("ECHOLOT_LIVE_TARGET") ?: "fmr")
+            fail("server accepted a session from an app above its window")
+        } catch (e: VersionRefused) {
+            println("too-new refused as expected: ${e.message}")
+        }
+    }
+}

echolot-app/core-engine/src/test/kotlin/app/echo_lot/engine/LiveDownstreamTest.kt

@@ -0,0 +1,68 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package app.echo_lot.engine
+
+import app.echo_lot.measurement.TestStatus
+import app.echo_lot.measurement.TestType
+import app.echo_lot.protocol.ControlClient
+import app.echo_lot.protocol.ProbeSession
+import kotlin.test.Test as JTest
+import kotlin.test.assertEquals
+import kotlin.test.assertNotNull
+import kotlin.test.assertTrue
+
+/**
+ * Runs DownstreamMeasurement against a LIVE server and checks the *documents* it produces, not
+ * just that packets moved: the tests must carry recomputable metrics and land on the right test
+ * types, because that is what an archived run is read back as. Self-skips without ECHOLOT_LIVE_*.
+ */
+class LiveDownstreamTest {
+
+    private val url = System.getenv("ECHOLOT_LIVE_URL")
+    private val pin = System.getenv("ECHOLOT_LIVE_PIN")
+    private val cred = System.getenv("ECHOLOT_LIVE_CRED")
+    private val udp = System.getenv("ECHOLOT_LIVE_UDP")
+    private val target = System.getenv("ECHOLOT_LIVE_TARGET") ?: "fmr"
+
+    @JTest
+    fun producesDownstreamTestsAndFindings() {
+        if (url == null || pin == null || cred == null || udp == null) {
+            println("LiveDownstreamTest skipped (no ECHOLOT_LIVE_* env)"); return
+        }
+        val control = ControlClient(url, setOf(pin))
+        val session = control.createSession(cred, target)
+        val (host, port) = udp.split(":").let { it[0] to it[1].toInt() }
+
+        val (tests, findings) = ProbeSession(cred, session, host, port).use { ps ->
+            ps.echo() // prime: the grant binds to the source the server has actually observed
+            DownstreamMeasurement(SystemIdSource())
+                .run(cred, session.sessionId, control, ps, sessionRef = "sess-1")
+        }
+        control.deleteSession(cred, session.sessionId)
+
+        for (t in tests) println("${t.type}  status=${t.status}  metrics=${t.metrics}")
+        for (f in findings) println("finding ${f.code} [${f.severity}] ${f.title}")
+
+        assertEquals(3, tests.size, "expected pmtud_down, frag_delivery and a downstream train")
+        val byType = tests.associateBy { it.type }
+
+        val pmtud = assertNotNull(byType[TestType.MTU_PMTUD_DOWN], "no mtu.pmtud_down test")
+        assertTrue(pmtud.status == TestStatus.OK || pmtud.status == TestStatus.PARTIAL,
+            "DF probe did not deliver anything: ${pmtud.status}")
+        val pathMtu = pmtud.metrics?.get("path_mtu_bytes")?.toString()?.toIntOrNull()
+        assertNotNull(pathMtu, "pmtud_down must report a path MTU")
+        assertTrue(pathMtu in 576..9000, "implausible downstream path MTU: $pathMtu")
+        println("downstream path MTU = $pathMtu bytes")
+
+        val frag = assertNotNull(byType[TestType.MTU_FRAG_DELIVERY], "no mtu.frag_delivery test")
+        assertNotNull(frag.metrics?.get("largest_delivered_bytes"))
+
+        val train = assertNotNull(byType[TestType.TRAIN_UDP_DOWNSTREAM], "no downstream train")
+        assertNotNull(train.evidence, "a train without columnar evidence is not recomputable")
+        val received = train.metrics?.get("received")?.toString()?.toIntOrNull() ?: 0
+        assertTrue(received > 0, "no downstream train packets arrived")
+        println("downstream train: $received received, loss=${train.metrics?.get("loss_pct")}, " +
+            "reordered=${train.metrics?.get("reordered_packets")}")
+    }
+}

echolot-app/core-engine/src/test/kotlin/app/echo_lot/engine/LiveMeasurementTest.kt

@@ -47,8 +47,11 @@ class LiveMeasurementTest {
 
         assertEquals(1, doc.serverSessions.size)
         assertTrue(doc.serverSessions[0].capabilities.contains("udp-probe"))
-        val test = doc.tests.single()
+        // A full run is the echo train plus the three downstream tests; assert on the one this
-        assertEquals(TestType.TRAIN_UDP_UPDOWN, test.type)
+        // test is about rather than on the count, so adding a measurement is not a test edit.
+        for (t in doc.tests) println("  ${t.type} → ${t.status}")
+        for (f in doc.findings) println("  finding ${f.code} [${f.severity}] ${f.title}")
+        val test = doc.tests.first { it.type == TestType.TRAIN_UDP_UPDOWN }
         assertTrue(test.status == TestStatus.OK || test.status == TestStatus.PARTIAL,
             "expected replies from live server, got ${test.status}")
 

echolot-app/core-measurement/bin/main/app/echo_lot/measurement/Document.kt

@@ -1,100 +0,0 @@
-// SPDX-FileCopyrightText: 2026 Echolot contributors
-// SPDX-License-Identifier: GPL-3.0-or-later
-
-// Package measurement models one measurement run (measurement-schema.md) — the archived,
-// diffable, exportable unit. Design rules honored in the types: observation/interpretation
-// separated (tests[] vs findings[]), two clocks (wall RFC3339 for humans, *_mono_ns for math),
-// units in field names, columnar trains. params/evidence/metrics are per-test-type, so they are
-// carried as JsonObject (the probe engine fills them; consumers ignore unknown fields).
-package app.echo_lot.measurement
-
-import kotlinx.serialization.SerialName
-import kotlinx.serialization.Serializable
-import kotlinx.serialization.json.JsonObject
-
-@Serializable
-data class MeasurementDocument(
-    val schema: String = "echolot/measurement",
-    @SerialName("schema_version") val schemaVersion: String = "1.0.0",
-    val run: Run,
-    val networks: List<Network> = emptyList(),
-    @SerialName("server_sessions") val serverSessions: List<ServerSession> = emptyList(),
-    val tests: List<Test> = emptyList(),
-    val findings: List<Finding> = emptyList(),
-    val summary: Summary? = null,
-)
-
-@Serializable
-data class Run(
-    val id: String, // UUIDv7
-    val trigger: Trigger,
-    @SerialName("started_at") val startedAt: String, // RFC3339 UTC, human correlation only
-    @SerialName("ended_at") val endedAt: String? = null,
-    val clock: Clock,
-    val app: AppInfo,
-    val device: DeviceInfo,
-    val tiers: Tiers,
-    @SerialName("profiles_used") val profilesUsed: List<String> = emptyList(),
-    val notes: String? = null,
-)
-
-@Serializable
-enum class Trigger {
-    @SerialName("manual") MANUAL,
-    @SerialName("scheduled") SCHEDULED,
-    @SerialName("monitor") MONITOR,
-    @SerialName("peer") PEER,
-}
-
-/** The two-clock anchor: mono_origin_wall maps the monotonic epoch to a wall time for humans;
- *  all math uses *_mono_ns relative to that monotonic origin. */
-@Serializable
-data class Clock(
-    @SerialName("mono_origin_wall") val monoOriginWall: String,
-    @SerialName("ntp_offset_ms") val ntpOffsetMs: Double? = null,
-    @SerialName("ntp_offset_source") val ntpOffsetSource: String? = null,
-)
-
-@Serializable
-data class AppInfo(
-    val version: String,
-    val build: Int,
-    val git: String? = null,
-    val flavor: String? = null,
-)
-
-@Serializable
-data class DeviceInfo(
-    val manufacturer: String,
-    val model: String,
-    @SerialName("android_sdk") val androidSdk: Int,
-    @SerialName("android_release") val androidRelease: String,
-    @SerialName("security_patch") val securityPatch: String? = null,
-)
-
-/** What each tier was *available*; each test records what it *used*. */
-@Serializable
-data class Tiers(
-    val app: Boolean = true,
-    val shizuku: Boolean = false,
-    val root: Boolean = false,
-)
-
-@Serializable
-data class ServerSession(
-    val id: String,
-    @SerialName("profile_id") val profileId: String? = null,
-    @SerialName("profile_name") val profileName: String? = null,
-    @SerialName("control_url") val controlUrl: String,
-    @SerialName("server_version") val serverVersion: String? = null,
-    val capabilities: List<String> = emptyList(),
-    @SerialName("session_id") val sessionId: String,
-    val target: SessionTarget,
-)
-
-@Serializable
-data class SessionTarget(
-    val ip4: String? = null,
-    val ip6: String? = null,
-    @SerialName("udp_port") val udpPort: Int = 0,
-)

echolot-app/core-measurement/bin/main/app/echo_lot/measurement/Evidence.kt

@@ -1,85 +0,0 @@
-// SPDX-FileCopyrightText: 2026 Echolot contributors
-// SPDX-License-Identifier: GPL-3.0-or-later
-
-package app.echo_lot.measurement
-
-import kotlinx.serialization.SerialName
-import kotlinx.serialization.Serializable
-import kotlinx.serialization.json.Json
-import kotlinx.serialization.json.JsonObject
-import kotlinx.serialization.json.encodeToJsonElement
-
-/**
- * Typed builders for the per-test-type evidence shapes the schema fixes (§6.2/§6.3/§6.4). Probe
- * code fills these and folds them into [Test.evidence] via [toEvidence]; keeping them typed here
- * means the columnar/traceroute/resolver contracts live in one place.
- */
-
-@PublishedApi
-internal val evidenceJson = Json { encodeDefaults = true; explicitNulls = true }
-
-/** Serialize any typed evidence object into the JsonObject the Test envelope carries. */
-inline fun <reified T> T.toEvidence(): JsonObject =
-    evidenceJson.encodeToJsonElement(this) as JsonObject
-
-/**
- * Packet-train evidence (§6.2): columnar parallel arrays, one index per probe packet. Missing
- * observations are null at that index — a 10k-packet train stays in the hundreds of kB. Server
- * columns use the server session epoch; only differences within one clock are meaningful unless a
- * time.server_offset test maps them.
- */
-@Serializable
-data class TrainEvidence(
-    @SerialName("epoch_mono_ns") val epochMonoNs: Long,
-    val seq: List<Int>,
-    @SerialName("t_tx_ns") val tTxNs: List<Long?>,
-    @SerialName("t_srv_rx_ns") val tSrvRxNs: List<Long?> = emptyList(),
-    @SerialName("t_srv_tx_ns") val tSrvTxNs: List<Long?> = emptyList(),
-    @SerialName("t_rx_ns") val tRxNs: List<Long?>,
-    @SerialName("size_bytes") val sizeBytes: List<Int>,
-    @SerialName("dscp_sent") val dscpSent: Int? = null,
-    @SerialName("dscp_seen_by_server") val dscpSeenByServer: List<Int?> = emptyList(),
-    @SerialName("ecn_sent") val ecnSent: Int? = null,
-    @SerialName("ecn_seen_by_server") val ecnSeenByServer: List<Int?> = emptyList(),
-    @SerialName("ttl_seen_by_server") val ttlSeenByServer: List<Int?> = emptyList(),
-    @SerialName("evidence_truncated") val evidenceTruncated: Boolean = false,
-)
-
-/** Traceroute evidence (§6.3): fixed-tuple flow + per-TTL probe replies. */
-@Serializable
-data class TracerouteEvidence(val flow: Flow, val hops: List<Hop>)
-
-@Serializable
-data class Flow(
-    @SerialName("src_port") val srcPort: Int,
-    @SerialName("dst_port") val dstPort: Int,
-    @SerialName("fixed_tuple") val fixedTuple: Boolean = true,
-)
-
-@Serializable
-data class Hop(val ttl: Int, val probes: List<HopProbe>)
-
-@Serializable
-data class HopProbe(
-    @SerialName("reply_from") val replyFrom: String? = null,
-    @SerialName("rtt_ns") val rttNs: Long? = null,
-    val icmp: String? = null,
-    @SerialName("reply_ttl") val replyTtl: Int? = null,
-)
-
-/** Resolver under test (§6.4); every dns.* test carries this in params. */
-@Serializable
-data class ResolverSpec(
-    val source: ResolverSource,
-    val address: String? = null,
-    val port: Int = 53,
-    val transport: String, // do53-udp | do53-tcp | dot | doh
-    @SerialName("doh_url") val dohUrl: String? = null,
-)
-
-@Serializable
-enum class ResolverSource {
-    @SerialName("system") SYSTEM,
-    @SerialName("manual") MANUAL,
-    @SerialName("server-recursive") SERVER_RECURSIVE,
-}

echolot-app/core-measurement/bin/main/app/echo_lot/measurement/Finding.kt

@@ -1,68 +0,0 @@
-// SPDX-FileCopyrightText: 2026 Echolot contributors
-// SPDX-License-Identifier: GPL-3.0-or-later
-
-package app.echo_lot.measurement
-
-import kotlinx.serialization.SerialName
-import kotlinx.serialization.Serializable
-
-/** Interpretation with references back to evidence (measurement-schema.md §7.1). A finding with
- *  no evidence_refs is invalid — every finding must be re-derivable from the evidence alone. */
-@Serializable
-data class Finding(
-    val id: String, // UUIDv7
-    val code: String, // stable registry (findings-registry.md), lint-rule style
-    val category: Category,
-    val severity: Severity,
-    val confidence: Confidence,
-    @SerialName("network_ref") val networkRef: String? = null,
-    val title: String,
-    val description: String,
-    @SerialName("evidence_refs") val evidenceRefs: List<EvidenceRef>,
-    val recommendation: String? = null,
-) {
-    init {
-        require(evidenceRefs.isNotEmpty()) { "a finding must reference at least one piece of evidence" }
-    }
-}
-
-@Serializable
-data class EvidenceRef(val test: String, val pointer: String? = null)
-
-/** Fixed §7.2 categories; each maps to one traffic light. */
-@Serializable
-enum class Category {
-    @SerialName("connectivity") CONNECTIVITY,
-    @SerialName("dns") DNS,
-    @SerialName("nat") NAT,
-    @SerialName("mtu") MTU,
-    @SerialName("ipv6") IPV6,
-    @SerialName("security") SECURITY,
-    @SerialName("performance") PERFORMANCE,
-    @SerialName("local") LOCAL,
-    @SerialName("wifi") WIFI,
-}
-
-/** Ordered worst→best via [rank]; drives the §7.3 light mapping. */
-@Serializable
-enum class Severity(val rank: Int) {
-    @SerialName("critical") CRITICAL(4),
-    @SerialName("high") HIGH(3),
-    @SerialName("medium") MEDIUM(2),
-    @SerialName("low") LOW(1),
-    @SerialName("info") INFO(0);
-
-    /** §7.3: critical|high → red, medium|low → yellow, info → green. */
-    fun toLight(): Verdict = when (this) {
-        CRITICAL, HIGH -> Verdict.RED
-        MEDIUM, LOW -> Verdict.YELLOW
-        INFO -> Verdict.GREEN
-    }
-}
-
-@Serializable
-enum class Confidence {
-    @SerialName("high") HIGH,
-    @SerialName("medium") MEDIUM,
-    @SerialName("low") LOW,
-}

