android/app/src/main/AndroidManifest.xml

@@ -64,6 +64,25 @@
       declared here too because ReminderReceiver now depends on it directly. A
       permission this file relies on should be visible in this file.
     -->
+    <!--
+      Updating this app from the server it syncs with (M12 step 7).
+
+      REQUEST_INSTALL_PACKAGES lets the app hand an APK to the system installer at
+      all. It is NOT what makes an install look suspicious to on-device heuristics
+      — Mihon declares it too — the legacy ACTION_VIEW install intent was, and this
+      app uses a PackageInstaller session instead. See AppUpdate.kt and Scribe note
+      2437. The person must additionally grant "install unknown apps" in system
+      settings; the update card asks before downloading anything.
+
+      UPDATE_PACKAGES_WITHOUT_USER_ACTION (API 31+) is what removes the install
+      confirmation on the UPDATE path, and only there — Android will not let an app
+      silently put a NEW package on a device, which is correct. It also only applies
+      when the new build is signed with the same key as the installed one, which is
+      why signing had to land before any of this could work.
+    -->
+    <uses-permission android:name="android.permission.REQUEST_INSTALL_PACKAGES" />
+    <uses-permission android:name="android.permission.UPDATE_PACKAGES_WITHOUT_USER_ACTION" />
+
     <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
     <uses-permission android:name="android.permission.SCHEDULE_EXACT_ALARM" />
     <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />
@@ -98,6 +117,16 @@
           do not survive either a reboot or an app update, so without this a phone
           that restarts overnight would quietly stop reminding anyone of anything.
         -->
+        <!--
+          Where the system reports what happened to an install we committed. Not
+          exported: the only sender is the PendingIntent this app handed to
+          PackageInstaller. Without it a failed install would be indistinguishable
+          from someone declining the dialog (Scribe #2438).
+        -->
+        <receiver
+            android:name=".UpdateReceiver"
+            android:exported="false" />
+
         <receiver
             android:name=".ReminderReceiver"
             android:exported="false">

