diff --git a/docs/build-status.md b/docs/build-status.md
index 529241a..671c248 100644
--- a/docs/build-status.md
+++ b/docs/build-status.md
@@ -1054,3 +1054,31 @@ Four consequences that decide whether it is worth it:
 What keeps this possible: uploads are already stored byte-for-byte as received, and every index
 field is derived in one function (`runs.Put`). The thing to avoid is admin features that *require*
 reading content — those would have to be unbuilt later.
+
+### App sign-in, and an undisclosed dependency it surfaced (2026-08-01)
+The app can now sign in to the server's identity provider: authorization code with PKCE, a
+`Sign in` card in settings, and the `echolot://auth` redirect handled alongside the enrolment one
+(told apart by host, since one spends a token and the other completes an authorization).
+
+The detail that decides whether this works on a real phone: **the PKCE verifier is written to
+storage before the browser opens**, not held in memory. Handing control to a browser backgrounds
+the process and Android may kill it; the callback then arrives at a fresh process. An in-memory
+verifier works on a developer's device and fails under memory pressure, which is the worst way for
+a sign-in to break.
+
+Nothing from the IdP is retained. The ID token proves who is signing in, once, and the device
+credential authenticates everything after — no access tokens stored, no refresh tokens rotated.
+
+**A server remains entirely optional.** All eight probes are device-tier; `serverConfigured` gates
+only upload and the account. But answering that question exposed something worth fixing: two probes
+hardcode the reference deployment —
+
+```kotlin
+DnsCanaryProbe(canaryZone = "c.echo-lot.app", ...)   // "Hardcoded to the reference deployment"
+StunProbe(serverHost = "fmr-1.echo-lot.app")
+```
+
+so a user with no server of their own still sends DNS and STUN traffic to fmr without being told.
+For a tool that goes to this much trouble over what leaves the device, an undisclosed dependency on
+a third party's infrastructure is the wrong default. It should prefer the configured server, and be
+explicit when there is none. **Open.**
diff --git a/echolot-app/app/src/main/AndroidManifest.xml b/echolot-app/app/src/main/AndroidManifest.xml
index e29224f..130cfdb 100644
--- a/echolot-app/app/src/main/AndroidManifest.xml
+++ b/echolot-app/app/src/main/AndroidManifest.xml
@@ -39,6 +39,17 @@
                 <category android:name="android.intent.category.BROWSABLE" />
                 <data android:scheme="echolot" android:host="enroll" />
             </intent-filter>
+            <!--
+              Sign-in redirect. The browser hands the authorization code back through this, which
+              is exactly why the flow uses PKCE: any app may register this scheme, so the code
+              alone must not be enough to complete a sign-in.
+            -->
+            <intent-filter android:autoVerify="false">
+                <action android:name="android.intent.action.VIEW" />
+                <category android:name="android.intent.category.DEFAULT" />
+                <category android:name="android.intent.category.BROWSABLE" />
+                <data android:scheme="echolot" android:host="auth" />
+            </intent-filter>
         </activity>
 
         <provider
diff --git a/echolot-app/app/src/main/kotlin/app/echo_lot/app/Account.kt b/echolot-app/app/src/main/kotlin/app/echo_lot/app/Account.kt
new file mode 100644
index 0000000..584c163
--- /dev/null
+++ b/echolot-app/app/src/main/kotlin/app/echo_lot/app/Account.kt
@@ -0,0 +1,123 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package app.echo_lot.app
+
+import app.echo_lot.protocol.AuthInfo
+import app.echo_lot.protocol.ControlClient
+import app.echo_lot.protocol.OidcLogin
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.jsonObject
+import kotlinx.serialization.json.jsonPrimitive
+
+/**
+ * Signing in to the configured server's identity provider.
+ *
+ * The awkward part of a browser-based sign-in on Android is that the app is not running while it
+ * happens. Handing control to a browser puts this process in the background, where it may be
+ * killed at any moment; the callback then arrives at a fresh process with none of the state the
+ * exchange needs. So the PKCE verifier and state are written to storage before the browser opens,
+ * not held in memory — an in-memory value works on a developer's device and fails on a phone under
+ * memory pressure, which is the worst way for this to break.
+ *
+ * Nothing from the identity provider is kept afterwards. The ID token proves who is signing in,
+ * once; the device credential authenticates everything from then on.
+ */
+class Account(private val settings: Settings) {
+
+    private val json = Json { ignoreUnknownKeys = true }
+
+    sealed interface SignInStart {
+        /** Open this in a browser. */
+        data class Browser(val url: String) : SignInStart
+        data class Unavailable(val reason: String) : SignInStart
+    }
+
+    /** Fetches the server's auth configuration and builds the authorization URL. */
+    fun begin(): SignInStart {
+        if (!settings.serverConfigured) {
+            return SignInStart.Unavailable(
+                "Enrol with a server first — sign-in belongs to the server's identity provider."