echolot-app/core-measurement/bin/main/app/echo_lot/measurement/Network.kt

@@ -1,113 +0,0 @@
-// SPDX-FileCopyrightText: 2026 Echolot contributors
-// SPDX-License-Identifier: GPL-3.0-or-later
-
-package app.echo_lot.measurement
-
-import kotlinx.serialization.SerialName
-import kotlinx.serialization.Serializable
-import kotlinx.serialization.json.JsonObject
-
-/** One Android Network in play (measurement-schema.md §4). Shizuku-tier fields (route proto,
- *  lifetimes) are absent at app tier — absence means "not observed", never "not present". */
-@Serializable
-data class Network(
-    val id: String,
-    val transport: Transport,
-    @SerialName("interface") val iface: String? = null,
-    val link: Link,
-    val wifi: Wifi? = null,
-    val cellular: Cellular? = null,
-    val changes: List<NetworkChange> = emptyList(),
-)
-
-@Serializable
-enum class Transport {
-    @SerialName("wifi") WIFI,
-    @SerialName("cellular") CELLULAR,
-    @SerialName("ethernet") ETHERNET,
-    @SerialName("vpn") VPN,
-    @SerialName("other") OTHER,
-}
-
-@Serializable
-data class Link(
-    val mtu: Int? = null,
-    val addresses: List<Address> = emptyList(),
-    val routes: List<Route> = emptyList(),
-    val dns: DnsConfig? = null,
-    val dhcp: Dhcp? = null,
-    @SerialName("captive_portal") val captivePortal: CaptivePortal? = null,
-)
-
-@Serializable
-data class Address(
-    val addr: String, // ip4 | ip6 (logical type, §8)
-    @SerialName("prefix_len") val prefixLen: Int,
-    val scope: String? = null,
-    val flags: List<String> = emptyList(),
-    @SerialName("valid_lft_s") val validLftS: Long? = null,
-    @SerialName("pref_lft_s") val prefLftS: Long? = null,
-)
-
-@Serializable
-data class Route(
-    val dst: String,
-    val gateway: String? = null,
-    val iface: String? = null,
-    val proto: RouteProto? = null, // shizuku tier; null = not observed
-    @SerialName("expires_s") val expiresS: Long? = null,
-)
-
-@Serializable
-enum class RouteProto {
-    @SerialName("dhcp") DHCP,
-    @SerialName("ra") RA,
-    @SerialName("static") STATIC,
-    @SerialName("unknown") UNKNOWN,
-}
-
-@Serializable
-data class DnsConfig(
-    val servers: List<String> = emptyList(),
-    @SerialName("private_dns_mode") val privateDnsMode: String? = null,
-    @SerialName("private_dns_hostname") val privateDnsHostname: String? = null,
-    @SerialName("search_domains") val searchDomains: List<String> = emptyList(),
-    @SerialName("nat64_prefix") val nat64Prefix: String? = null,
-)
-
-@Serializable
-data class Dhcp(val server: String? = null, @SerialName("lease_s") val leaseS: Long? = null)
-
-@Serializable
-data class CaptivePortal(
-    val detected: Boolean = false,
-    @SerialName("api_url") val apiUrl: String? = null,
-    @SerialName("venue_url") val venueUrl: String? = null,
-)
-
-@Serializable
-data class Wifi(
-    val ssid: String? = null,          // ssid (logical type)
-    val bssid: String? = null,         // bssid (logical type)
-    @SerialName("rssi_dbm") val rssiDbm: Int? = null,
-    @SerialName("link_speed_mbps") val linkSpeedMbps: Int? = null,
-    @SerialName("frequency_mhz") val frequencyMhz: Int? = null,
-    @SerialName("channel_width_mhz") val channelWidthMhz: Int? = null,
-    val standard: String? = null,
-    val security: String? = null,
-    @SerialName("mac_randomization") val macRandomization: Boolean? = null,
-)
-
-@Serializable
-data class Cellular(
-    val rat: String? = null,
-    val operator: String? = null,
-    val band: String? = null,
-)
-
-@Serializable
-data class NetworkChange(
-    @SerialName("at_mono_ns") val atMonoNs: Long,
-    val kind: String, // lost | gained | link_changed
-    val detail: JsonObject? = null,
-)

echolot-app/core-measurement/bin/main/app/echo_lot/measurement/Summary.kt

@@ -1,90 +0,0 @@
-// SPDX-FileCopyrightText: 2026 Echolot contributors
-// SPDX-License-Identifier: GPL-3.0-or-later
-
-package app.echo_lot.measurement
-
-import kotlinx.serialization.SerialName
-import kotlinx.serialization.Serializable
-
-@Serializable
-enum class Verdict {
-    @SerialName("green") GREEN,
-    @SerialName("yellow") YELLOW,
-    @SerialName("red") RED,
-    @SerialName("inconclusive") INCONCLUSIVE,
-}
-
-@Serializable
-data class Summary(
-    val overall: Verdict,
-    val categories: Map<String, CategorySummary>,
-)
-
-@Serializable
-data class CategorySummary(
-    val verdict: Verdict,
-    @SerialName("worst_finding") val worstFinding: String? = null,
-    @SerialName("tests_run") val testsRun: Int,
-    @SerialName("tests_failed") val testsFailed: Int,
-)
-
-/**
- * Deterministic verdict derivation, fixed by measurement-schema.md §7.3:
- *
- * - A category's verdict = the light of its worst-severity finding
- *   (critical|high → red, medium|low → yellow, info/none → green).
- * - A category is `inconclusive` when > 50% of its tests are failed/unsupported.
- * - Overall = the worst category light; `inconclusive` only when ALL categories are.
- *
- * The mapping test-type → category comes from [TestType.category]. Only categories that have
- * findings or tests appear in the summary.
- */
-object Verdicts {
-
-    private fun isInconclusiveTest(s: TestStatus) =
-        s == TestStatus.FAILED || s == TestStatus.UNSUPPORTED
-
-    fun derive(tests: List<Test>, findings: List<Finding>): Summary {
-        val testsByCat = tests.groupBy { TestType.category(it.type) }
-        val findingsByCat = findings.groupBy { it.category }
-        val categories = (testsByCat.keys + findingsByCat.keys)
-
-        val perCat = LinkedHashMap<String, CategorySummary>()
-        for (cat in Category.entries) {
-            if (cat !in categories) continue
-            val catTests = testsByCat[cat].orEmpty()
-            val catFindings = findingsByCat[cat].orEmpty()
-
-            val failed = catTests.count { isInconclusiveTest(it.status) }
-            val inconclusive = catTests.isNotEmpty() && failed * 2 > catTests.size
-
-            val worst = catFindings.maxByOrNull { it.severity.rank }
-            val verdict = when {
-                inconclusive -> Verdict.INCONCLUSIVE
-                worst == null -> Verdict.GREEN
-                else -> worst.severity.toLight()
-            }
-            perCat[serialName(cat)] = CategorySummary(
-                verdict = verdict,
-                worstFinding = worst?.id,
-                testsRun = catTests.size,
-                testsFailed = failed,
-            )
-        }
-
-        val overall = deriveOverall(perCat.values)
-        return Summary(overall = overall, categories = perCat)
-    }
-
-    /** Overall = worst light; inconclusive only if every category is inconclusive. */
-    private fun deriveOverall(cats: Collection<CategorySummary>): Verdict {
-        if (cats.isEmpty()) return Verdict.INCONCLUSIVE
-        if (cats.all { it.verdict == Verdict.INCONCLUSIVE }) return Verdict.INCONCLUSIVE
-        val rank = mapOf(Verdict.GREEN to 0, Verdict.YELLOW to 1, Verdict.RED to 2)
-        // Non-inconclusive categories decide the overall light.
-        return cats.filter { it.verdict != Verdict.INCONCLUSIVE }
-            .maxByOrNull { rank.getValue(it.verdict) }!!.verdict
-    }
-
-    private fun serialName(cat: Category): String = cat.name.lowercase()
-}

echolot-app/core-measurement/bin/main/app/echo_lot/measurement/Test.kt

@@ -1,149 +0,0 @@
-// SPDX-FileCopyrightText: 2026 Echolot contributors
-// SPDX-License-Identifier: GPL-3.0-or-later
-
-package app.echo_lot.measurement
-
-import kotlinx.serialization.SerialName
-import kotlinx.serialization.Serializable
-import kotlinx.serialization.json.JsonObject
-
-/** The generic test envelope (measurement-schema.md §6). params must fully reproduce the test;
- *  evidence is append-only raw truth; metrics must be recomputable from evidence. All three are
- *  per-test-type JSON, so they are carried as JsonObject. */
-@Serializable
-data class Test(
-    val id: String, // UUIDv7
-    val type: String, // TestType registry (§6.1)
-    @SerialName("network_ref") val networkRef: String? = null,
-    @SerialName("session_ref") val sessionRef: String? = null, // null for local-only tests
-    val tier: Tier,
-    @SerialName("started_mono_ns") val startedMonoNs: Long,
-    @SerialName("ended_mono_ns") val endedMonoNs: Long,
-    val status: TestStatus,
-    val error: TestError? = null,
-    val params: JsonObject? = null,
-    val evidence: JsonObject? = null,
-    val metrics: JsonObject? = null,
-)
-
-@Serializable
-enum class Tier {
-    @SerialName("app") APP,
-    @SerialName("shizuku") SHIZUKU,
-    @SerialName("root") ROOT,
-}
-
-@Serializable
-enum class TestStatus {
-    @SerialName("ok") OK,
-    @SerialName("failed") FAILED,
-    @SerialName("unsupported") UNSUPPORTED,
-    @SerialName("skipped") SKIPPED,
-    @SerialName("partial") PARTIAL,
-}
-
-@Serializable
-data class TestError(val code: String, val detail: String? = null)
-
-/**
- * The v1 test-type registry (§6.1). String constants (dotted, family-first) so probe code and the
- * server's measurement-schema test-type registry stay aligned. [category] maps a type to one of
- * the fixed §7.2 categories for verdict rollup.
- */
-object TestType {
-    // link
-    const val LINK_SNAPSHOT = "link.snapshot"
-    const val LINK_DHCP_RENEWAL_WATCH = "link.dhcp_renewal_watch"
-    const val LINK_IP_MONITOR = "link.ip_monitor"
-    /** Who advertises IPv6 on this link (+ gateway identity). Registry addition, v1.1. */
-    const val LINK_RA_SOURCE = "link.ra_source"
-    // net — connectivity validation (reproduces Android's NetworkMonitor generate_204 checks)
-    const val NET_CAPTIVE_PORTAL = "net.captive_portal"
-    // icmp
-    const val ICMP_PING4 = "icmp.ping4"
-    const val ICMP_PING6 = "icmp.ping6"
-    // trace
-    const val TRACEROUTE_UDP4 = "traceroute.udp4"
-    const val TRACEROUTE_UDP6 = "traceroute.udp6"
-    const val TRACEROUTE_ICMP4 = "traceroute.icmp4"
-    const val TRACEROUTE_ICMP6 = "traceroute.icmp6"
-    // train
-    const val TRAIN_UDP_UPDOWN = "train.udp_updown"
-    // mtu
-    const val MTU_PMTUD_UP = "mtu.pmtud_up"
-    const val MTU_PMTUD_DOWN = "mtu.pmtud_down"
-    const val MTU_BLACKHOLE = "mtu.blackhole"
-    const val MTU_MSS_OBSERVED = "mtu.mss_observed"
-    const val MTU_FRAG_DELIVERY = "mtu.frag_delivery"
-    // nat
-    const val NAT_STUN_5780 = "nat.stun_5780"
-    const val NAT_MAPPING_LIFETIME_UDP = "nat.mapping_lifetime_udp"
-    const val NAT_MAPPING_LIFETIME_TCP = "nat.mapping_lifetime_tcp"
-    const val NAT_HAIRPIN = "nat.hairpin"
-    const val NAT_CONNECT_BACK = "nat.connect_back"
-    const val NAT_CGNAT_DETECT = "nat.cgnat_detect"
-    // dns
-    const val DNS_RESOLVER_INVENTORY = "dns.resolver_inventory"
-    const val DNS_CANARY = "dns.canary"
-    const val DNS_INTERCEPTION = "dns.interception"
-    const val DNS_TTL_INTEGRITY = "dns.ttl_integrity"
-    const val DNS_ANSWER_INTEGRITY = "dns.answer_integrity"
-    const val DNS_DNSSEC = "dns.dnssec"
-    const val DNS_NXDOMAIN_WILDCARD = "dns.nxdomain_wildcard"
-    const val DNS_REBIND_FILTER = "dns.rebind_filter"
-    const val DNS_AAAA_FILTER = "dns.aaaa_filter"
-    const val DNS_DNS64 = "dns.dns64"
-    const val DNS_COMPARE = "dns.compare"
-    // sec
-    const val SEC_TLS_REFERENCE = "sec.tls_reference"
-    const val SEC_CLIENTHELLO_ECHO = "sec.clienthello_echo"
-    const val SEC_HTTP_ECHO = "sec.http_echo"
-    const val SEC_SNI_FILTER = "sec.sni_filter"
-    const val SEC_DSCP_ECN_SURVIVAL = "sec.dscp_ecn_survival"
-    const val SEC_ARP_WATCH = "sec.arp_watch"
-    // port
-    const val PORT_REACH_SWEEP = "port.reach_sweep"
-    const val PORT_UDP_USABILITY = "port.udp_usability"
-    // perf
-    const val PERF_THROUGHPUT_TCP = "perf.throughput_tcp"
-    const val PERF_THROUGHPUT_UDP = "perf.throughput_udp"
-    const val PERF_BUFFERBLOAT = "perf.bufferbloat"
-    const val PERF_RRC_LATENCY = "perf.rrc_latency"
-    // v6
-    const val V6_DUALSTACK_COMPARE = "v6.dualstack_compare"
-    const val V6_HAPPY_EYEBALLS = "v6.happy_eyeballs"
-    const val V6_BROKENNESS = "v6.brokenness"
-    const val V6_NAT64_CLAT = "v6.nat64_clat"
-    // wifi
-    const val WIFI_ENVIRONMENT_SCAN = "wifi.environment_scan"
-    const val WIFI_ROAM_LOG = "wifi.roam_log"
-    const val WIFI_SIGNAL_LOG = "wifi.signal_log"
-    // local
-    const val LOCAL_MDNS_INVENTORY = "local.mdns_inventory"
-    const val LOCAL_SSDP_INVENTORY = "local.ssdp_inventory"
-    const val LOCAL_LLMNR_INVENTORY = "local.llmnr_inventory"
-    const val LOCAL_GATEWAY_SERVICES = "local.gateway_services"
-    const val LOCAL_NTP = "local.ntp"
-    // peer
-    const val PEER_REACHABILITY = "peer.reachability"
-    const val PEER_ISOLATION = "peer.isolation"
-    const val PEER_MULTICAST = "peer.multicast"
-    const val PEER_LAN_TRAIN = "peer.lan_train"
-    const val PEER_LEASE_DIFF = "peer.lease_diff"
-    // time
-    const val TIME_SERVER_OFFSET = "time.server_offset"
-
-    /** Maps a dotted test type to its §7.2 category for verdict rollup. */
-    fun category(type: String): Category = when (type.substringBefore('.')) {
-        "link", "icmp", "trace", "traceroute", "train", "port", "time", "net" -> Category.CONNECTIVITY
-        "dns" -> Category.DNS
-        "nat" -> Category.NAT
-        "mtu" -> Category.MTU
-        "v6" -> Category.IPV6
-        "sec" -> Category.SECURITY
-        "perf" -> Category.PERFORMANCE
-        "local", "peer" -> Category.LOCAL
-        "wifi" -> Category.WIFI
-        else -> Category.CONNECTIVITY
-    }
-}

echolot-app/core-measurement/bin/test/app/echo_lot/measurement/SerializationTest.kt