android/app/src/main/java/com/fabledsword/thoughtsync/AppUpdate.kt

@@ -0,0 +1,151 @@
+package com.fabledsword.thoughtsync
+
+import android.content.Context
+import android.content.Intent
+import android.content.IntentSender
+import android.content.pm.PackageInstaller
+import android.net.Uri
+import android.os.Build
+import android.provider.Settings
+import android.util.Log
+import java.io.File
+
+/**
+ * Replacing this app with a newer build of itself.
+ *
+ * ## A PackageInstaller session, not an install intent
+ *
+ * The obvious route — `ACTION_VIEW` on the APK with
+ * `application/vnd.android.package-archive` — is the one on-device install
+ * heuristics are tuned against, and it is what produces the "bypassing Android
+ * security" warning the operator saw on Minstrel (Scribe note 2437). It also never
+ * tells the OS that this app is the legitimate updater of its own package, and it
+ * returns nothing: a failed install is indistinguishable from a person dismissing
+ * the dialog.
+ *
+ * A session says who is doing what. On Android 12+ it can also declare that no user
+ * action is required, which — paired with `UPDATE_PACKAGES_WITHOUT_USER_ACTION` —
+ * removes the confirmation dialog entirely on the UPDATE path. Not on a first
+ * install: the OS will not let an app quietly put a NEW package on a device, which
+ * is right.
+ *
+ * Two things from that same research that are NOT done here, deliberately:
+ * `setRequestUpdateOwnership` was chased and turned out to be a red herring, and
+ * `REQUEST_INSTALL_PACKAGES` is not the differentiator either — Mihon declares it
+ * too. The mechanism was the whole difference.
+ *
+ * ## The outcome comes back
+ *
+ * `commit` takes an `IntentSender`; the system reports the result to
+ * [UpdateReceiver], which is why a failure can be shown rather than guessed at.
+ */
+object AppUpdate {
+    private const val TAG = "ThoughtSyncUpdate"
+
+    /** This build's versionCode — what the server's is compared against. */
+    fun installedVersionCode(context: Context): Long =
+        runCatching {
+            context.packageManager.getPackageInfo(context.packageName, 0).longVersionCode
+        }.getOrDefault(0L)
+
+    /**
+     * Whether this app may install packages at all.
+     *
+     * A separate grant from anything in the manifest, and one only the person can
+     * give. Checked before offering an update rather than after downloading 55 MiB.
+     */
+    fun canInstall(context: Context): Boolean = context.packageManager.canRequestPackageInstalls()
+
+    /** The settings page where that grant lives, scoped to this app. */
+    fun installPermissionSettings(context: Context): Intent =
+        Intent(Settings.ACTION_MANAGE_UNKNOWN_APP_SOURCES)
+            .setData(Uri.fromParts("package", context.packageName, null))
+            .addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
+
+    /** Where a download goes: app-private, so no storage permission is involved. */
+    fun downloadTarget(context: Context): File = File(context.cacheDir, "update.apk")
+
+    /**
+     * Hand the APK to the system installer.
+     *
+     * Streamed into the session rather than passed as a path or a content URI —
+     * the session takes bytes, which is also why no FileProvider is needed here.
+     *
+     * Returns the failure to show, or null when the install was handed over
+     * successfully. "Handed over" is the honest word: the real outcome arrives
+     * later at [UpdateReceiver], because a commit that the system accepts can still
+     * fail afterwards.
+     */
+    fun install(
+        context: Context,
+        apk: File,
+    ): String? {
+        val installer = context.packageManager.packageInstaller
+        var sessionId = -1
+        return try {
+            val params =
+                PackageInstaller.SessionParams(PackageInstaller.SessionParams.MODE_FULL_INSTALL)
+            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.S) {
+                // Only honoured on an UPDATE of an app signed with the same key —
+                // exactly our case, and the reason the signing work had to land
+                // first. Android ignores it for anything else rather than failing,
+                // so there is no need to guard on which it is.
+                params.setRequireUserAction(PackageInstaller.SessionParams.USER_ACTION_NOT_REQUIRED)
+            }
+
+            sessionId = installer.createSession(params)
+            installer.openSession(sessionId).use { session ->
+                writeApk(session, apk)
+                session.commit(statusSender(context))
+            }
+            null
+        } catch (e: Exception) {
+            // Broad on purpose: createSession throws IOException, openWrite throws,
+            // and the framework raises SecurityException for a revoked grant. All of
+            // them mean one thing to the person — it did not install — and none of
+            // them should take the app down.
+            Log.w(TAG, "could not start the install session", e)
+            if (sessionId != -1) runCatching { installer.abandonSession(sessionId) }
+            e.message ?: "The update could not be installed."
+        }
+    }
+
+    /**
+     * Stream the APK into the session.
+     *
+     * Its own function only because the two nested `use` blocks read badly inline —
+     * and detekt agreed, which is fair: a stream inside a session inside a try is
+     * three things to hold at once.
+     */
+    private fun writeApk(
+        session: PackageInstaller.Session,
+        apk: File,
+    ) {
+        session.openWrite(WRITE_NAME, 0, apk.length()).use { out ->
+            apk.inputStream().use { it.copyTo(out) }
+            // Before close: the session must have the bytes on disk, not sitting in
+            // a buffer, or commit can be handed a short file.
+            session.fsync(out)
+        }
+    }
+
+    private fun statusSender(context: Context): IntentSender {
+        val intent =
+            Intent(context, UpdateReceiver::class.java).setAction(UpdateReceiver.ACTION_INSTALLED)
+        // MUTABLE, and this is the one place it is correct: the system fills the
+        // result extras in before delivering it. An immutable one would arrive with
+        // no status at all.
+        val flags =
+            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.S) {
+                android.app.PendingIntent.FLAG_UPDATE_CURRENT or
+                    android.app.PendingIntent.FLAG_MUTABLE
+            } else {
+                android.app.PendingIntent.FLAG_UPDATE_CURRENT
+            }
+        return android.app.PendingIntent
+            .getBroadcast(context, 0, intent, flags)
+            .intentSender
+    }
+
+    private const val WRITE_NAME = "thoughtsync-update"
+}