+            )
+        }
+        val auth = runCatching { client().profile(settings.serverCredential).auth }.getOrNull()
+            ?: return SignInStart.Unavailable("Could not reach the server to ask how to sign in.")
+
+        auth.discoveryError?.let {
+            // The distinction matters: "the operator configured an IdP that is not answering" is
+            // their problem to fix, and is not the same as "this server has no accounts".
+            return SignInStart.Unavailable("The server's identity provider is not responding: $it")
+        }
+        if (!auth.enabled) {
+            return SignInStart.Unavailable("This server does not offer accounts.")
+        }
+        return try {
+            val pending = OidcLogin.begin(auth)
+            // Written before the browser opens, because after that this process may not survive.
+            settings.pendingVerifier = pending.verifier
+            settings.pendingState = pending.state
+            SignInStart.Browser(pending.authorizationUrl)
+        } catch (t: Throwable) {
+            SignInStart.Unavailable(t.message ?: "Could not start sign-in.")
+        }
+    }
+
+    /**
+     * Completes sign-in from the `echolot://auth` redirect.
+     *
+     * Blocking; callers run it off the main thread.
+     */
+    fun complete(callbackUri: String): String {
+        val verifier = settings.pendingVerifier
+        val state = settings.pendingState
+        // Cleared first, whatever happens next: these are single-use, and leaving them behind
+        // would let a later callback be completed against a flow nobody started.
+        settings.clearPendingAuth()
+
+        if (verifier.isBlank() || state.isBlank()) {
+            return "That sign-in did not start on this device."
+        }
+        return try {
+            val auth = client().profile(settings.serverCredential).auth
+            val idToken = OidcLogin.complete(
+                auth, OidcLogin.Pending("", verifier, state), callbackUri,
+            )
+            val reply = client().linkAccount(settings.serverCredential, idToken)
+            val o = json.parseToJsonElement(reply).jsonObject
+            val name = o["display_name"]?.jsonPrimitive?.content ?: "signed in"
+            settings.accountName = name
+            settings.accountId = o["account_id"]?.jsonPrimitive?.content ?: ""
+            val admin = o["admin"]?.jsonPrimitive?.content == "true"
+            "Signed in as $name" + if (admin) " (administrator)" else ""
+        } catch (e: OidcLogin.LoginFailed) {
+            e.message ?: "Sign-in failed."
+        } catch (t: Throwable) {
+            "Sign-in failed: ${t.message ?: t.javaClass.simpleName}"
+        }
+    }
+
+    /** Signs out. The device stays enrolled — signing out should not cost an enrolment. */
+    fun signOut(): String = try {
+        client().unlinkAccount(settings.serverCredential)
+        settings.accountName = ""
+        settings.accountId = ""
+        "Signed out. This device is still enrolled."
+    } catch (t: Throwable) {
+        "Could not sign out: ${t.message ?: t.javaClass.simpleName}"
+    }
+
+    /** Asks the server who it thinks is signed in, so the UI is not trusting stale local state. */
+    fun refresh(): String? = runCatching {
+        val o = json.parseToJsonElement(client().accountStatus(settings.serverCredential)).jsonObject
+        val signedIn = o["signed_in"]?.jsonPrimitive?.content == "true"
+        settings.accountName = if (signedIn) {
+            o["display_name"]?.jsonPrimitive?.content ?: ""
+        } else {
+            ""
+        }
+        settings.accountName.takeIf { it.isNotBlank() }
+    }.getOrNull()
+
+    private fun client() =
+        ControlClient(settings.serverUrl, setOf(settings.serverPin), BuildConfig.APP_SEMVER)
+}
diff --git a/echolot-app/app/src/main/kotlin/app/echo_lot/app/MainActivity.kt b/echolot-app/app/src/main/kotlin/app/echo_lot/app/MainActivity.kt
index 4c2506b..207e3f0 100644
--- a/echolot-app/app/src/main/kotlin/app/echo_lot/app/MainActivity.kt
+++ b/echolot-app/app/src/main/kotlin/app/echo_lot/app/MainActivity.kt
@@ -63,13 +63,24 @@ class MainActivity : ComponentActivity() {
                     // An echolot://enroll link (QR scan, or a link the operator sent) opens the
                     // app straight into settings with the enrollment already done, so the user
                     // sees the result rather than a form they still have to fill in.