@@ -1,71 +0,0 @@
-// SPDX-FileCopyrightText: 2026 Echolot contributors
-// SPDX-License-Identifier: GPL-3.0-or-later
-
-package app.echo_lot.measurement
-
-import kotlinx.serialization.json.Json
-import kotlin.test.Test as JTest
-import kotlin.test.assertEquals
-import kotlin.test.assertTrue
-
-class SerializationTest {
-
-    private val json = Json { ignoreUnknownKeys = true; encodeDefaults = true }
-
-    @JTest
-    fun documentRoundTrips() {
-        val doc = MeasurementDocument(
-            run = Run(
-                id = "0198c5f2-0000-7000-8000-000000000000",
-                trigger = Trigger.MANUAL,
-                startedAt = "2026-07-31T14:03:21.114Z",
-                clock = Clock(monoOriginWall = "2026-07-31T14:03:21.114Z"),
-                app = AppInfo(version = "0.1.0", build = 1),
-                device = DeviceInfo("OnePlus", "CPH2747", 36, "16"),
-                tiers = Tiers(app = true, shizuku = true),
-            ),
-            networks = listOf(
-                Network(
-                    id = "net-1", transport = Transport.WIFI, iface = "wlan0",
-                    link = Link(mtu = 1500, addresses = listOf(Address("192.0.2.23", 24, "global"))),
-                    wifi = Wifi(ssid = "example", rssiDbm = -54),
-                ),
-            ),
-            tests = listOf(
-                Test(
-                    id = "t-1", type = TestType.ICMP_PING4, networkRef = "net-1", tier = Tier.APP,
-                    startedMonoNs = 0, endedMonoNs = 38_000_000, status = TestStatus.OK,
-                    evidence = TrainEvidence(
-                        epochMonoNs = 0, seq = listOf(0, 1), tTxNs = listOf(0L, 20_000_000L),
-                        tRxNs = listOf(16_500_000L, null), sizeBytes = listOf(64, 64),
-                    ).toEvidence(),
-                ),
-            ),
-        )
-
-        val encoded = json.encodeToString(MeasurementDocument.serializer(), doc)
-        val decoded = json.decodeFromString(MeasurementDocument.serializer(), encoded)
-        assertEquals(doc.run.id, decoded.run.id)
-        assertEquals(Transport.WIFI, decoded.networks[0].transport)
-        assertEquals(TestType.ICMP_PING4, decoded.tests[0].type)
-        // snake_case field names on the wire
-        assertTrue(encoded.contains("\"schema_version\""))
-        assertTrue(encoded.contains("\"mono_origin_wall\""))
-        assertTrue(encoded.contains("\"t_tx_ns\""))
-        // null preserved at train index 1
-        assertTrue(encoded.contains("[16500000,null]"))
-    }
-
-    @JTest
-    fun findingRequiresEvidence() {
-        try {
-            Finding(
-                id = "f-1", code = "x", category = Category.DNS, severity = Severity.INFO,
-                confidence = Confidence.LOW, title = "t", description = "d", evidenceRefs = emptyList(),
-            )
-            throw AssertionError("expected IllegalArgumentException for empty evidence_refs")
-        } catch (e: IllegalArgumentException) {
-            // expected — a finding with no evidence is invalid (§7.1)
-        }
-    }
-}

echolot-app/core-measurement/bin/test/app/echo_lot/measurement/VerdictsTest.kt

@@ -1,109 +0,0 @@
-// SPDX-FileCopyrightText: 2026 Echolot contributors
-// SPDX-License-Identifier: GPL-3.0-or-later
-
-package app.echo_lot.measurement
-
-import kotlin.test.Test as JTest
-import kotlin.test.assertEquals
-
-class VerdictsTest {
-
-    private fun test(type: String, status: TestStatus, id: String = type): Test =
-        Test(id = id, type = type, tier = Tier.APP, startedMonoNs = 0, endedMonoNs = 1, status = status)
-
-    private fun finding(cat: Category, sev: Severity, id: String = "f-$cat-$sev"): Finding =
-        Finding(
-            id = id, code = "x.$cat", category = cat, severity = sev, confidence = Confidence.HIGH,
-            title = "t", description = "d", evidenceRefs = listOf(EvidenceRef("some-test")),
-        )
-
-    @JTest
-    fun categoryLightFromWorstSeverity() {
-        val tests = listOf(test(TestType.DNS_CANARY, TestStatus.OK))
-        val findings = listOf(
-            finding(Category.DNS, Severity.LOW),
-            finding(Category.DNS, Severity.HIGH), // worst → red
-            finding(Category.DNS, Severity.INFO),
-        )
-        val s = Verdicts.derive(tests, findings)
-        assertEquals(Verdict.RED, s.categories["dns"]!!.verdict)
-        assertEquals("f-DNS-HIGH", s.categories["dns"]!!.worstFinding)
-        assertEquals(Verdict.RED, s.overall)
-    }
-
-    @JTest
-    fun noFindingsIsGreen() {
-        val s = Verdicts.derive(listOf(test(TestType.MTU_BLACKHOLE, TestStatus.OK)), emptyList())
-        assertEquals(Verdict.GREEN, s.categories["mtu"]!!.verdict)
-        assertEquals(Verdict.GREEN, s.overall)
-    }
-
-    @JTest
-    fun mediumAndLowAreYellow() {
-        val s = Verdicts.derive(
-            listOf(test(TestType.SEC_HTTP_ECHO, TestStatus.OK)),
-            listOf(finding(Category.SECURITY, Severity.MEDIUM)),
-        )
-        assertEquals(Verdict.YELLOW, s.categories["security"]!!.verdict)
-    }
-
-    @JTest
-    fun majorityFailedIsInconclusive() {
-        // 2 of 3 dns tests failed → > 50% → inconclusive, even with a finding present.
-        val tests = listOf(
-            test(TestType.DNS_CANARY, TestStatus.FAILED, "a"),
-            test(TestType.DNS_TTL_INTEGRITY, TestStatus.UNSUPPORTED, "b"),
-            test(TestType.DNS_COMPARE, TestStatus.OK, "c"),
-        )
-        val s = Verdicts.derive(tests, listOf(finding(Category.DNS, Severity.HIGH)))
-        assertEquals(Verdict.INCONCLUSIVE, s.categories["dns"]!!.verdict)
-        assertEquals(2, s.categories["dns"]!!.testsFailed)
-        assertEquals(3, s.categories["dns"]!!.testsRun)
-    }
-
-    @JTest
-    fun exactlyHalfFailedIsNotInconclusive() {
-        // 1 of 2 failed → not > 50% → the finding decides.
-        val tests = listOf(
-            test(TestType.NAT_HAIRPIN, TestStatus.FAILED, "a"),
-            test(TestType.NAT_CONNECT_BACK, TestStatus.OK, "b"),
-        )
-        val s = Verdicts.derive(tests, listOf(finding(Category.NAT, Severity.CRITICAL)))
-        assertEquals(Verdict.RED, s.categories["nat"]!!.verdict)
-    }
-
-    @JTest
-    fun overallIsWorstCategory() {
-        val tests = listOf(
-            test(TestType.DNS_CANARY, TestStatus.OK, "d"),
-            test(TestType.MTU_BLACKHOLE, TestStatus.OK, "m"),
-        )
-        val findings = listOf(
-            finding(Category.DNS, Severity.MEDIUM),   // yellow
-            finding(Category.MTU, Severity.CRITICAL), // red
-        )
-        val s = Verdicts.derive(tests, findings)
-        assertEquals(Verdict.RED, s.overall)
-    }
-
-    @JTest
-    fun overallInconclusiveOnlyWhenAllAre() {
-        val tests = listOf(
-            test(TestType.DNS_CANARY, TestStatus.FAILED, "d"),    // dns inconclusive
-            test(TestType.MTU_BLACKHOLE, TestStatus.OK, "m"),     // mtu green
-        )
-        val s = Verdicts.derive(tests, emptyList())
-        assertEquals(Verdict.INCONCLUSIVE, s.categories["dns"]!!.verdict)
-        assertEquals(Verdict.GREEN, s.categories["mtu"]!!.verdict)
-        assertEquals(Verdict.GREEN, s.overall) // not all inconclusive → mtu decides
-    }
-
-    @JTest
-    fun categoryMappingCoversFamilies() {
-        assertEquals(Category.CONNECTIVITY, TestType.category(TestType.TRACEROUTE_UDP4))
-        assertEquals(Category.IPV6, TestType.category(TestType.V6_BROKENNESS))
-        assertEquals(Category.LOCAL, TestType.category(TestType.PEER_MULTICAST))
-        assertEquals(Category.PERFORMANCE, TestType.category(TestType.PERF_BUFFERBLOAT))
-        assertEquals(Category.CONNECTIVITY, TestType.category(TestType.TIME_SERVER_OFFSET))
-    }
-}

echolot-app/core-measurement/src/main/kotlin/app/echo_lot/measurement/Test.kt

@@ -69,6 +69,8 @@ object TestType {
     const val TRACEROUTE_ICMP6 = "traceroute.icmp6"
     // train
     const val TRAIN_UDP_UPDOWN = "train.udp_updown"
+    /** Server-to-client train under a §3.4 grant: the direction a round trip cannot separate. */
+    const val TRAIN_UDP_DOWNSTREAM = "train.udp_downstream"
     // mtu
     const val MTU_PMTUD_UP = "mtu.pmtud_up"
     const val MTU_PMTUD_DOWN = "mtu.pmtud_down"

echolot-app/core-protocol/src/main/kotlin/app/echo_lot/protocol/Compat.kt

@@ -0,0 +1,202 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package app.echo_lot.protocol
+
+/**
+ * Version compatibility, the client side of the same question the server asks about us.
+ *
+ * Versions are SemVer, but what is really being checked is whether the peer speaks a wire protocol
+ * and schema this build understands; the version is a proxy, and it only works because the
+ * breaking axis is bumped when the contract changes. Bounds therefore sit at breaking boundaries
+ * rather than at individual releases — a server patch release must never make the app refuse to
+ * talk to it.
+ *
+ * Mirrors `server/internal/compat`. Two implementations of one rule is a duplication worth
+ * accepting: each side must be able to state and enforce its own limits without asking the other,
+ * which is the entire point of a compatibility check.
+ */
+data class SemVer(
+    val major: Int,
+    val minor: Int,
+    val patch: Int,
+    val pre: String = "",
+) : Comparable<SemVer> {
+
+    override fun compareTo(other: SemVer): Int {
+        (major - other.major).let { if (it != 0) return it.coerceIn(-1, 1) }
+        (minor - other.minor).let { if (it != 0) return it.coerceIn(-1, 1) }
+        (patch - other.patch).let { if (it != 0) return it.coerceIn(-1, 1) }
+        // A pre-release sorts below the same version without one (SemVer §11).
+        return when {
+            pre == other.pre -> 0
+            pre.isEmpty() -> 1
+            other.pre.isEmpty() -> -1
+            else -> pre.compareTo(other.pre).coerceIn(-1, 1)
+        }
+    }
+
+    override fun toString(): String = "$major.$minor.$patch" + if (pre.isEmpty()) "" else "-$pre"
+
+    /**
+     * The first version that may break compatibility with this one. Below 1.0.0 the minor is the
+     * breaking axis (SemVer §4), so 0.4.2's next break is 0.5.0 — treating it as 1.0.0 would let
+     * this build accept a peer it cannot actually talk to.
+     */
+    fun nextBreaking(): SemVer =
+        if (major == 0) SemVer(0, minor + 1, 0) else SemVer(major + 1, 0, 0)
+
+    companion object {
+        /** Accepts "1.2.3", "v1.2.3" and namespaced tags like "server-v1.2.3". Null if unusable. */
+        fun parse(raw: String?): SemVer? {
+            var s = raw?.trim().orEmpty()
+            if (s.isEmpty()) return null
+            // Strip a tag prefix ending in "v", guarded on the prefix having no digits so a
+            // pre-release identifier containing a "v" is left alone.
+            val v = s.lastIndexOf('v')
+            if (v >= 0 && v + 1 < s.length && s[v + 1].isDigit() && s.take(v).none { it.isDigit() }) {
+                s = s.substring(v + 1)
+            }
+            s = s.substringBefore('+')
+            val pre = s.substringAfter('-', "")
+            val core = s.substringBefore('-')
+            val parts = core.split(".")
+            if (parts.size != 3) return null
+            val nums = parts.map { it.toIntOrNull() ?: return null }
+            if (nums.any { it < 0 }) return null
+            return SemVer(nums[0], nums[1], nums[2], pre)
+        }
+    }
+}
+
+/** [min, max): minimum inclusive, maximum exclusive. A null max is unbounded. */
+data class VersionRange(val min: SemVer, val max: SemVer? = null) {
+    operator fun contains(v: SemVer): Boolean = v >= min && (max == null || v < max)
+    override fun toString(): String = ">= $min" + (max?.let { ", < $it" } ?: "")
+
+    companion object {
+        fun of(min: String, max: String?): VersionRange {
+            val lo = requireNotNull(SemVer.parse(min)) { "bad minimum version: $min" }
+            val hi = max?.takeIf { it.isNotBlank() }?.let {
+                requireNotNull(SemVer.parse(it)) { "bad maximum version: $it" }
+            }
+            require(hi == null || hi > lo) { "maximum $max is not above minimum $min" }
+            return VersionRange(lo, hi)
+        }
+    }
+}
+
+/**
+ * What this build of the app requires of a server, and what it tells servers about itself.
+ *
+ * Two separate questions, deliberately not conflated:
+ *
+ *  - **Protocol version** — can these builds talk at all? This is the correctness axis, and its
+ *    breaking boundary is enforced strictly.
+ *  - **Release-version window** — should they, per policy? A coarse safety net over the peer's
+ *    SemVer, with bounds at breaking boundaries so a patch release never strands anyone. Both
+ *    sides publish their own, and the operator can tighten the server's.
+ *
+ * `MIN_SERVER` is 0.4.2 for a concrete reason, not caution: below it a multi-homed server sent
+ * granted traffic from an address the session never used, so every downstream packet was dropped
+ * in transit and reported as 100 % downstream loss. A confidently wrong measurement is worse than
+ * a refused one, so talking to those builds is not something to allow "just in case".
+ */
+object Compat {
+    /** Header the app sets on every control-plane request. */
+    const val APP_VERSION_HEADER = "X-Echolot-App-Version"
+
+    /** The wire contract (probe-protocol.md) this build implements. */
+    const val PROTOCOL_VERSION = "1.0.0"
+
+    const val MIN_SERVER = "0.4.2"
+
+    /** Exclusive. The next breaking series is refused until this app is taught about it. */
+    const val MAX_SERVER = "1.0.0"
+
+    val serverRange: VersionRange = VersionRange.of(MIN_SERVER, MAX_SERVER)
+
+    enum class Verdict { OK, PROTOCOL_MISMATCH, SERVER_TOO_OLD, SERVER_TOO_NEW, APP_REFUSED, UNKNOWN }
+
+    /**
+     * The result of checking a server, with a message written for the person holding the phone.
+     * [usable] is what callers branch on; [message] is what they show.
+     */
+    data class Result(val verdict: Verdict, val message: String?) {
+        val usable: Boolean get() = verdict == Verdict.OK || verdict == Verdict.UNKNOWN
+    }
+
+    /**
+     * Checks a profile both ways: is the server within our range, and are we within the server's.
+     *
+     * Asking both is the point of advertising the window in the profile. Discovering that the
+     * server will refuse us only when a measurement fails halfway through is a much worse
+     * experience than being told before the run starts.
+     */
+    fun check(profile: Profile, appVersion: String): Result {
+        // The protocol version is the axis that decides whether these two builds *can* talk;
+        // the release-version window below is the operator's policy about whether they *may*.
+        // Checking the real thing first means a mismatch is reported as what it is.
+        val ours = SemVer.parse(PROTOCOL_VERSION)!!
+        val theirs = SemVer.parse(profile.compat.protocolVersion)
+        if (theirs != null && theirs >= ours.nextBreaking()) {
+            return Result(
+                Verdict.PROTOCOL_MISMATCH,
+                "This server speaks probe protocol ${profile.compat.protocolVersion}; this app " +
+                    "speaks $PROTOCOL_VERSION and does not understand that revision. Update the app.",
+            )
+        }
+        if (theirs != null && ours >= theirs.nextBreaking()) {
+            return Result(
+                Verdict.PROTOCOL_MISMATCH,
+                "This server speaks probe protocol ${profile.compat.protocolVersion}, which this " +
+                    "app ($PROTOCOL_VERSION) has moved past. Update the server.",
+            )
+        }
+
+        val server = SemVer.parse(profile.serverVersion)
+            ?: return Result(
+                Verdict.UNKNOWN,
+                "Server did not report a usable version (\"${profile.serverVersion}\") — " +
+                    "continuing without a compatibility check.",
+            )
+
+        if (server < serverRange.min) {
+            return Result(
+                Verdict.SERVER_TOO_OLD,
+                "This server runs $server; Echolot needs $serverRange. " +
+                    "Measurements against older servers can be wrong rather than merely missing, " +
+                    "so update the server.",
+            )
+        }
+        if (serverRange.max != null && server >= serverRange.max) {
+            return Result(
+                Verdict.SERVER_TOO_NEW,
+                "This server runs $server, which is newer than this app understands " +
+                    "($serverRange). Update the app.",
+            )
+        }
+
+        // And the server's own view of us.
+        val app = SemVer.parse(appVersion)
+        val serverWantsMin = SemVer.parse(profile.compat.appMin)
+        val serverWantsMax = SemVer.parse(profile.compat.appMax)
+        if (app != null && serverWantsMin != null) {
+            if (app < serverWantsMin) {
+                return Result(
+                    Verdict.APP_REFUSED,
+                    "This server only accepts Echolot $serverWantsMin or newer; this app is " +
+                        "$app. Update the app.",
+                )
+            }
+            if (serverWantsMax != null && app >= serverWantsMax) {
+                return Result(
+                    Verdict.APP_REFUSED,
+                    "This server refuses Echolot $serverWantsMax and newer; this app is $app. " +
+                        "Use an older app, or a server that has caught up.",
+                )
+            }
+        }
+        return Result(Verdict.OK, null)
+    }
+}

echolot-app/core-protocol/src/main/kotlin/app/echo_lot/protocol/ControlClient.kt

@@ -4,9 +4,21 @@
 package app.echo_lot.protocol
 
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.jsonObject
+import kotlinx.serialization.json.jsonPrimitive
 import java.net.URL
 import javax.net.ssl.HttpsURLConnection
 