android/app/src/main/java/com/fabledsword/thoughtsync/MainActivity.kt

@@ -34,6 +34,7 @@ import com.fabledsword.thoughtsync.ui.SyncScreen
 import com.fabledsword.thoughtsync.ui.SyncState
 import com.fabledsword.thoughtsync.ui.SyncViewModel
 import com.fabledsword.thoughtsync.ui.ThoughtSyncTheme
+import com.fabledsword.thoughtsync.ui.UpdateViewModel
 import com.fabledsword.thoughtsync.ui.olderThan
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.launch
@@ -150,6 +151,7 @@ private fun App(
     var composing by rememberSaveable { mutableStateOf(false) }
     var showingSync by rememberSaveable { mutableStateOf(false) }
 
+    val update: UpdateViewModel = viewModel(factory = UpdateViewModel.factory(core, context))
     val settings = remember(context) { SyncSettings(context) }
     var automatic by remember { mutableStateOf(settings.automatic) }
 
@@ -179,6 +181,11 @@ private fun App(
                     automatic = it
                     settings.automatic = it
                 },
+                update = update.state,
+                onCheckUpdate = update::check,
+                onInstallUpdate = update::downloadAndInstall,
+                onDismissUpdateError = update::dismissError,
+                onInstallOutcome = update::consumeInstallOutcome,
             )
 
         Screen.EDITOR ->

android/app/src/main/java/com/fabledsword/thoughtsync/UpdateOutcome.kt

@@ -0,0 +1,37 @@
+package com.fabledsword.thoughtsync
+
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.setValue
+
+/**
+ * The last thing the system said about an install, waiting to be shown.
+ *
+ * A process-wide holder because the two ends cannot reach each other any other
+ * way: [UpdateReceiver] is constructed by the system, and the view model that
+ * wants the answer is owned by the composition. The alternative — a bound service
+ * or a broadcast the UI also listens for — is more machinery for one nullable
+ * string.
+ *
+ * Safe as snapshot state: `onReceive` runs on the main thread, which is where
+ * Compose expects its state to be written.
+ */
+object UpdateOutcome {
+    /** `error == null` means it went through, or the person declined. */
+    data class Result(
+        val error: String?,
+    )
+
+    /** Null until the system has said something about an install we committed. */
+    var latest: Result? by mutableStateOf(null)
+        private set
+
+    fun report(error: String?) {
+        latest = Result(error)
+    }
+
+    /** Called once the UI has shown it, so a later install starts from silence. */
+    fun clear() {
+        latest = null
+    }
+}