-                    val enrollUri = intent?.takeIf { it.action == Intent.ACTION_VIEW }?.dataString
+                    // Both deep links land here. They are told apart by host, so a sign-in
+                    // redirect is never mistaken for an enrolment link — one spends a token, the
+                    // other completes an authorization, and confusing them would fail obscurely.
+                    val incoming = intent?.takeIf { it.action == Intent.ACTION_VIEW }?.dataString
+                    val authUri = incoming?.takeIf { it.startsWith("echolot://auth") }
+                    val enrollUri = incoming?.takeIf { it.startsWith("echolot://enroll") }
                     androidx.compose.runtime.LaunchedEffect(enrollUri) {
                         if (enrollUri != null) {
                             vm.enroll(enrollUri)
                             screen = Screen.SETTINGS
                         }
                     }
+                    androidx.compose.runtime.LaunchedEffect(authUri) {
+                        if (authUri != null) {
+                            vm.completeSignIn(authUri)
+                            screen = Screen.SETTINGS
+                        }
+                    }
                     androidx.compose.runtime.LaunchedEffect(autorun) {
                         if (autorun) vm.run(devUpload = true)
                     }
@@ -107,6 +118,18 @@ class MainActivity : ComponentActivity() {
                             lifecycleScope.launch { preview = vm.previewNewestRun() }
                         },
                         onCheckServer = vm::checkServer,
+                        accountName = vm.accountName,
+                        onSignIn = {
+                            vm.beginSignIn { url ->
+                                // A plain VIEW intent rather than a Custom Tab: the browser is
+                                // where the user's existing IdP session already lives, and
+                                // androidx.browser would be a dependency for a rounded corner.
+                                runCatching {
+                                    startActivity(Intent(Intent.ACTION_VIEW, android.net.Uri.parse(url)))
+                                }
+                            }
+                        },
+                        onSignOut = vm::signOut,
                         onEnroll = vm::enroll,
                         serverStatus = vm.state.archiveStatus,
                         onBack = { screen = Screen.RUN },
diff --git a/echolot-app/app/src/main/kotlin/app/echo_lot/app/RunViewModel.kt b/echolot-app/app/src/main/kotlin/app/echo_lot/app/RunViewModel.kt
index 8d54cab..b60040f 100644
--- a/echolot-app/app/src/main/kotlin/app/echo_lot/app/RunViewModel.kt
+++ b/echolot-app/app/src/main/kotlin/app/echo_lot/app/RunViewModel.kt
@@ -215,6 +215,44 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
      *  history screen has been opened - the two disagreeing read as data loss. */
     fun archivedRunCount(): Int = store.list().size
 
+    private val account = Account(settings)
+
+    /** Name of whoever is signed in on this device, for the settings screen. */
+    var accountName by mutableStateOf(settings.accountName)
+        private set
+
+    /** Starts sign-in; the caller opens the returned URL in a browser. */
+    fun beginSignIn(open: (String) -> Unit) {
+        viewModelScope.launch {
+            state = state.copy(archiveStatus = "contacting the server …")
+            when (val r = withContext(Dispatchers.IO) { account.begin() }) {
+                is Account.SignInStart.Browser -> {
+                    state = state.copy(archiveStatus = "continue in your browser …")
+                    open(r.url)
+                }
+                is Account.SignInStart.Unavailable ->
+                    state = state.copy(archiveStatus = r.reason)
+            }
+        }
+    }
+
+    /** Completes sign-in from the echolot://auth redirect. */
+    fun completeSignIn(callbackUri: String) {
+        viewModelScope.launch {
+            val msg = withContext(Dispatchers.IO) { account.complete(callbackUri) }
+            accountName = settings.accountName
+            state = state.copy(archiveStatus = msg)
+        }
+    }
+
+    fun signOut() {
+        viewModelScope.launch {
+            val msg = withContext(Dispatchers.IO) { account.signOut() }
+            accountName = settings.accountName
+            state = state.copy(archiveStatus = msg)
+        }
+    }
+
     /** Redeems an enrollment link, from a paste or from an echolot:// deep link. */
     fun enroll(link: String, deviceName: String? = android.os.Build.MODEL) {
         viewModelScope.launch {
diff --git a/echolot-app/app/src/main/kotlin/app/echo_lot/app/Settings.kt b/echolot-app/app/src/main/kotlin/app/echo_lot/app/Settings.kt
index 56f9473..1703ef6 100644
--- a/echolot-app/app/src/main/kotlin/app/echo_lot/app/Settings.kt
+++ b/echolot-app/app/src/main/kotlin/app/echo_lot/app/Settings.kt
@@ -104,6 +104,36 @@ class Settings(context: Context) {
     val serverConfigured: Boolean
         get() = serverUrl.isNotBlank() && serverPin.isNotBlank() && serverCredential.isNotBlank()
 
+    // ---- account ---------------------------------------------------------------------
+
+    /**
+     * The PKCE verifier and state for a sign-in that is out at the browser.