+/** The server declined to store this run, per the operator's policy. Not a transport failure. */
+class UploadRefused(message: String) : Exception(message)
+
+/**
+ * The server refused this app's version. Distinct from a transport failure and from an auth
+ * failure: nothing about the request was wrong, the two builds simply do not go together, and the
+ * message says which versions do.
+ */
+class VersionRefused(message: String) : Exception(message)
+
 /**
  * The control-plane client (probe-protocol.md §2): enrollment, profile, sessions — over
  * SPKI-pinned HTTPS. Uses HttpsURLConnection (available since Android API 1, unlike
@@ -16,11 +28,14 @@ import javax.net.ssl.HttpsURLConnection
  *
  * @param controlUrl e.g. "https://fmr-1.echo-lot.app:8443"
  * @param pins       the `pin-sha256` value(s) from the enrollment QR (base64, no prefix)
+ * @param appVersion this build's SemVer, sent on every request so the server can refuse a build
+ *                   it cannot serve *before* a measurement half-runs (BuildConfig.VERSION_NAME).
  */
-/** The server declined to store this run, per the operator's policy. Not a transport failure. */
+class ControlClient(
-class UploadRefused(message: String) : Exception(message)
+    private val controlUrl: String,
-
+    pins: Set<String>,
-class ControlClient(private val controlUrl: String, pins: Set<String>) {
+    private val appVersion: String = "",
+) {
 
     private val json = Json { ignoreUnknownKeys = true }
     private val socketFactory = Pinning.sslContext(pins).socketFactory
@@ -33,12 +48,42 @@ class ControlClient(private val controlUrl: String, pins: Set<String>) {
         conn.connectTimeout = 10_000
         conn.readTimeout = 10_000
         credential?.let { conn.setRequestProperty("Authorization", "Bearer $it") }
+        if (appVersion.isNotBlank()) conn.setRequestProperty(Compat.APP_VERSION_HEADER, appVersion)
         return conn
     }
 
+    /**
+     * 426 is the server saying "your version, not your request". Raised as a distinct exception
+     * from every call site so callers never report it as a network error — the whole value of the
+     * check is that the failure is legible.
+     */
+    private fun checkVersion(conn: HttpsURLConnection, body: String) {
+        if (conn.responseCode == 426) throw VersionRefused(extractError(body) ?: body.take(200))
+    }
+
+    /**
+     * Pulls the "error" string out of a JSON body.
+     *
+     * Parsed rather than pattern-matched: an encoder may legitimately escape characters in the
+     * message (Go escapes ">" by default), and a regex hands the user "needs \u003e= 0.2.0".
+     * The parser knows how to undo every escape; a regex would have to be taught each one.
+     */
+    private fun extractError(body: String): String? = runCatching {
+        json.parseToJsonElement(body).jsonObject["error"]?.jsonPrimitive?.content
+    }.getOrNull()
+
+    /**
+     * Reads the response body, and turns a 426 into [VersionRefused] first.
+     *
+     * Every call goes through here, so the version check cannot be forgotten at a new call site —
+     * the alternative (a check per method) is exactly the kind of thing that gets missed once and
+     * then reports "upload failed: 426" to a user for a year.
+     */
     private fun body(conn: HttpsURLConnection): String {
         val stream = if (conn.responseCode in 200..299) conn.inputStream else conn.errorStream
-        return stream?.bufferedReader()?.use { it.readText() } ?: ""
+        val text = stream?.bufferedReader()?.use { it.readText() } ?: ""
+        checkVersion(conn, text)
+        return text
     }
 
     private fun writeJson(conn: HttpsURLConnection, payload: String) {
@@ -66,21 +111,24 @@ class ControlClient(private val controlUrl: String, pins: Set<String>) {
         val conn = open("/v1/enroll", "POST", null)
         conn.setRequestProperty("Authorization", "Bearer $token")
         writeJson(conn, if (name != null) """{"name":${jstr(name)}}""" else "{}")
-        check(conn.responseCode == 201) { "enroll failed: ${conn.responseCode} ${body(conn)}" }
+        val text = body(conn) // reads and raises VersionRefused on 426
-        return json.decodeFromString(EnrollResponse.serializer(), body(conn))
+        check(conn.responseCode == 201) { "enroll failed: ${conn.responseCode} $text" }
+        return json.decodeFromString(EnrollResponse.serializer(), text)
     }
 
     fun profile(credential: String): Profile {
         val conn = open("/v1/profile", "GET", credential)
-        check(conn.responseCode == 200) { "profile failed: ${conn.responseCode} ${body(conn)}" }
+        val text = body(conn)
-        return json.decodeFromString(Profile.serializer(), body(conn))
+        check(conn.responseCode == 200) { "profile failed: ${conn.responseCode} $text" }
+        return json.decodeFromString(Profile.serializer(), text)
     }
 
     fun createSession(credential: String, target: String): SessionResponse {
         val conn = open("/v1/sessions", "POST", credential)
         writeJson(conn, """{"target":${jstr(target)}}""")
-        check(conn.responseCode == 201) { "session failed: ${conn.responseCode} ${body(conn)}" }
+        val text = body(conn)
-        return json.decodeFromString(SessionResponse.serializer(), body(conn))
+        check(conn.responseCode == 201) { "session failed: ${conn.responseCode} $text" }
+        return json.decodeFromString(SessionResponse.serializer(), text)
     }
 
     /**
@@ -111,7 +159,7 @@ class ControlClient(private val controlUrl: String, pins: Set<String>) {
         val body = body(conn)
         when (conn.responseCode) {
             in 200..299 -> return body
-            403 -> throw UploadRefused(body)
+            403 -> throw UploadRefused(extractError(body) ?: body.take(200))
             413 -> throw UploadRefused("run is larger than this server accepts: $body")
             else -> error("upload failed: ${conn.responseCode} $body")
         }
@@ -120,14 +168,16 @@ class ControlClient(private val controlUrl: String, pins: Set<String>) {
     /** Lists this device's runs stored on the server. */
     fun listRuns(credential: String): String {
         val conn = open("/v1/runs", "GET", credential)
-        check(conn.responseCode == 200) { "list runs failed: ${conn.responseCode}" }
+        val text = body(conn)
-        return body(conn)
+        check(conn.responseCode == 200) { "list runs failed: ${conn.responseCode} $text" }
+        return text
     }
 
     fun getRun(credential: String, runId: String): String {
         val conn = open("/v1/runs/$runId", "GET", credential)
-        check(conn.responseCode == 200) { "get run failed: ${conn.responseCode}" }
+        val text = body(conn)
-        return body(conn)
+        check(conn.responseCode == 200) { "get run failed: ${conn.responseCode} $text" }
+        return text
     }
 
     fun deleteRun(credential: String, runId: String) {
@@ -136,8 +186,9 @@ class ControlClient(private val controlUrl: String, pins: Set<String>) {
 
     fun observations(credential: String, sessionId: String): String {
         val conn = open("/v1/sessions/$sessionId/observations", "GET", credential)
-        check(conn.responseCode == 200) { "observations failed: ${conn.responseCode}" }
+        val text = body(conn)
-        return body(conn)
+        check(conn.responseCode == 200) { "observations failed: ${conn.responseCode} $text" }
+        return text
     }
 
     fun deleteSession(credential: String, sessionId: String) {

echolot-app/core-protocol/src/main/kotlin/app/echo_lot/protocol/Model.kt

@@ -56,6 +56,16 @@ data class UploadPolicy(
     }
 }
 
+/** The server's declaration of what it speaks and which app versions it will serve. */
+@Serializable
+data class CompatInfo(
+    @SerialName("protocol_version") val protocolVersion: String = "",
+    @SerialName("schema_version") val schemaVersion: String = "",
+    @SerialName("app_min") val appMin: String = "",
+    /** Exclusive; empty means the server sets no upper bound. */
+    @SerialName("app_max") val appMax: String = "",
+)
+
 @Serializable
 data class Profile(
     @SerialName("profile_version") val profileVersion: Int = 0,
@@ -67,6 +77,7 @@ data class Profile(
     @SerialName("server_selftest") val serverSelftest: SelfTest? = null,
     val pins: List<String> = emptyList(),
     val uploads: UploadPolicy = UploadPolicy(),
+    val compat: CompatInfo = CompatInfo(),
 ) {
     fun supports(capability: String) = capability in capabilities
 }

echolot-app/core-protocol/src/main/kotlin/app/echo_lot/protocol/ProbeSession.kt

@@ -12,6 +12,11 @@ import java.util.Base64
  * A data-plane session (probe-protocol.md §3): derives the session key, then sends signed ELT1
  * packets to the server's UDP endpoint and reads back verified responses. One session ↔ one
  * server target. Blocking; the caller owns threading.
+ *
+ * One instance per server session, for its whole lifetime. Sequence numbers start at zero here
+ * while the server's anti-replay window (§3.2) keeps counting, so a second instance sharing a
+ * session id has all its packets discarded as replays — and, because the server then never
+ * records the new source, any granted send still targets the socket that was closed.
  */
 class ProbeSession(
     private val credential: String,

echolot-app/core-protocol/src/test/kotlin/app/echo_lot/protocol/CompatTest.kt

@@ -0,0 +1,162 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package app.echo_lot.protocol
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNotNull
+import kotlin.test.assertNull
+import kotlin.test.assertTrue
+
+/**
+ * The client half of the compatibility rule. Deliberately mirrors `compat_test.go`: the two
+ * implementations must agree on where the boundaries are, or one side refuses a peer the other
+ * accepts and the disagreement surfaces as an inexplicable failure in the field.
+ */
+class CompatTest {
+
+    @Test
+    fun parsesTheFormsThatActuallyReachUs() {
+        assertEquals(SemVer(1, 2, 3), SemVer.parse("1.2.3"))
+        assertEquals(SemVer(1, 2, 3), SemVer.parse("v1.2.3"))
+        assertEquals(SemVer(0, 4, 2), SemVer.parse("server-v0.4.2"))
+        assertEquals(SemVer(0, 2, 0), SemVer.parse(" 0.2.0 "))
+        assertEquals(SemVer(1, 0, 0, "rc1"), SemVer.parse("1.0.0-rc1"))
+        assertEquals(SemVer(1, 0, 0), SemVer.parse("1.0.0+build.7"))
+        assertEquals(SemVer(1, 0, 0, "rc1"), SemVer.parse("1.0.0-rc1+meta"))
+        // A pre-release identifier containing a "v" is not a tag prefix.
+        assertEquals(SemVer(1, 2, 3, "rcv1"), SemVer.parse("1.2.3-rcv1"))
+
+        for (bad in listOf("", "dev", "1.2", "1.2.3.4", "x.y.z", "-1.0.0", "1.2.beta", null)) {
+            assertNull(SemVer.parse(bad), "should not parse: $bad")
+        }
+    }
+
+    @Test
+    fun ordersPreReleasesBelowTheirRelease() {
+        fun lt(a: String, b: String) {
+            val x = assertNotNull(SemVer.parse(a))
+            val y = assertNotNull(SemVer.parse(b))
+            assertTrue(x < y, "$a should sort below $b")
+            assertTrue(y > x)
+        }
+        lt("0.9.9", "1.0.0")
+        lt("1.0.0", "1.0.1")
+        lt("1.0.0", "1.1.0")
+        lt("1.0.0-rc1", "1.0.0")
+        lt("1.0.0-rc1", "1.0.0-rc2")
+        assertEquals(0, SemVer.parse("1.2.3")!!.compareTo(SemVer.parse("v1.2.3")!!))
+    }
+
+    // Below 1.0.0 the minor is the breaking axis. Must match Go's NextBreaking exactly.
+    @Test
+    fun nextBreakingUsesTheMinorBelowOne() {
+        assertEquals("0.5.0", SemVer.parse("0.4.2")!!.nextBreaking().toString())
+        assertEquals("0.1.0", SemVer.parse("0.0.9")!!.nextBreaking().toString())
+        assertEquals("2.0.0", SemVer.parse("1.2.3")!!.nextBreaking().toString())
+    }
+
+    @Test
+    fun rangeIsMinInclusiveMaxExclusive() {
+        val r = VersionRange.of("0.2.0", "1.0.0")
+        for (s in listOf("0.2.0", "0.2.1", "0.9.9", "1.0.0-rc1")) {
+            assertTrue(SemVer.parse(s)!! in r, "$s should be inside $r")
+        }
+        for (s in listOf("0.1.9", "1.0.0", "1.0.1", "2.0.0")) {
+            assertTrue(SemVer.parse(s)!! !in r, "$s should be outside $r")
+        }
+        assertTrue(SemVer.parse("99.0.0")!! in VersionRange.of("0.2.0", null), "empty max is unbounded")
+    }
+
+    private fun profile(serverVersion: String, appMin: String = "0.2.0", appMax: String = "1.0.0") =
+        Profile(
+            serverVersion = serverVersion,
+            compat = CompatInfo(protocolVersion = "1.0.0", appMin = appMin, appMax = appMax),
+        )
+
+    @Test
+    fun acceptsAServerInsideTheWindow() {
+        val r = Compat.check(profile("0.4.2"), appVersion = "0.2.0")
+        assertEquals(Compat.Verdict.OK, r.verdict)
+        assertTrue(r.usable)
+        assertNull(r.message)
+    }
+
+    // 0.4.2 is the minimum for a concrete reason: older multi-homed servers mis-address granted
+    // sends and the client reports 100% downstream loss that never happened.
+    @Test
+    fun refusesAServerBelowTheMinimum() {
+        val r = Compat.check(profile("0.4.1"), appVersion = "0.2.0")
+        assertEquals(Compat.Verdict.SERVER_TOO_OLD, r.verdict)
+        assertTrue(!r.usable)
+        assertTrue(r.message!!.contains("0.4.1") && r.message!!.contains("0.4.2"),
+            "the message must name both versions: ${r.message}")
+    }
+
+    @Test
+    fun refusesAServerFromANewerBreakingSeries() {
+        val r = Compat.check(profile("1.0.0"), appVersion = "0.2.0")
+        assertEquals(Compat.Verdict.SERVER_TOO_NEW, r.verdict)
+        assertTrue(r.message!!.contains("Update the app"), "should tell the user what to do")
+    }
+
+    // Learning that the server will refuse us only when a measurement fails halfway is a much
+    // worse experience than being told before the run starts — so the check goes both ways.
+    @Test
+    fun detectsThatTheServerWouldRefuseThisApp() {
+        val old = Compat.check(profile("0.4.2", appMin = "0.5.0"), appVersion = "0.2.0")
+        assertEquals(Compat.Verdict.APP_REFUSED, old.verdict)
+        assertTrue(old.message!!.contains("0.5.0"))
+
+        val tooNew = Compat.check(profile("0.4.2", appMax = "0.3.0"), appVersion = "0.4.0")
+        assertEquals(Compat.Verdict.APP_REFUSED, tooNew.verdict)
+    }
+
+    // A development build reports something unparseable. Locking a developer out of their own
+    // server would be a poor trade for a check meant to make failures clearer.
+    @Test
+    fun unknownVersionsAreUsableWithAnExplanation() {
+        val r = Compat.check(profile("dev"), appVersion = "0.2.0")
+        assertEquals(Compat.Verdict.UNKNOWN, r.verdict)
+        assertTrue(r.usable, "an unidentifiable server must not be treated as incompatible")
+        assertNotNull(r.message)
+    }
+
+    @Test
+    fun aServerThatDeclaresNoWindowIsNotTreatedAsRefusingUs() {
+        // An older server predating the compat block sends nothing; absence must not read as
+        // a restriction.
+        val r = Compat.check(Profile(serverVersion = "0.4.2"), appVersion = "0.2.0")
+        assertEquals(Compat.Verdict.OK, r.verdict)
+    }
+
+    // The protocol version decides whether the builds *can* talk; the release window is only
+    // policy about whether they *may*. A protocol break must be reported as a protocol break,
+    // even when both release versions sit comfortably inside their windows.
+    @Test
+    fun aProtocolBreakIsReportedAsOne() {
+        val newer = Profile(
+            serverVersion = "0.9.0",
+            compat = CompatInfo(protocolVersion = "2.0.0", appMin = "0.2.0", appMax = "1.0.0"),
+        )
+        val r = Compat.check(newer, appVersion = "0.2.0")
+        assertEquals(Compat.Verdict.PROTOCOL_MISMATCH, r.verdict)
+        assertTrue(r.message!!.contains("2.0.0"))
+
+        // Same protocol series, different patch: fine. A protocol bugfix must not split a fleet.
+        val samePatch = Profile(
+            serverVersion = "0.9.0",
+            compat = CompatInfo(protocolVersion = "1.0.4", appMin = "0.2.0", appMax = "1.0.0"),
+        )
+        assertEquals(Compat.Verdict.OK, Compat.check(samePatch, appVersion = "0.2.0").verdict)
+    }
+
+    @Test
+    fun thisBuildsOwnBoundsAreWellFormed() {
+        val r = Compat.serverRange
+        assertEquals(SemVer.parse(Compat.MIN_SERVER), r.min)
+        assertEquals(SemVer.parse(Compat.MAX_SERVER), r.max)
+        assertTrue(r.min < r.max!!, "the built-in window must be non-empty")
+    }
+}