android/app/src/main/java/com/fabledsword/thoughtsync/UpdateReceiver.kt

@@ -0,0 +1,77 @@
+package com.fabledsword.thoughtsync
+
+import android.content.BroadcastReceiver
+import android.content.Context
+import android.content.Intent
+import android.content.pm.PackageInstaller
+import android.util.Log
+
+/**
+ * What the system says about an install we committed.
+ *
+ * Without this the app would `commit` and learn nothing — a failed install would
+ * look exactly like a person deciding not to go ahead, and the update card would
+ * sit there claiming an update is available with no explanation of why nothing
+ * happened. That was the specific complaint recorded against Minstrel's first
+ * attempt (Scribe #2438).
+ *
+ * The result is written to [UpdateOutcome] rather than notified: the app is on
+ * screen when this fires — someone just tapped Update — so the place to say it is
+ * the card they are looking at.
+ */
+class UpdateReceiver : BroadcastReceiver() {
+    override fun onReceive(
+        context: Context,
+        intent: Intent,
+    ) {
+        if (intent.action != ACTION_INSTALLED) return
+
+        val status = intent.getIntExtra(PackageInstaller.EXTRA_STATUS, Int.MIN_VALUE)
+        val message = intent.getStringExtra(PackageInstaller.EXTRA_STATUS_MESSAGE)
+
+        when (status) {
+            PackageInstaller.STATUS_PENDING_USER_ACTION -> {
+                // Android wants a confirmation. This is the ORDINARY path below API
+                // 31, and the path on 31+ whenever the OS declines to skip the
+                // dialog — which it may, and is entitled to.
+                val confirm =
+                    @Suppress("DEPRECATION")
+                    intent.getParcelableExtra<Intent>(Intent.EXTRA_INTENT)
+                if (confirm == null) {
+                    UpdateOutcome.report("Android asked for confirmation but sent no way to give it.")
+                    return
+                }
+                // NEW_TASK because a receiver has no activity of its own to start
+                // from. The app is in the foreground, so this surfaces immediately.
+                confirm.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
+                runCatching { context.startActivity(confirm) }
+                    .onFailure {
+                        Log.w(TAG, "could not show the install confirmation", it)
+                        UpdateOutcome.report("Android's install confirmation could not be shown.")
+                    }
+            }
+
+            PackageInstaller.STATUS_SUCCESS -> {
+                // Rarely seen: a successful self-update replaces this process, so
+                // the app is usually gone before it can act on this.
+                Log.i(TAG, "update installed")
+                UpdateOutcome.report(null)
+            }
+
+            PackageInstaller.STATUS_FAILURE_ABORTED ->
+                // Someone declined. Not an error, and saying "install failed" for a
+                // deliberate choice is how an app sounds broken when it is not.
+                UpdateOutcome.report(null)
+
+            else -> {
+                Log.w(TAG, "install failed: status=$status message=$message")
+                UpdateOutcome.report(message ?: "The update did not install.")
+            }
+        }
+    }
+
+    companion object {
+        const val ACTION_INSTALLED = "com.fabledsword.thoughtsync.UPDATE_INSTALLED"
+        private const val TAG = "ThoughtSyncUpdate"
+    }
+}

android/app/src/main/java/com/fabledsword/thoughtsync/ui/SyncPairing.kt

@@ -4,7 +4,6 @@ import android.os.Build
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Row
 import androidx.compose.foundation.layout.fillMaxWidth
-import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.text.KeyboardActions
 import androidx.compose.foundation.text.KeyboardOptions
 import androidx.compose.material3.Button
@@ -120,10 +119,14 @@ fun UnlinkedPanel(
         Button(
             onClick = { credentials?.let { onLink(url, it) } },
             enabled = credentials != null && !state.busy,
-            modifier = Modifier.padding(bottom = 24.dp),
         ) {
             Text(stringResource(R.string.sync_connect))
         }
+
+        // Where app updates come from, said here rather than left as a gap. This
+        // device has no update path at all until it is linked, and a Check button
+        // that always found nothing would be worse than the sentence.
+        UnlinkedUpdateNote()
     }
 }
 

android/app/src/main/java/com/fabledsword/thoughtsync/ui/SyncScreen.kt

@@ -36,6 +36,7 @@ import androidx.compose.ui.res.stringResource
 import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.unit.dp
 import com.fabledsword.thoughtsync.R
+import com.fabledsword.thoughtsync.UpdateOutcome
 
 /**
  * Opt-in server pairing.
@@ -64,6 +65,11 @@ fun SyncScreen(
     onDismissRevokeNotice: () -> Unit,
     automatic: Boolean,
     onAutomaticChange: (Boolean) -> Unit,
+    update: UpdateState,
+    onCheckUpdate: () -> Unit,
+    onInstallUpdate: () -> Unit,
+    onDismissUpdateError: () -> Unit,
+    onInstallOutcome: (UpdateOutcome.Result) -> Unit,
 ) {
     Scaffold(
         topBar = {
@@ -99,6 +105,11 @@ fun SyncScreen(
                         onUnlink = onUnlink,
                         automatic = automatic,
                         onAutomaticChange = onAutomaticChange,
+                        update = update,
+                        onCheckUpdate = onCheckUpdate,
+                        onInstallUpdate = onInstallUpdate,
+                        onDismissUpdateError = onDismissUpdateError,
+                        onInstallOutcome = onInstallOutcome,
                     )
                 else ->
                     UnlinkedPanel(
@@ -122,6 +133,11 @@ private fun LinkedPanel(
     onUnlink: () -> Unit,
     automatic: Boolean,
     onAutomaticChange: (Boolean) -> Unit,
+    update: UpdateState,
+    onCheckUpdate: () -> Unit,
+    onInstallUpdate: () -> Unit,
+    onDismissUpdateError: () -> Unit,
+    onInstallOutcome: (UpdateOutcome.Result) -> Unit,
 ) {
     var confirmingUnlink by remember { mutableStateOf(false) }
 