+     *
+     * Persisted rather than held in memory because handing control to a browser backgrounds this
+     * process, and Android may kill it before the callback returns. An in-memory value works on a
+     * developer's device and fails on a phone under memory pressure.
+     */
+    var pendingVerifier: String
+        get() = prefs.getString(PENDING_VERIFIER, "") ?: ""
+        set(v) = prefs.edit().putString(PENDING_VERIFIER, v).apply()
+
+    var pendingState: String
+        get() = prefs.getString(PENDING_STATE, "") ?: ""
+        set(v) = prefs.edit().putString(PENDING_STATE, v).apply()
+
+    fun clearPendingAuth() = prefs.edit().remove(PENDING_VERIFIER).remove(PENDING_STATE).apply()
+
+    /** Display name of whoever is signed in on this device; empty when nobody is. */
+    var accountName: String
+        get() = prefs.getString(ACCOUNT_NAME, "") ?: ""
+        set(v) = prefs.edit().putString(ACCOUNT_NAME, v).apply()
+
+    var accountId: String
+        get() = prefs.getString(ACCOUNT_ID, "") ?: ""
+        set(v) = prefs.edit().putString(ACCOUNT_ID, v).apply()
+
+    val signedIn: Boolean get() = accountName.isNotBlank()
+
     private fun hex(s: String) = ByteArray(s.length / 2) {
         ((Character.digit(s[it * 2], 16) shl 4) or Character.digit(s[it * 2 + 1], 16)).toByte()
     }
@@ -120,5 +150,9 @@ class Settings(context: Context) {
         const val SERVER_URL = "server_url"
         const val SERVER_PIN = "server_pin"
         const val SERVER_CRED = "server_credential"
+        const val PENDING_VERIFIER = "pending_auth_verifier"
+        const val PENDING_STATE = "pending_auth_state"
+        const val ACCOUNT_NAME = "account_name"
+        const val ACCOUNT_ID = "account_id"
     }
 }
diff --git a/echolot-app/app/src/main/kotlin/app/echo_lot/app/SettingsScreen.kt b/echolot-app/app/src/main/kotlin/app/echo_lot/app/SettingsScreen.kt
index f82bb00..6ff9a45 100644
--- a/echolot-app/app/src/main/kotlin/app/echo_lot/app/SettingsScreen.kt
+++ b/echolot-app/app/src/main/kotlin/app/echo_lot/app/SettingsScreen.kt
@@ -49,6 +49,9 @@ fun SettingsScreen(
     onDeleteAll: () -> Unit,
     onPreviewUpload: () -> Unit,
     onCheckServer: () -> Unit,
+    accountName: String,
+    onSignIn: () -> Unit,
+    onSignOut: () -> Unit,
     onEnroll: (String) -> Unit,
     serverStatus: String?,
     onBack: () -> Unit,
@@ -152,6 +155,38 @@ fun SettingsScreen(
             }
         }
 
+        // ---- account ----
+        Card(Modifier.fillMaxWidth()) {
+            Column(Modifier.padding(14.dp), verticalArrangement = Arrangement.spacedBy(8.dp)) {
+                Text("Account", style = MaterialTheme.typography.titleMedium)
+                if (accountName.isNotBlank()) {
+                    Text("Signed in as $accountName", style = MaterialTheme.typography.bodyMedium)
+                    Text(
+                        "Runs from every device signed in to this account share one history.",
+                        style = MaterialTheme.typography.bodySmall,
+                    )
+                    TextButton(onClick = onSignOut) { Text("Sign out") }
+                } else {
+                    Text(
+                        "Signing in is optional. It links this device to an account on your " +
+                            "server, so several devices share one history — and some servers only " +
+                            "accept uploads from a signed-in device.",
+                        style = MaterialTheme.typography.bodySmall,
+                    )
+                    Button(onClick = onSignIn, enabled = settings.serverConfigured) {
+                        Text("Sign in")
+                    }
+                    if (!settings.serverConfigured) {
+                        Text(
+                            "Enrol with a server first — the account belongs to the server, not " +
+                                "to the app.",
+                            style = MaterialTheme.typography.bodySmall,
+                        )
+                    }
+                }
+            }
+        }
+
         // ---- upload ----
         Card(Modifier.fillMaxWidth()) {
             Column(Modifier.padding(14.dp), verticalArrangement = Arrangement.spacedBy(8.dp)) {