server/cmd/echolot-server/main.go

@@ -36,6 +36,7 @@ import (
 	"time"
 
 	"echo-lot.app/server/internal/canarydns"
+	"echo-lot.app/server/internal/compat"
 	"echo-lot.app/server/internal/config"
 	"echo-lot.app/server/internal/control"
 	"echo-lot.app/server/internal/dataplane"
@@ -131,6 +132,15 @@ func serve(cfg *config.Config) error {
 			"retention_days", cfg.UploadRetentionDays, "max_runs_per_device", cfg.UploadMaxRuns)
 	}
 
+	// A malformed window is fatal rather than ignored: an operator who set a restriction must
+	// not end up running without one because of a typo.
+	appRange, err := compat.ParseRange(cfg.MinAppVersion, cfg.MaxAppVersion)
+	if err != nil {
+		return fmt.Errorf("app version window: %w", err)
+	}
+	slog.Info("client compatibility", "accepts_app", appRange.String(),
+		"protocol", control.ProtocolVersion, "schema", control.SchemaVersion)
+
 	ctl := &control.Server{
 		Store: st, Sessions: sessions, Name: cfg.Name,
 		UDPPort: mustPort(firstAddr(cfg.UDPListen)), TCPPort: mustPort(firstAddr(cfg.TCPListen)),
@@ -140,6 +150,7 @@ func serve(cfg *config.Config) error {
 		BigSend:     dp.BigSend,
 		TCPRecent:   func(ip string) any { return tcpSrv.RecentFor(ip) },
 		Runs:        runStore,
+		AppRange:    appRange,
 	}
 
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)

server/internal/compat/compat.go

@@ -0,0 +1,200 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+// Package compat decides whether two Echolot builds should talk to each other.
+//
+// Versions are SemVer. What is actually being checked, though, is not "is this release recent"
+// but "does this peer speak a wire protocol and schema I understand" — the version is a proxy for
+// that, and the proxy only holds because we bump the breaking axis when the contract changes.
+// So the bounds here are set at *breaking boundaries*, not at every release: a patch bump must
+// never strand a fleet, and the range must not need editing to ship a bugfix.
+//
+// The one rule that shapes the rest: refusing a peer must still tell it why. A client that cannot
+// reach the profile endpoint cannot learn what version it should be, so it has nothing to show its
+// user but a network error. GET /v1/profile is therefore always reachable, whatever the range says.
+package compat
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+// Version is a parsed SemVer. Build metadata is discarded (it is explicitly not part of
+// precedence); pre-release is kept and compared, because "1.0.0-rc1" must sort below "1.0.0".
+type Version struct {
+	Major, Minor, Patch int
+	Pre                 string
+}
+
+// Parse accepts "1.2.3", "v1.2.3" and our namespaced release tags ("server-v1.2.3"), because
+// those are the forms that actually reach this code: a tag, a --version output, and an HTTP
+// header written by three different pieces of software.
+func Parse(s string) (Version, bool) {
+	s = strings.TrimSpace(s)
+	// Strip a tag prefix ending in "v" ("v1.2.3", "server-v1.2.3"). Guarded on the prefix having
+	// no digits so a pre-release identifier that happens to contain a "v" is left alone.
+	if i := strings.LastIndexByte(s, 'v'); i >= 0 && i+1 < len(s) &&
+		s[i+1] >= '0' && s[i+1] <= '9' && !strings.ContainsAny(s[:i], "0123456789") {
+		s = s[i+1:]
+	}
+	if plus := strings.IndexByte(s, '+'); plus >= 0 {
+		s = s[:plus]
+	}
+	var pre string
+	if dash := strings.IndexByte(s, '-'); dash >= 0 {
+		pre, s = s[dash+1:], s[:dash]
+	}
+	parts := strings.Split(s, ".")
+	if len(parts) != 3 {
+		return Version{}, false
+	}
+	out := Version{Pre: pre}
+	for i, p := range parts {
+		n, err := strconv.Atoi(p)
+		if err != nil || n < 0 {
+			return Version{}, false
+		}
+		switch i {
+		case 0:
+			out.Major = n
+		case 1:
+			out.Minor = n
+		case 2:
+			out.Patch = n
+		}
+	}
+	return out, true
+}
+
+func (v Version) String() string {
+	s := fmt.Sprintf("%d.%d.%d", v.Major, v.Minor, v.Patch)
+	if v.Pre != "" {
+		s += "-" + v.Pre
+	}
+	return s
+}
+
+// Compare returns -1, 0 or 1. A pre-release sorts below the same version without one (SemVer §11);
+// two pre-releases compare lexically, which is close enough for the identifiers we use.
+func (v Version) Compare(o Version) int {
+	for _, d := range []int{v.Major - o.Major, v.Minor - o.Minor, v.Patch - o.Patch} {
+		if d != 0 {
+			return sign(d)
+		}
+	}
+	switch {
+	case v.Pre == o.Pre:
+		return 0
+	case v.Pre == "":
+		return 1
+	case o.Pre == "":
+		return -1
+	case v.Pre < o.Pre:
+		return -1
+	}
+	return 1
+}
+
+func (v Version) Less(o Version) bool { return v.Compare(o) < 0 }
+
+// NextBreaking is the first version that may break compatibility with v.
+//
+// Below 1.0.0 the minor is the breaking axis (SemVer §4: anything may change in 0.y), so 0.4.2's
+// next break is 0.5.0, not 1.0.0. Getting this wrong in the permissive direction would let a
+// 0.5 server accept a 0.4 app that cannot speak to it.
+func (v Version) NextBreaking() Version {
+	if v.Major == 0 {
+		return Version{Major: 0, Minor: v.Minor + 1}
+	}
+	return Version{Major: v.Major + 1}
+}
+
+// Range is [Min, Max): minimum inclusive, maximum exclusive. An unset Max means unbounded.
+//
+// Exclusive on the upper end because the useful bound is always "the version that broke it",
+// and writing that literally ("< 1.0.0") is unambiguous in a way that "<= 0.999.999" is not.
+type Range struct {
+	Min    Version
+	Max    Version
+	HasMax bool
+}
+
+func (r Range) Contains(v Version) bool {
+	if v.Less(r.Min) {
+		return false
+	}
+	if r.HasMax && !v.Less(r.Max) {
+		return false
+	}
+	return true
+}
+
+func (r Range) String() string {
+	if !r.HasMax {
+		return ">= " + r.Min.String()
+	}
+	return ">= " + r.Min.String() + ", < " + r.Max.String()
+}
+
+// ParseRange builds a range from two strings; an empty max means unbounded. A malformed bound is
+// an error rather than a silently ignored one: a typo in an operator's config must not quietly
+// turn a restriction off.
+func ParseRange(min, max string) (Range, error) {
+	lo, ok := Parse(min)
+	if !ok {
+		return Range{}, fmt.Errorf("bad minimum version %q", min)
+	}
+	if strings.TrimSpace(max) == "" {
+		return Range{Min: lo}, nil
+	}
+	hi, ok := Parse(max)
+	if !ok {
+		return Range{}, fmt.Errorf("bad maximum version %q", max)
+	}
+	if hi.Less(lo) || hi.Compare(lo) == 0 {
+		return Range{}, fmt.Errorf("maximum %s is not above minimum %s", max, min)
+	}
+	return Range{Min: lo, Max: hi, HasMax: true}, nil
+}
+
+// Verdict is the outcome of a compatibility check.
+type Verdict int
+
+const (
+	OK Verdict = iota
+	TooOld
+	TooNew
+	// Unknown means the peer did not say, or said something unparseable — a development build,
+	// or a client too old to send its version at all.
+	Unknown
+)
+
+// Check reports whether peer falls in r, and explains the answer in words meant for a person.
+// The message is deliberately actionable: it names both versions and what to do about it, since
+// it is the only thing the user on the other end will see.
+func Check(peer string, r Range, peerName string) (Verdict, string) {
+	v, ok := Parse(peer)
+	if !ok {
+		return Unknown, fmt.Sprintf("%s did not report a usable version (%q); proceeding without a compatibility check", peerName, peer)
+	}
+	switch {
+	case v.Less(r.Min):
+		return TooOld, fmt.Sprintf("%s %s is older than this build supports (needs %s). Update the %s.",
+			peerName, v, r, peerName)
+	case r.HasMax && !v.Less(r.Max):
+		return TooNew, fmt.Sprintf("%s %s is newer than this build supports (accepts %s). Update this side, or use a version of the %s within that range.",
+			peerName, v, r, peerName)
+	}
+	return OK, ""
+}
+
+func sign(d int) int {
+	if d < 0 {
+		return -1
+	}
+	if d > 0 {
+		return 1
+	}
+	return 0
+}

server/internal/compat/compat_test.go

@@ -0,0 +1,156 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package compat
+
+import "testing"
+
+func TestParseAcceptsTheFormsThatActuallyReachUs(t *testing.T) {
+	cases := map[string]Version{
+		"1.2.3":          {Major: 1, Minor: 2, Patch: 3},
+		"v1.2.3":         {Major: 1, Minor: 2, Patch: 3},
+		"server-v0.4.2":  {Minor: 4, Patch: 2},
+		" 0.2.0 ":        {Minor: 2},
+		"1.0.0-rc1":      {Major: 1, Pre: "rc1"},
+		"1.0.0+build.7":  {Major: 1},
+		"1.0.0-rc1+meta": {Major: 1, Pre: "rc1"},
+	}
+	for in, want := range cases {
+		got, ok := Parse(in)
+		if !ok || got != want {
+			t.Errorf("Parse(%q) = %v,%v; want %v", in, got, ok, want)
+		}
+	}
+	// A pre-release identifier containing a "v" must not be mistaken for a tag prefix.
+	if got, ok := Parse("1.2.3-rcv1"); !ok || got.Pre != "rcv1" || got.Major != 1 {
+		t.Errorf("Parse(1.2.3-rcv1) = %v,%v", got, ok)
+	}
+	for _, bad := range []string{"", "dev", "1.2", "1.2.3.4", "x.y.z", "-1.0.0", "1.2.beta"} {
+		if _, ok := Parse(bad); ok {
+			t.Errorf("Parse(%q) should have failed", bad)
+		}
+	}
+}
+
+func TestCompareOrdersPreReleasesBelowTheirRelease(t *testing.T) {
+	lt := func(a, b string) {
+		t.Helper()
+		x, _ := Parse(a)
+		y, _ := Parse(b)
+		if x.Compare(y) != -1 || y.Compare(x) != 1 {
+			t.Errorf("expected %s < %s", a, b)
+		}
+	}
+	lt("0.9.9", "1.0.0")
+	lt("1.0.0", "1.0.1")
+	lt("1.0.0", "1.1.0")
+	lt("1.0.0-rc1", "1.0.0")
+	lt("1.0.0-rc1", "1.0.0-rc2")
+	a, _ := Parse("1.2.3")
+	b, _ := Parse("v1.2.3")
+	if a.Compare(b) != 0 {
+		t.Error("the same version written two ways must compare equal")
+	}
+}
+
+// Below 1.0.0 the minor is the breaking axis. Treating 1.0.0 as the next break for a 0.x build
+// would let a 0.5 server accept a 0.4 client it cannot actually talk to.
+func TestNextBreakingUsesTheMinorBelowOne(t *testing.T) {
+	cases := map[string]string{
+		"0.4.2": "0.5.0",
+		"0.0.9": "0.1.0",
+		"1.2.3": "2.0.0",
+		"2.0.0": "3.0.0",
+	}
+	for in, want := range cases {
+		v, _ := Parse(in)
+		if got := v.NextBreaking().String(); got != want {
+			t.Errorf("NextBreaking(%s) = %s, want %s", in, got, want)
+		}
+	}
+}
+
+func TestRangeIsMinInclusiveMaxExclusive(t *testing.T) {
+	r, err := ParseRange("0.2.0", "1.0.0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	in := []string{"0.2.0", "0.2.1", "0.9.9", "1.0.0-rc1"}
+	out := []string{"0.1.9", "1.0.0", "1.0.1", "2.0.0"}
+	for _, s := range in {
+		v, _ := Parse(s)
+		if !r.Contains(v) {
+			t.Errorf("%s should be inside %s", s, r)
+		}
+	}
+	for _, s := range out {
+		v, _ := Parse(s)
+		if r.Contains(v) {
+			t.Errorf("%s should be outside %s", s, r)
+		}
+	}
+}
+
+func TestUnboundedRangeHasNoCeiling(t *testing.T) {
+	r, err := ParseRange("0.2.0", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	v, _ := Parse("99.0.0")
+	if !r.Contains(v) {
+		t.Error("an empty maximum must mean unbounded")
+	}
+}
+
+// A typo in an operator's config must not silently disable the restriction it was meant to set.
+func TestMalformedBoundsAreErrorsNotSilentPermissiveness(t *testing.T) {
+	for _, c := range [][2]string{
+		{"nonsense", "1.0.0"},
+		{"0.2.0", "nonsense"},
+		{"1.0.0", "0.9.0"}, // max below min
+		{"1.0.0", "1.0.0"}, // empty window: nothing could ever satisfy it
+	} {
+		if _, err := ParseRange(c[0], c[1]); err == nil {
+			t.Errorf("ParseRange(%q, %q) should have failed", c[0], c[1])
+		}
+	}
+}
+
+func TestCheckExplainsItself(t *testing.T) {
+	r, _ := ParseRange("0.2.0", "1.0.0")
+
+	if v, msg := Check("0.5.0", r, "app"); v != OK || msg != "" {
+		t.Errorf("in-range check should pass silently: %v %q", v, msg)
+	}
+	v, msg := Check("0.1.0", r, "app")
+	if v != TooOld {
+		t.Fatalf("want TooOld, got %v", v)
+	}
+	for _, want := range []string{"0.1.0", "0.2.0", "Update"} {
+		if !contains(msg, want) {
+			t.Errorf("the refusal must name %q so the user can act on it: %q", want, msg)
+		}
+	}
+	if v, _ := Check("1.4.0", r, "app"); v != TooNew {
+		t.Errorf("want TooNew, got %v", v)
+	}
+
+	// A development build reports "dev". Locking developers out of their own server would be a
+	// poor trade for a check that exists to prevent confusing failures.
+	if v, msg := Check("dev", r, "server"); v != Unknown || msg == "" {
+		t.Errorf("unparseable version should be Unknown with an explanation, got %v %q", v, msg)
+	}
+}
+
+func contains(h, n string) bool {
+	return len(h) >= len(n) && (h == n || len(n) == 0 || indexOf(h, n) >= 0)
+}
+
+func indexOf(h, n string) int {
+	for i := 0; i+len(n) <= len(h); i++ {
+		if h[i:i+len(n)] == n {
+			return i
+		}
+	}
+	return -1
+}