@@ -214,6 +230,15 @@ private fun LinkedPanel(
         Notice(tone = Tone.ERROR, title = stringResource(R.string.sync_failed_title), body = it)
     }
 
+    // The app itself comes from this server too, not just the notes.
+    UpdateCard(
+        state = update,
+        onCheck = onCheckUpdate,
+        onInstall = onInstallUpdate,
+        onDismissError = onDismissUpdateError,
+        onOutcome = onInstallOutcome,
+    )
+
     Text(
         text = stringResource(R.string.sync_footer),
         style = MaterialTheme.typography.bodySmall,

android/app/src/main/java/com/fabledsword/thoughtsync/ui/UpdateCard.kt

@@ -0,0 +1,135 @@
+package com.fabledsword.thoughtsync.ui
+
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.padding
+import androidx.compose.material3.Button
+import androidx.compose.material3.LinearProgressIndicator
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.platform.LocalContext
+import androidx.compose.ui.res.stringResource
+import androidx.compose.ui.unit.dp
+import com.fabledsword.thoughtsync.AppUpdate
+import com.fabledsword.thoughtsync.R
+import com.fabledsword.thoughtsync.UpdateOutcome
+
+/**
+ * Updating the app from the server it is linked to.
+ *
+ * Lives on the sync screen because that is what it IS — the server hands out the
+ * client as well as the notes. Putting it in a settings screen of its own would
+ * separate two halves of one relationship.
+ *
+ * Nothing here appears on an unlinked device; [UnlinkedUpdateNote] says why in one
+ * line instead, so the absence reads as a consequence of not being linked rather
+ * than as a missing feature.
+ */
+@Composable
+fun UpdateCard(
+    state: UpdateState,
+    onCheck: () -> Unit,
+    onInstall: () -> Unit,
+    onDismissError: () -> Unit,
+    onOutcome: (UpdateOutcome.Result) -> Unit,
+) {
+    val context = LocalContext.current
+
+    // The system answers an install through a BroadcastReceiver, which has no way
+    // back into a view model. This is the seam.
+    UpdateOutcome.latest?.let { result ->
+        LaunchedEffect(result) { onOutcome(result) }
+    }
+
+    Column(modifier = Modifier.fillMaxWidth().padding(top = 4.dp)) {
+        Text(
+            text = stringResource(R.string.update_installed_version, state.installedVersion),
+            style = MaterialTheme.typography.bodySmall,
+            color = MaterialTheme.colorScheme.onSurfaceVariant,
+        )
+
+        val available = state.available
+        if (available != null) {
+            Text(
+                text =
+                    stringResource(
+                        R.string.update_available,
+                        available.version,
+                        available.size / BYTES_PER_MB,
+                    ),
+                style = MaterialTheme.typography.bodyMedium,
+            )
+        } else if (state.upToDate) {
+            Text(
+                text = stringResource(R.string.update_current),
+                style = MaterialTheme.typography.bodySmall,
+                color = MaterialTheme.colorScheme.onSurfaceVariant,
+            )
+        }
+
+        if (state.working) {
+            LinearProgressIndicator(modifier = Modifier.fillMaxWidth().padding(vertical = 6.dp))
+        }
+
+        Row(horizontalArrangement = Arrangement.spacedBy(8.dp)) {
+            if (available == null) {
+                TextButton(onClick = onCheck, enabled = !state.busy) {
+                    Text(stringResource(R.string.update_check))
+                }
+            } else {
+                Button(onClick = onInstall, enabled = !state.busy) {
+                    Text(stringResource(R.string.update_install))
+                }
+            }
+        }
+
+        // Android's "install unknown apps" grant is separate from anything in the
+        // manifest and only the person can give it. Said BEFORE a download rather
+        // than after, so nobody spends 55 MiB to be told no.
+        if (available != null && !AppUpdate.canInstall(context)) {
+            Notice(
+                tone = Tone.WARN,
+                title = stringResource(R.string.update_permission_title),
+                body = stringResource(R.string.update_permission_body),
+                actionLabel = stringResource(R.string.update_permission_action),
+                onAction = { context.startActivity(AppUpdate.installPermissionSettings(context)) },
+            )
+        }
+
+        state.error?.let {
+            Notice(
+                tone = Tone.ERROR,
+                title = stringResource(R.string.update_failed_title),
+                body = it,
+                onDismiss = onDismissError,
+            )
+        }
+    }
+}
+
+/**
+ * The one line an unlinked device gets.
+ *
+ * Updates arrive from a linked server, so there is genuinely nothing to offer
+ * here — and a Check button that always found nothing would be worse than saying
+ * so.
+ */
+@Composable
+fun UnlinkedUpdateNote() {
+    Text(
+        text = stringResource(R.string.update_needs_server),
+        style = MaterialTheme.typography.bodySmall,
+        color = MaterialTheme.colorScheme.onSurfaceVariant,
+        // Carries the bottom breathing room the connect button used to provide,
+        // now that it is the last thing on the unlinked screen.
+        modifier = Modifier.padding(top = 8.dp, bottom = 24.dp),
+    )
+}
+
+private const val BYTES_PER_MB = 1024 * 1024