server/internal/config/config.go

@@ -58,6 +58,11 @@ type Config struct {
 	UploadMaxRuns       int    // ECHOLOT_UPLOAD_MAX_RUNS / --upload-max-runs (per device)
 	UploadMinAnon       string // ECHOLOT_UPLOAD_MIN_ANONYMIZATION / --upload-min-anonymization
 
+	// Client compatibility window. Bounds are SemVer; an empty maximum means unbounded. The
+	// defaults sit at breaking boundaries, so shipping a patch never requires changing them.
+	MinAppVersion string // ECHOLOT_MIN_APP_VERSION / --min-app-version
+	MaxAppVersion string // ECHOLOT_MAX_APP_VERSION / --max-app-version (exclusive)
+
 	// Mode
 	Docker bool // --docker (or autodetected; env ECHOLOT_DOCKER=1 forces)
 }
@@ -106,6 +111,8 @@ func Load(args []string) (*Config, *Actions, error) {
 	fs.IntVar(&c.UploadRetentionDays, "upload-retention-days", envInt("UPLOAD_RETENTION_DAYS", 90), "delete uploaded runs older than this; 0 disables")
 	fs.IntVar(&c.UploadMaxRuns, "upload-max-runs", envInt("UPLOAD_MAX_RUNS", 200), "keep at most this many runs per device; 0 disables")
 	fs.StringVar(&c.UploadMinAnon, "upload-min-anonymization", envOr("UPLOAD_MIN_ANONYMIZATION", "full"), "least anonymization accepted: full|balanced|strict")
+	fs.StringVar(&c.MinAppVersion, "min-app-version", envOr("MIN_APP_VERSION", "0.2.0"), "oldest app version this server will serve (SemVer, inclusive)")
+	fs.StringVar(&c.MaxAppVersion, "max-app-version", envOr("MAX_APP_VERSION", "1.0.0"), "first app version this server will refuse (SemVer, exclusive); empty = unbounded")
 	fs.BoolVar(&c.Docker, "docker", envOr("DOCKER", "") == "1", "force container mode (config from env, no systemd/self-update)")
 
 	fs.BoolVar(&a.InstallSystemd, "install-systemd", false, "install a systemd unit for this binary and exit")

server/internal/control/compat_gate_test.go

@@ -0,0 +1,113 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package control
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"echo-lot.app/server/internal/compat"
+	"echo-lot.app/server/internal/store"
+)
+
+func rangeOrDie(t *testing.T, min, max string) compat.Range {
+	t.Helper()
+	r, err := compat.ParseRange(min, max)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return r
+}
+
+func TestGateRefusesOutOfRangeApps(t *testing.T) {
+	s := &Server{AppRange: rangeOrDie(t, "0.2.0", "1.0.0")}
+	reached := false
+	h := s.requireCompatibleApp(func(w http.ResponseWriter, _ *http.Request) {
+		reached = true
+		w.WriteHeader(http.StatusOK)
+	})
+
+	for _, tc := range []struct {
+		version  string
+		wantCode int
+		wantThru bool
+	}{
+		{"0.2.0", http.StatusOK, true}, // exactly the minimum is in range
+		{"0.9.9", http.StatusOK, true},
+		{"0.1.9", http.StatusUpgradeRequired, false}, // too old
+		{"1.0.0", http.StatusUpgradeRequired, false}, // maximum is exclusive
+		{"2.0.0", http.StatusUpgradeRequired, false}, // too new
+		{"", http.StatusOK, true},                    // unknown: allowed, see below
+		{"dev", http.StatusOK, true},                 // development build
+	} {
+		reached = false
+		req := httptest.NewRequest("GET", "/v1/sessions", nil)
+		if tc.version != "" {
+			req.Header.Set(AppVersionHeader, tc.version)
+		}
+		rec := httptest.NewRecorder()
+		h(rec, req)
+		if rec.Code != tc.wantCode || reached != tc.wantThru {
+			t.Errorf("app %q: got code=%d reached=%v, want code=%d reached=%v",
+				tc.version, rec.Code, reached, tc.wantCode, tc.wantThru)
+		}
+	}
+}
+
+// A refusal that does not say what version to install is only marginally better than a timeout.
+func TestRefusalNamesTheAcceptedWindow(t *testing.T) {
+	s := &Server{AppRange: rangeOrDie(t, "0.2.0", "1.0.0")}
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest("POST", "/v1/runs", nil)
+	req.Header.Set(AppVersionHeader, "0.1.0")
+	s.requireCompatibleApp(func(http.ResponseWriter, *http.Request) {})(rec, req)
+
+	var body map[string]any
+	if err := json.Unmarshal(rec.Body.Bytes(), &body); err != nil {
+		t.Fatalf("refusal body is not JSON: %v", err)
+	}
+	for _, key := range []string{"error", "app_version", "accepts_app", "protocol_version"} {
+		if body[key] == nil || body[key] == "" {
+			t.Errorf("refusal omits %q, so the client cannot explain itself: %v", key, body)
+		}
+	}
+	if !strings.Contains(body["accepts_app"].(string), "0.2.0") {
+		t.Errorf("accepts_app should state the minimum: %v", body["accepts_app"])
+	}
+}
+
+// The profile is how a refused client learns which version it needs. Gating it would leave the
+// user with a network error instead of an answer, which defeats the whole check.
+func TestProfileIsReachableRegardlessOfVersion(t *testing.T) {
+	st, err := store.Open(t.TempDir())
+	if err != nil {
+		t.Fatal(err)
+	}
+	s := &Server{
+		AppRange: rangeOrDie(t, "9.0.0", ""), // nothing current could satisfy this
+		Store:    st,
+	}
+	req := httptest.NewRequest("GET", "/v1/profile", nil)
+	req.Header.Set(AppVersionHeader, "0.1.0")
+	rec := httptest.NewRecorder()
+	s.Handler().ServeHTTP(rec, req)
+
+	// The request carries no credential, so the handler answers 401 — the point is that it is
+	// the *handler* answering, not the version gate turning it into 426.
+	if rec.Code == http.StatusUpgradeRequired {
+		t.Fatal("the profile endpoint must never be gated on app version")
+	}
+}
+
+func TestZeroValueRangeFallsBackToTheBuiltInDefault(t *testing.T) {
+	s := &Server{} // nothing configured
+	got := s.appRange()
+	want := DefaultAppRange()
+	if got.Min != want.Min || got.HasMax != want.HasMax || got.Max != want.Max {
+		t.Fatalf("appRange() = %s, want the built-in default %s", got, want)
+	}
+}

server/internal/control/control.go

@@ -24,6 +24,7 @@ import (
 	"strings"
 	"time"
 
+	"echo-lot.app/server/internal/compat"
 	"echo-lot.app/server/internal/dataplane"
 	"echo-lot.app/server/internal/runs"
 	"echo-lot.app/server/internal/session"
@@ -70,22 +71,87 @@ type Server struct {
 	// server's own egress isn't full-MTU, client MTU results measure the
 	// server, not the client.
 	ProvenGood func() (mtuOK, sysctlOK bool)
+
+	// AppRange is the app-version window this server will serve. Zero value means the built-in
+	// default (see DefaultAppRange).
+	AppRange compat.Range
+}
+
+// AppVersionHeader is how a client states its version. A client too old to send it is treated as
+// unknown rather than refused: the check exists to turn confusing failures into clear ones, and
+// refusing something we cannot identify achieves the opposite.
+const AppVersionHeader = "X-Echolot-App-Version"
+
+// ProtocolVersion is the wire contract (probe-protocol.md) this build implements. It is what the
+// version window is really about; the release version is only a proxy for it.
+const ProtocolVersion = "1.0.0"
+
+// SchemaVersion is the measurement-document format this server can store.
+const SchemaVersion = "1.0.0"
+
+// DefaultAppRange: everything from the first app that speaks this protocol up to — but not
+// including — the next breaking series. Bounds sit at breaking boundaries so shipping a patch
+// never requires touching this.
+func DefaultAppRange() compat.Range {
+	r, err := compat.ParseRange("0.2.0", "1.0.0")
+	if err != nil {
+		panic("built-in app range is malformed: " + err.Error())
+	}
+	return r
+}
+
+func (s *Server) appRange() compat.Range {
+	if s.AppRange.Min == (compat.Version{}) && !s.AppRange.HasMax {
+		return DefaultAppRange()
+	}
+	return s.AppRange
+}
+
+// requireCompatibleApp wraps a handler with the version window.
+//
+// Deliberately NOT applied to GET /v1/profile: that is where a client learns which version it
+// should be. Gating it would leave a refused client with nothing to show its user but a timeout,
+// which is precisely the confusion this check exists to remove.
+func (s *Server) requireCompatibleApp(next http.HandlerFunc) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		verdict, msg := compat.Check(r.Header.Get(AppVersionHeader), s.appRange(), "app")
+		switch verdict {
+		case compat.TooOld, compat.TooNew:
+			slog.Info("refused incompatible app", "app_version", r.Header.Get(AppVersionHeader),
+				"accepts", s.appRange().String(), "path", r.URL.Path)
+			// 426 says exactly this and nothing else; the body carries the window so the app can
+			// show the user the number to reach, not just that it failed.
+			writeJSON(w, http.StatusUpgradeRequired, map[string]any{
+				"error":            msg,
+				"app_version":      r.Header.Get(AppVersionHeader),
+				"accepts_app":      s.appRange().String(),
+				"server_version":   Version,
+				"protocol_version": ProtocolVersion,
+			})
+			return
+		}
+		next(w, r)
+	}
 }
 
 func (s *Server) Handler() http.Handler {
 	mux := http.NewServeMux()
-	mux.HandleFunc("POST /v1/enroll", s.enroll)
+	// Always reachable, whatever the version window says: this is how a client discovers the
+	// window it has to satisfy.
 	mux.HandleFunc("GET /v1/profile", s.profile)
-	mux.HandleFunc("POST /v1/sessions", s.newSession)
+
-	mux.HandleFunc("DELETE /v1/sessions/{id}", s.deleteSession)
+	gate := s.requireCompatibleApp
-	mux.HandleFunc("GET /v1/sessions/{id}/observations", s.observations)
+	mux.HandleFunc("POST /v1/enroll", gate(s.enroll))
-	mux.HandleFunc("POST /v1/sessions/{id}/actions", s.actions)
+	mux.HandleFunc("POST /v1/sessions", gate(s.newSession))
-	mux.HandleFunc("POST /v1/echo", s.httpEcho)
+	mux.HandleFunc("DELETE /v1/sessions/{id}", gate(s.deleteSession))
-	mux.HandleFunc("GET /v1/tls-reference", s.tlsReference)
+	mux.HandleFunc("GET /v1/sessions/{id}/observations", gate(s.observations))
-	mux.HandleFunc("POST /v1/runs", s.uploadRun)
+	mux.HandleFunc("POST /v1/sessions/{id}/actions", gate(s.actions))
-	mux.HandleFunc("GET /v1/runs", s.listRuns)
+	mux.HandleFunc("POST /v1/echo", gate(s.httpEcho))
-	mux.HandleFunc("GET /v1/runs/{id}", s.getRun)
+	mux.HandleFunc("GET /v1/tls-reference", gate(s.tlsReference))
-	mux.HandleFunc("DELETE /v1/runs/{id}", s.deleteRun)
+	mux.HandleFunc("POST /v1/runs", gate(s.uploadRun))
+	mux.HandleFunc("GET /v1/runs", gate(s.listRuns))
+	mux.HandleFunc("GET /v1/runs/{id}", gate(s.getRun))
+	mux.HandleFunc("DELETE /v1/runs/{id}", gate(s.deleteRun))
 	// TODO(spec §5): frag_send, throughput (both build on the same grant machinery)
 	return mux
 }
@@ -364,7 +430,12 @@ func bearer(r *http.Request) string {
 func writeJSON(w http.ResponseWriter, code int, v any) {
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(code)
-	_ = json.NewEncoder(w).Encode(v)
+	enc := json.NewEncoder(w)
+	// Go escapes <, > and & by default, for JSON embedded in HTML. This is an API, and the
+	// escaping is actively harmful here: a refusal message reading "needs >= 0.2.0" is what
+	// the user ends up seeing. Nothing we emit is ever interpolated into a page.
+	enc.SetEscapeHTML(false)
+	_ = enc.Encode(v)
 }
 
 // enroll redeems a single-use enrollment token for a device credential (§2.1).
@@ -424,6 +495,14 @@ func (s *Server) profile(w http.ResponseWriter, r *http.Request) {
 		// The app needs the upload rules before it offers the switch: whether uploads are
 		// accepted at all, and how much identifying detail it must strip first.
 		"uploads": s.uploadPolicy(),
+		// What this build speaks, and which app versions it will serve. A client checks the
+		// server side of the same question against its own bounds.
+		"compat": map[string]any{
+			"protocol_version": ProtocolVersion,
+			"schema_version":   SchemaVersion,
+			"app_min":          s.appRange().Min.String(),
+			"app_max":          maxOrEmpty(s.appRange()),
+		},
 	})
 }
 
@@ -574,3 +653,12 @@ func (s *Server) deleteRun(w http.ResponseWriter, r *http.Request) {
 	}
 	w.WriteHeader(http.StatusNoContent)
 }
+
+// maxOrEmpty renders an unbounded ceiling as "" rather than as a sentinel version, so a client
+// reading the profile cannot mistake a placeholder for a real bound.
+func maxOrEmpty(r compat.Range) string {
+	if !r.HasMax {
+		return ""
+	}
+	return r.Max.String()
+}

server/internal/dataplane/udp.go

@@ -177,6 +177,7 @@ func (s *Server) mtuAck(conn *net.UDPConn, raddr netip.AddrPort, sess *session.S
 }
 