android/app/src/main/java/com/fabledsword/thoughtsync/ui/UpdateViewModel.kt

@@ -0,0 +1,128 @@
+package com.fabledsword.thoughtsync.ui
+
+import android.content.Context
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.setValue
+import androidx.lifecycle.ViewModel
+import androidx.lifecycle.ViewModelProvider
+import androidx.lifecycle.viewModelScope
+import com.fabledsword.thoughtsync.AppUpdate
+import com.fabledsword.thoughtsync.UpdateOutcome
+import com.fabledsword.thoughtsync.core.ClientUpdate
+import com.fabledsword.thoughtsync.core.ThoughtSync
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.withContext
+
+/** Everything the update card renders from. */
+data class UpdateState(
+    /** What is running now. Shown even when there is nothing to update to. */
+    val installedVersion: Long = 0,
+    val checking: Boolean = false,
+    /** Only ever set to something NEWER — the core does that comparison. */
+    val available: ClientUpdate? = null,
+    /** A check completed and found nothing. Distinct from "not checked yet". */
+    val upToDate: Boolean = false,
+    val working: Boolean = false,
+    val error: String? = null,
+) {
+    val busy: Boolean get() = checking || working
+}
+
+/**
+ * Updating the app from the server it syncs with.
+ *
+ * **Linked-only, and said out loud.** The app is local-first and completely usable
+ * having never touched a server, so an unlinked install has no update path at all.
+ * The card says that rather than offering a Check button that silently finds
+ * nothing — the same lesson as the desktop's unlink copy (issue 2110).
+ *
+ * The core does the network work, not this class: the device token lives in the
+ * Rust store and pulling it into Kotlin to make an HTTP call would spread the one
+ * secret this app holds across two languages for no gain.
+ */
+class UpdateViewModel(
+    private val core: ThoughtSync,
+    /**
+     * MUST be the application context — it outlives this view model, and holding an
+     * Activity here is the textbook way to leak a window.
+     */
+    private val context: Context,
+) : ViewModel() {
+    var state by mutableStateOf(UpdateState(installedVersion = AppUpdate.installedVersionCode(context)))
+        private set
+
+    /** Ask the linked server what it has. */
+    fun check() {
+        viewModelScope.launch {
+            state = state.copy(checking = true, error = null, upToDate = false)
+            state =
+                try {
+                    val found = core.clientUpdate(state.installedVersion)
+                    state.copy(checking = false, available = found, upToDate = found == null)
+                } catch (e: Exception) {
+                    // Broad by intent, as everywhere the core is called: it reports
+                    // every failure as one error type carrying a message written to
+                    // be read, and a failed check must not take the screen down.
+                    state.copy(checking = false, error = e.message ?: FALLBACK)
+                }
+        }
+    }
+
+    /**
+     * Download the update and hand it to the system installer.
+     *
+     * One action rather than two buttons: nobody wants a downloaded APK sitting
+     * around as an intermediate state they have to think about.
+     */
+    fun downloadAndInstall() {
+        viewModelScope.launch {
+            state = state.copy(working = true, error = null)
+            UpdateOutcome.clear()
+            val failure =
+                try {
+                    val target = AppUpdate.downloadTarget(context)
+                    core.downloadClientUpdate(target.absolutePath)
+                    // Off the main thread: this streams ~55 MiB into the session.
+                    withContext(Dispatchers.IO) { AppUpdate.install(context, target) }
+                } catch (e: Exception) {
+                    e.message ?: FALLBACK
+                }
+            // `working` stays TRUE on success: the install is still in flight, and
+            // on a silent update this process is about to be replaced. Clearing it
+            // here would flash "ready" a moment before the app disappears.
+            state =
+                if (failure == null) state else state.copy(working = false, error = failure)
+        }
+    }
+
+    /**
+     * Take whatever the system finally said about the install.
+     *
+     * Called from the composition, because the answer arrives at a BroadcastReceiver
+     * the system owns and there is no other way back into this class.
+     */
+    fun consumeInstallOutcome(result: UpdateOutcome.Result) {
+        UpdateOutcome.clear()
+        state = state.copy(working = false, error = result.error)
+    }
+
+    fun dismissError() {
+        state = state.copy(error = null)
+    }
+
+    companion object {
+        fun factory(
+            core: ThoughtSync,
+            context: Context,
+        ): ViewModelProvider.Factory =
+            object : ViewModelProvider.Factory {
+                @Suppress("UNCHECKED_CAST")
+                override fun <T : ViewModel> create(modelClass: Class<T>): T =
+                    UpdateViewModel(core, context.applicationContext) as T
+            }
+    }
+}
+
+private const val FALLBACK = "The update couldn't be checked."