 // Observation block (spec §3.3), fixed 40 bytes appended to the RESP header:
+//
 //	 0  8  t_rx_ns (server clock, process epoch)
 //	 8  8  t_tx_ns
 //	16 16  observed source IP (v4-mapped when v4)

web/README.md

@@ -0,0 +1,74 @@
+# Echolot website (`web/`)
+
+Minimal single-page site for [echo-lot.app](https://echo-lot.app), served from Cloudflare
+Workers. Static files in `public/` are served straight from the edge; the tiny Worker in
+`src/index.js` only runs for paths that aren't files:
+
+| Path          | Behavior                                                                 |
+| ------------- | ------------------------------------------------------------------------ |
+| `/apk`        | 302 → newest `.apk` asset of the latest Gitea release (QR-code friendly) |
+| `/apk.sha256` | 302 → the matching `.sha256` asset                                       |
+| `/api/latest` | JSON `{version, published_at, apk, sha256}` — the homepage's version readout |
+| `/fdroid`, `/source` | 302 → the URLs configured in `wrangler.jsonc` vars               |
+
+The latest release is resolved from the Gitea API **at request time** (edge-cached 5 min), so
+publishing a release — `git tag v0.2.0 && git push origin v0.2.0`, which triggers
+`.gitea/workflows/release.yml` — is the only release step. The site never needs a redeploy for
+a new version, and empty/unreachable values fall back to the homepage instead of 404ing.
+
+Light/dark follows the OS (`prefers-color-scheme`), no toggle, no JS required for it. Colors
+come from the branding palette (teal = instrument, single amber point = finding).
+
+`public/assets/` (favicon, wordmark, social preview) are **copies** of `../assets/branding/` —
+that directory is the source of truth; re-copy after any branding change.
+
+## Deploy
+
+Everything is driven by [wrangler](https://developers.cloudflare.com/workers/wrangler/), config
+in `wrangler.jsonc`. No build step, no node_modules to commit.
+
+### One-time setup
+
+1. In the Cloudflare dashboard, add **echo-lot.app** as a zone (and point the domain's
+   nameservers at Cloudflare). The `routes` in `wrangler.jsonc` use `custom_domain: true`, so
+   wrangler creates the DNS records for `echo-lot.app` and `www` automatically on first deploy —
+   the zone just has to exist in the same account.
+2. Auth, either flavor:
+   - **Interactive:** `npx wrangler login` (opens the browser once, stores an OAuth token).
+   - **API token (also what CI uses):** dashboard → My Profile → API Tokens → create from the
+     **"Edit Cloudflare Workers"** template. Then:
+
+     ```
+     $env:CLOUDFLARE_API_TOKEN  = "..."   # PowerShell; export ... on POSIX
+     $env:CLOUDFLARE_ACCOUNT_ID = "..."   # dashboard → Workers & Pages, right sidebar
+     ```
+
+### Deploy
+
+```
+cd web
+npx wrangler@4 deploy
+```
+
+That's it — uploads `src/index.js` + the `public/` assets, wires the custom domains. Useful
+extras: `npx wrangler dev` (local preview at localhost:8787), `npx wrangler tail` (live logs),
+`npx wrangler versions list`.
+
+### CI deploy (Gitea Actions)
+
+`.gitea/workflows/deploy-site.yml` runs `wrangler deploy` on every push to `main`/`master` that
+touches `web/`. It stays inert until you add two repo secrets (Settings → Actions → Secrets):
+`CLOUDFLARE_API_TOKEN` and `CLOUDFLARE_ACCOUNT_ID` (same values as above).
+
+Cloudflare's raw REST API (`PUT /accounts/:id/workers/scripts/...`) exists, but the assets
+upload needs a manifest/session dance that wrangler already implements — use wrangler even in
+automation.
+
+## Config knobs (`wrangler.jsonc` → `vars`)
+
+- `GITEA_REPO_API` — Gitea repo API base; releases must be publicly readable.
+- `DOWNLOAD_URL` — manual `/apk` fallback while Gitea is unreachable.
+- `FDROID_URL` — set when the F-Droid listing exists; until then `/fdroid` loops home.
+- `SOURCE_URL` — public source mirror for the footer + `/source`.
+
+Vars are plain (non-secret) config; change + `wrangler deploy` to apply.

web/public/assets/icon.svg

@@ -0,0 +1,24 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 96 96">
+  <defs>
+    <linearGradient id="tile" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0" stop-color="#0E2433"/>
+      <stop offset="1" stop-color="#071522"/>
+    </linearGradient>
+  </defs>
+  <rect width="96" height="96" rx="21" fill="url(#tile)"/>
+  <!-- the network, at rest -->
+  <g fill="#1E4A5C">
+    <circle cx="24" cy="24" r="2.6"/><circle cx="48" cy="24" r="2.6"/><circle cx="72" cy="24" r="2.6"/>
+    <circle cx="24" cy="48" r="2.6"/>                                <circle cx="72" cy="48" r="2.6"/>
+    <circle cx="24" cy="72" r="2.6"/><circle cx="48" cy="72" r="2.6"/><circle cx="72" cy="72" r="2.6"/>
+  </g>
+  <!-- one node, under examination -->
+  <circle cx="48" cy="48" r="8" fill="none" stroke="#FFB454" stroke-opacity="0.3" stroke-width="2"/>
+  <circle cx="48" cy="48" r="4.5" fill="#FFB454"/>
+  <g fill="none" stroke="#35E0C4" stroke-width="3.5" stroke-linecap="round" stroke-linejoin="round">
+    <path d="M 33 41 V 33 H 41"/>
+    <path d="M 55 33 H 63 V 41"/>
+    <path d="M 63 55 V 63 H 55"/>
+    <path d="M 41 63 H 33 V 55"/>
+  </g>
+</svg>

web/public/assets/wordmark-on-dark.svg

@@ -0,0 +1,17 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="-10 -92 433 104">
+  <!-- "echolot" — hand-drawn monoline letterforms (no font dependency). For dark grounds. -->
+  <g fill="none" stroke="#E8F4F2" stroke-width="11" stroke-linecap="round">
+    <path d="M 0 -24 H 48"/>
+    <path d="M 48 -24 A 24 24 0 1 0 40.97 -7.03"/>
+    <path d="M 110.97 -40.97 A 24 24 0 1 0 110.97 -7.03"/>
+    <path d="M 140 -76 V 0"/>
+    <path d="M 140 -24 A 24 24 0 0 1 188 -24 L 188 0"/>
+    <circle cx="234" cy="-24" r="24"/>
+    <path d="M 280 -76 V 0"/>
+    <circle cx="326" cy="-24" r="24" stroke="#35E0C4"/>
+    <path d="M 372 -48 H 402"/>
+    <path d="M 387 -68 V 0"/>
+  </g>
+  <!-- the finding -->
+  <circle cx="326" cy="-24" r="6.5" fill="#FFB454"/>
+</svg>

web/public/assets/wordmark-on-light.svg

@@ -0,0 +1,17 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="-10 -92 433 104">
+  <!-- "echolot" — hand-drawn monoline letterforms (no font dependency). For light grounds. -->
+  <g fill="none" stroke="#1B3540" stroke-width="11" stroke-linecap="round">
+    <path d="M 0 -24 H 48"/>
+    <path d="M 48 -24 A 24 24 0 1 0 40.97 -7.03"/>
+    <path d="M 110.97 -40.97 A 24 24 0 1 0 110.97 -7.03"/>
+    <path d="M 140 -76 V 0"/>
+    <path d="M 140 -24 A 24 24 0 0 1 188 -24 L 188 0"/>
+    <circle cx="234" cy="-24" r="24"/>
+    <path d="M 280 -76 V 0"/>
+    <circle cx="326" cy="-24" r="24" stroke="#0E9384"/>
+    <path d="M 372 -48 H 402"/>
+    <path d="M 387 -68 V 0"/>
+  </g>
+  <!-- the finding -->
+  <circle cx="326" cy="-24" r="6.5" fill="#E08A1E"/>
+</svg>

web/public/index.html

@@ -5,103 +5,124 @@
 <head>
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
-<title>Echolot — depth soundings for your local network</title>
+<title>Echolot — measure, don't guess</title>
 <meta name="description" content="Free Android app for detecting and debugging local network issues: rogue DHCP, broken IPv6 RAs, MTU black holes, multicast loss, lying DNS. No root required.">
 <meta property="og:title" content="Echolot">
-<meta property="og:description" content="Depth soundings for your local network. F/OSS Android network diagnostics — no root required.">
+<meta property="og:description" content="Measure, don't guess. F/OSS Android network diagnostics — no root required.">
 <meta property="og:url" content="https://echo-lot.app/">
-<link rel="icon" href="data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 32 32'%3E%3Crect width='32' height='32' fill='%23071A29'/%3E%3Cg fill='none' stroke='%23FFB454' stroke-width='2'%3E%3Ccircle cx='16' cy='16' r='3' fill='%23FFB454' stroke='none'/%3E%3Cpath d='M16 6a10 10 0 0 1 10 10'/%3E%3Cpath d='M16 1a15 15 0 0 1 15 15' opacity='.5'/%3E%3C/g%3E%3C/svg%3E">
+<meta property="og:image" content="https://echo-lot.app/assets/social-preview.png">
+<meta name="theme-color" media="(prefers-color-scheme: dark)" content="#071522">
+<meta name="theme-color" media="(prefers-color-scheme: light)" content="#F2F6F7">
+<link rel="icon" href="/assets/icon.svg" type="image/svg+xml">
+<link rel="icon" href="/assets/icon.png" type="image/png" sizes="512x512">
+<link rel="apple-touch-icon" href="/assets/icon.png">
 <style>
+  /* Branding: teal is always the instrument (links, brackets, controls);
+     the single amber point is the finding (the focused node, the version
+     readout). Dark = the instrument's own display; light = the same tokens
+     on paper. Palette from assets/branding/. */
   :root {
-    --depth-0:  #0B2437;   /* surface */
+    color-scheme: dark;
-    --depth-1:  #092031;   /* photic */
+    --bg-0:   #0C2130;   /* top of page */
-    --depth-2:  #071A29;   /* mid */
+    --bg-1:   #071522;   /* abyss — page floor, panel ground */
-    --depth-3:  #051320;   /* floor */
+    --tile:   #0E2433;   /* raised surfaces */
-    --foam:     #DCE9F1;   /* primary text */
+    --foam:   #E8F4F2;   /* primary text */
-    --slate:    #8AA5B8;   /* secondary text */
+    --slate:  #7DA2AC;   /* secondary text */
-    --grid:     #16374E;   /* hairlines, chart grid */
+    --caption:#5E8B96;   /* mono captions, from the banner */
-    --ping:     #FFB454;   /* the one accent: sonar amber */
+    --grid:   #10303F;   /* hairlines */
-    --ok:       #7BC98F;   /* verdict green, chips only */
+    --rest:   #1E4A5C;   /* the network, at rest */
+    --teal:   #35E0C4;   /* instrument */
+    --on-teal:#04212B;   /* text on teal */
+    --amber:  #FFB454;   /* the finding */
     --mono: "Cascadia Code", "SF Mono", Consolas, "Liberation Mono", Menlo, monospace;
     --sans: "Segoe UI", system-ui, -apple-system, "Helvetica Neue", Arial, sans-serif;
   }
+  @media (prefers-color-scheme: light) {
+    :root {
+      color-scheme: light;
+      --bg-0:   #F2F6F7;
+      --bg-1:   #E4ECEF;
+      --tile:   #EBF1F3;
+      --foam:   #1B3540;   /* ink, from wordmark-on-light */
+      --slate:  #47656F;
+      --caption:#5E8B96;
+      --grid:   #C4D3D8;
+      --rest:   #9FB8C1;
+      --teal:   #0E9384;   /* instrument, printable contrast */
+      --on-teal:#F5FBFA;
+      --amber:  #C77413;   /* finding ink */
+    }
+  }
   * { box-sizing: border-box; margin: 0; }
   html { scroll-behavior: smooth; }
   body {
     font-family: var(--sans);
     color: var(--foam);
-    background: linear-gradient(var(--depth-0), var(--depth-1) 30%, var(--depth-2) 65%, var(--depth-3));
+    background: linear-gradient(var(--bg-0), var(--bg-1) 70%);
+    min-height: 100vh;
     line-height: 1.6;
     -webkit-font-smoothing: antialiased;
   }
-  a { color: var(--ping); text-decoration-thickness: 1px; text-underline-offset: 3px; }
+  a { color: var(--teal); text-decoration-thickness: 1px; text-underline-offset: 3px; }
   a:hover { text-decoration-thickness: 2px; }
-  :focus-visible { outline: 2px solid var(--ping); outline-offset: 3px; border-radius: 2px; }
+  :focus-visible { outline: 2px solid var(--teal); outline-offset: 3px; border-radius: 2px; }
 
   .col { max-width: 46rem; margin: 0 auto; padding: 0 1.25rem; }
 
-  /* Depth ruler: fixed left margin scale, desktop only. Marks are set per-section
+  /* Graduated rule: the tick motif from the banner, vertical. Fixed left
-     by scroll position purely decoratively — it is a ruler, not navigation. */
+     margin, desktop only, purely decorative. */
-  .ruler {
+  .rule {
     position: fixed; top: 0; bottom: 0; left: 0; width: 3.5rem;
     border-right: 1px solid var(--grid);
-    font-family: var(--mono); font-size: .65rem; color: var(--slate);
     display: none;
   }
-  @media (min-width: 72rem) { .ruler { display: block; } }
+  @media (min-width: 72rem) { .rule { display: block; } }
-  .ruler span {
+  .rule::after {
-    position: absolute; right: .5rem; transform: translateY(-50%);
+    content: ""; position: absolute; right: 0; top: 0; bottom: 0; width: .4rem;
-  }
+    background: repeating-linear-gradient(to bottom, var(--rest) 0 1.5px, transparent 1.5px 60px);
-  .ruler span::after {
-    content: ""; position: absolute; right: -.55rem; top: 50%;
-    width: .35rem; height: 1px; background: var(--slate);
   }
 
   header.hero { padding: 4.5rem 0 3rem; }
-  .wordmark {
+  .wordmark { display: block; height: 30px; width: auto; }
-    font-family: var(--mono); font-size: .8rem; letter-spacing: .35em;
-    text-transform: uppercase; color: var(--slate);
-  }
-  .wordmark b { color: var(--ping); font-weight: 600; }
   h1 {
     font-size: clamp(1.9rem, 5vw, 3rem);
     font-weight: 650; letter-spacing: -.02em; line-height: 1.15;
-    margin: 1rem 0 .75rem; max-width: 30ch;
+    margin: 1.75rem 0 .75rem; max-width: 30ch;
   }
   .hero p.lede { color: var(--slate); max-width: 52ch; font-size: 1.05rem; }
   .hero p.lede strong { color: var(--foam); font-weight: 600; }
 
-  /* Echogram: the signature. A chart-recorder trace of ping RTTs; the sweep
+  /* Focus panel: the signature, straight from the mark. The network at rest,
-     line is the sounder, the profile is the "seabed" the echoes draw. */
+     one node under examination — teal brackets are the instrument, the amber
-  figure.echogram {
+     point is the finding. */
+  figure.focus {
     margin: 2.5rem 0 0; border: 1px solid var(--grid); border-radius: 4px;
-    background:
+    background: var(--tile);
-      repeating-linear-gradient(to right, transparent 0 39px, var(--grid) 39px 40px),
-      repeating-linear-gradient(to bottom, transparent 0 31px, var(--grid) 31px 32px),
-      var(--depth-3);
     position: relative; overflow: hidden;
   }
-  .echogram svg { display: block; width: 100%; height: auto; }
+  .focus svg { display: block; width: 100%; height: auto; }
-  .echogram figcaption {
+  .focus .rest-node { fill: var(--rest); }
+  .focus .bracket   { fill: none; stroke: var(--teal); stroke-width: 3; stroke-linecap: round; stroke-linejoin: round; }
+  .focus .finding   { fill: var(--amber); }
+  .focus .halo      { fill: none; stroke: var(--amber); stroke-width: 2; opacity: .3; }
+  .focus .readout   { font-family: var(--mono); font-size: 11px; fill: var(--slate); }
+  .focus .readout .flag { fill: var(--amber); }
+  .focus .lead      { stroke: var(--grid); stroke-width: 1; }
+  .focus figcaption {
     position: absolute; top: .5rem; left: .75rem;
-    font-family: var(--mono); font-size: .65rem; color: var(--slate);
+    font-family: var(--mono); font-size: .65rem; color: var(--caption);
   }
-  .sweep {
+  @keyframes examine { 0%, 100% { opacity: .3; } 50% { opacity: .1; } }
-    position: absolute; top: 0; bottom: 0; width: 1px;
+  .focus .halo { animation: examine 3.2s ease-in-out infinite; }
-    background: var(--ping); opacity: .8;
-    box-shadow: 0 0 8px var(--ping);
-    animation: sweep 7s linear infinite;
-  }
-  @keyframes sweep { from { left: 0; } to { left: 100%; } }
   @media (prefers-reduced-motion: reduce) {
-    .sweep { animation: none; left: 62%; }
+    .focus .halo { animation: none; }
     html { scroll-behavior: auto; }
   }
 
   section { padding: 3.5rem 0 0; }
   .eyebrow {
     font-family: var(--mono); font-size: .7rem; letter-spacing: .25em;
-    text-transform: uppercase; color: var(--ping);
+    text-transform: uppercase; color: var(--teal);
   }
   h2 { font-size: 1.35rem; font-weight: 650; margin: .5rem 0 1rem; letter-spacing: -.01em; }
   section > .col > p { color: var(--slate); max-width: 58ch; }
@@ -129,21 +150,26 @@
     border: 1px solid var(--grid); border-radius: 3px; padding: .35rem .6rem;
     color: var(--slate);
   }
-  .tier b { color: var(--ok); font-weight: 600; }
+  .tier b { color: var(--teal); font-weight: 600; }
 
   /* Install */
-  .buttons { display: flex; gap: .75rem; flex-wrap: wrap; margin: 1.5rem 0 1rem; }
+  .release {
+    font-family: var(--mono); font-size: .8rem; color: var(--slate);
+    margin-top: 1.25rem;
+  }
+  .release b { color: var(--amber); font-weight: 600; }
+  .buttons { display: flex; gap: .75rem; flex-wrap: wrap; margin: 1rem 0 1rem; }
   .btn {
     display: inline-block; padding: .7rem 1.3rem; border-radius: 4px;
     font-weight: 600; text-decoration: none; font-size: .95rem;
   }
-  .btn.primary { background: var(--ping); color: var(--depth-3); }
+  .btn.primary { background: var(--teal); color: var(--on-teal); }
   .btn.primary:hover { filter: brightness(1.08); }
   .btn.ghost { border: 1px solid var(--grid); color: var(--foam); }
   .btn.ghost:hover { border-color: var(--slate); }
   .note {
     font-size: .85rem; color: var(--slate);
-    border-left: 2px solid var(--ping); padding-left: .9rem; max-width: 52ch;
+    border-left: 2px solid var(--teal); padding-left: .9rem; max-width: 52ch;
   }
   .checksum { font-family: var(--mono); font-size: .75rem; color: var(--slate); margin-top: 1rem; }
 
@@ -157,45 +183,51 @@
 </head>
 <body>
 
-<div class="ruler" aria-hidden="true">
+<div class="rule" aria-hidden="true"></div>
-  <span style="top:6%">0 m</span>
-  <span style="top:28%">─ 20</span>
-  <span style="top:50%">─ 40</span>
-  <span style="top:72%">─ 60</span>
-  <span style="top:94%">─ 80</span>
-</div>
 
 <header class="hero">
   <div class="col">
-    <p class="wordmark"><b>●</b> echo·lot <span aria-hidden="true">/ˈɛçolo:t/ — echo sounder</span></p>
+    <picture>
-    <h1>Depth soundings for your local network.</h1>
+      <source srcset="/assets/wordmark-on-dark.svg" media="(prefers-color-scheme: dark)">
-    <p class="lede">An echo sounder maps the seabed by timing returns. <strong>Echolot</strong> does the
+      <img class="wordmark" src="/assets/wordmark-on-light.svg" alt="echolot" width="125" height="30">
-    same to your network: free Android diagnostics for the layer where things actually break —
+    </picture>
+    <h1>Measure, don't guess.</h1>
+    <p class="lede">Free Android diagnostics for the layer where networks actually break.
+    <strong>Echolot</strong> takes the failure you can feel and pins it to a fact you can show —
     <strong>no root required</strong>. Built for people who know what a neighbor table is.</p>
 
-    <figure class="echogram">
+    <figure class="focus">
-      <figcaption>trace · icmp.ping4 · rtt ms ↓ / t →</figcaption>
+      <figcaption>focus · dhcp.rogue_detect · tier:app</figcaption>
-      <svg viewBox="0 0 720 190" role="img" aria-label="Chart-recorder style trace of ping round-trip times, drawn like a sonar seabed profile">
+      <svg viewBox="0 0 720 240" role="img" aria-label="A grid of network nodes at rest; one node is framed by viewfinder brackets, highlighted as a finding: two DHCP servers answered the same DISCOVER">
-        <!-- echo returns: the profile -->
+        <!-- the network, at rest -->
-        <polyline fill="none" stroke="#FFB454" stroke-width="1.5" opacity=".9"
+        <g class="rest-node">
-          points="0,138 40,136 80,139 120,135 160,137 200,141 240,138 260,120 280,96 300,88 320,94 340,118 360,134 400,136 440,133 480,158 500,171 520,168 540,150 560,139 600,137 640,140 680,136 720,138"/>
+          <circle cx="120" cy="60" r="3"/><circle cx="240" cy="60" r="3"/><circle cx="360" cy="60" r="3"/><circle cx="480" cy="60" r="3"/><circle cx="600" cy="60" r="3"/>
-        <!-- second, fainter return (multipath) -->
+          <circle cx="120" cy="120" r="3"/><circle cx="480" cy="120" r="3"/><circle cx="600" cy="120" r="3"/>
-        <polyline fill="none" stroke="#FFB454" stroke-width="1" opacity=".25"
+          <circle cx="120" cy="180" r="3"/><circle cx="240" cy="180" r="3"/><circle cx="360" cy="180" r="3"/><circle cx="480" cy="180" r="3"/><circle cx="600" cy="180" r="3"/>
-          points="0,148 40,146 80,149 120,145 160,147 200,151 240,148 260,132 280,110 300,101 320,107 340,129 360,144 400,146 440,143 480,168 500,180 520,177 540,160 560,149 600,147 640,150 680,146 720,148"/>
+        </g>
-        <!-- dropped probes -->
+        <!-- one node, under examination -->
-        <g fill="#8AA5B8" font-family="monospace" font-size="9">
+        <circle class="halo" cx="240" cy="120" r="11"/>
-          <text x="497" y="30">×</text><text x="507" y="30">×</text>
+        <circle class="finding" cx="240" cy="120" r="5"/>
-          <text x="288" y="30">▲ spike: wifi→cell handover</text>
+        <g class="bracket">
+          <path d="M 222 111 V 102 H 231"/>
+          <path d="M 249 102 H 258 V 111"/>
+          <path d="M 258 129 V 138 H 249"/>
+          <path d="M 231 138 H 222 V 129"/>
+        </g>
+        <!-- the readout -->
+        <line class="lead" x1="262" y1="120" x2="296" y2="120"/>
+        <g class="readout">
+          <text x="304" y="112">DISCOVER → 2 OFFERs</text>
+          <text x="304" y="130">192.168.1.1 gw · <tspan class="flag">192.168.1.223 — who is this?</tspan></text>
         </g>
       </svg>
-      <div class="sweep" aria-hidden="true"></div>
     </figure>
   </div>
 </header>
 
 <section id="what">
   <div class="col">
-    <p class="eyebrow">What it sounds out</p>
+    <p class="eyebrow">What it measures</p>
     <h2>Signal bars lie. Timings don't.</h2>
     <p>Most wifi apps show you signal strength and call it a diagnosis. The failures that ruin
     home and office networks live deeper: a second DHCP server nobody admits to, IPv6 router
@@ -234,15 +266,16 @@
   <div class="col">
     <p class="eyebrow">Install</p>
     <h2>Get Echolot</h2>
+    <p class="release" id="release" hidden></p>
     <div class="buttons">
-      <a class="btn primary" href="/apk">Download APK</a>
+      <a class="btn primary" id="dl-btn" href="/apk">Download APK</a>
       <a class="btn ghost" href="/fdroid">F-Droid</a>
     </div>
-    <p class="note"><strong>Pre-release.</strong> The capability prober is running on real
+    <p class="note" id="prerelease-note"><strong>Pre-release.</strong> The capability prober is
-    hardware; the production app is under construction. These links go live with the first
+    running on real hardware; the production app is under construction. These links go live with
-    release — until then they loop back here. No mailing list, no tracker: check back, or watch
+    the first release — until then they loop back here. No mailing list, no tracker: check back,
-    the <a href="/source">repository</a>.</p>
+    or watch the <a href="/source">repository</a>.</p>
-    <p class="checksum">releases will ship with sha256sums + a signing key you can pin</p>
+    <p class="checksum" id="checksum">releases will ship with sha256sums + a signing key you can pin</p>
   </div>
 </section>
 
@@ -253,5 +286,41 @@
   </div>
 </footer>
 
+<script>
+// Release readout: asks this site's own Worker (/api/latest, which proxies the
+// Gitea "latest release" API, edge-cached). Progressive enhancement — with no
+// JS, no network, or no release yet, the static pre-release copy above stands.
+(async () => {
+  let rel;
+  try {
+    const res = await fetch("/api/latest");
+    if (!res.ok) return;
+    rel = await res.json();
+  } catch { return; }
+  if (!rel || !rel.available) return;
+
+  const line = document.getElementById("release");
+  const ver = document.createElement("b");
+  ver.textContent = rel.version;
+  line.append("» latest ", ver);
+  const date = (rel.published_at || "").slice(0, 10);
+  if (date) line.append(" · " + date);
+  if (rel.apk && rel.apk.size) {
+    line.append(" · " + (rel.apk.size / 1048576).toFixed(1) + " MiB");
+  }
+  line.hidden = false;
+
+  document.getElementById("prerelease-note").hidden = true;
+  if (rel.sha256) {
+    const c = document.getElementById("checksum");
+    c.textContent = "";
+    const a = document.createElement("a");
+    a.href = "/apk.sha256";
+    a.textContent = "sha256";
+    c.append(a, " · verify before you sideload");
+  }
+})();
+</script>
+
 </body>
 </html>

web/src/index.js

@@ -3,28 +3,32 @@
 
 // Everything under public/ is served straight from the edge without invoking
 // this Worker. The Worker exists for the short stable URLs (/apk, /fdroid,
-// /source) — short enough for a QR code — and to resolve "/apk" to the newest
+// /source) — short enough for a QR code — and for /api/latest, which the
-// release asset at request time, so tagging a release in Gitea is the only
+// homepage uses to show the current version. Both resolve the newest release
-// publish step. No site redeploy, no URL to update.
+// from the Gitea API at request time, so tagging a release in Gitea is the
+// only publish step. No site redeploy, no URL to update.
 
 const STATIC_ROUTES = {
   "/fdroid": "FDROID_URL",
   "/source": "SOURCE_URL",
 };
 
-// Resolve the newest APK from the Gitea "latest release" API. Cached at the
+// Fetch the Gitea "latest release" object. Cached at the edge for 5 minutes so
-// edge for 5 minutes so a release becomes visible quickly, while Gitea sees
+// a new release becomes visible quickly, while Gitea sees at most one API hit
-// at most one API hit per POP per 5 min regardless of download traffic.
+// per POP per 5 min regardless of traffic. Returns null on any failure —
-async function latestApkUrl(env) {
+// callers degrade to fallbacks rather than surfacing errors.
+async function latestRelease(env) {
   if (!env.GITEA_REPO_API) return null;
   const res = await fetch(`${env.GITEA_REPO_API}/releases/latest`, {
     headers: { Accept: "application/json", "User-Agent": "echolot-site" },
     cf: { cacheTtl: 300, cacheEverything: true },
   });
   if (!res.ok) return null;
-  const rel = await res.json();
+  return res.json();
-  const apk = rel.assets?.find((a) => a.name?.endsWith(".apk"));
+}
-  return apk?.browser_download_url ?? null;
+
+function asset(rel, suffix) {
+  return rel?.assets?.find((a) => a.name?.endsWith(suffix)) ?? null;
 }
 
 function redirect(location) {
@@ -38,21 +42,50 @@ function redirect(location) {
   });
 }
 
+function json(body, maxAge) {
+  return new Response(JSON.stringify(body), {
+    headers: {
+      "Content-Type": "application/json",
+      "Cache-Control": `public, max-age=${maxAge}`,
+    },
+  });
+}
+
 export default {
   async fetch(request, env) {
     const { pathname } = new URL(request.url);
     const path = pathname.replace(/\/$/, "");
     const fallback = new URL("/#install", request.url).toString();
 
-    if (path === "/apk" || path === "/download") {
+    if (path === "/apk" || path === "/download" || path === "/apk.sha256") {
-      // Order: live Gitea release → manual override → install section.
+      const suffix = path === "/apk.sha256" ? ".sha256" : ".apk";
       let target = null;
       try {
-        target = await latestApkUrl(env);
+        target = asset(await latestRelease(env), suffix)?.browser_download_url;
       } catch {
         // Gitea unreachable — fall through rather than 500 on a download link.
       }
-      return redirect(target || env.DOWNLOAD_URL || fallback);
+      const override = suffix === ".apk" ? env.DOWNLOAD_URL : null;
+      return redirect(target || override || fallback);
+    }
+
+    if (path === "/api/latest") {
+      let rel = null;
+      try {
+        rel = await latestRelease(env);
+      } catch {}
+      const apk = asset(rel, ".apk");
+      if (!rel || !apk) return json({ available: false }, 60);
+      return json(
+        {
+          available: true,
+          version: rel.tag_name,
+          published_at: rel.published_at,
+          apk: { name: apk.name, size: apk.size, url: apk.browser_download_url },
+          sha256: Boolean(asset(rel, ".sha256")),
+        },
+        300,
+      );
     }
 
     const varName = STATIC_ROUTES[path];

web/wrangler.jsonc

@@ -22,14 +22,14 @@
   // its route fall back to the homepage's install section, so nothing 404s
   // before the first release exists.
   "vars": {
-    // Gitea repo API base, e.g. "https://git.example.net/api/v1/repos/mram/echolot".
+    // Gitea repo API base. The repo (or at least its releases) must be
-    // The repo (or at least its releases) must be publicly readable.
+    // publicly readable for /apk and /api/latest to resolve.
-    "GITEA_REPO_API": "",
+    "GITEA_REPO_API": "https://git.rambossek.at/api/v1/repos/EchoLot/echolot",
-    // Manual override / fallback while GITEA_REPO_API is unset or unreachable.
+    // Manual override / fallback while GITEA_REPO_API is unreachable.
     "DOWNLOAD_URL": "",
     // F-Droid listing, once it exists: https://f-droid.org/packages/app.echo_lot.app/
     "FDROID_URL": "",
     // Public source URL, the footer + /source target.
-    "SOURCE_URL": ""
+    "SOURCE_URL": "https://git.rambossek.at/EchoLot/echolot"
   }
 }