android/app/src/main/res/values/strings.xml

@@ -123,6 +123,16 @@
     <string name="sync_automatic">Sync automatically</string>
     <string name="sync_automatic_on">Checks about every 15 minutes, and whenever you open the app.</string>
     <string name="sync_automatic_off">Only when you pull the board down or tap Sync now.</string>
+    <string name="update_installed_version">This app is build %1$d.</string>
+    <string name="update_available">Build %1$s is available (%2$d MB).</string>
+    <string name="update_current">You\'re on the newest build this server has.</string>
+    <string name="update_check">Check for an update</string>
+    <string name="update_install">Update</string>
+    <string name="update_failed_title">The update didn\'t install</string>
+    <string name="update_permission_title">Android needs your permission</string>
+    <string name="update_permission_body">ThoughtSync has to be allowed to install apps before it can update itself. This is a one-time setting.</string>
+    <string name="update_permission_action">Allow installing</string>
+    <string name="update_needs_server">App updates come from a server you connect. Until then, install new builds yourself.</string>
     <string name="sync_footer">Your notes live on this device either way — syncing just keeps a server copy in step, so your other devices can catch up.</string>
     <string name="sync_failed_title">Sync failed</string>
     <string name="sync_rejected_title">The server wouldn\'t accept some changes</string>

android/config/detekt.yml

@@ -64,6 +64,11 @@ exceptions:
     #   * the reminder BroadcastReceiver — same argument, one step worse: it can be
     #     woken at 3am by an alarm or by BOOT_COMPLETED, and every path inside it
     #     has already logged its own failure by the time this catches anything.
+    #   * the self-updater — the install path throws IOException from three
+    #     different calls and SecurityException when the "install unknown apps"
+    #     grant has been revoked since it was checked. All of them mean one thing
+    #     to the person ("it did not install"), and none should take the app down
+    #     while it is holding their notes.
     #
     # Scoped to those paths rather than disabled globally: elsewhere the rule is
     # right and still applies.
@@ -72,3 +77,4 @@ exceptions:
       - "**/ThoughtSyncApplication.kt"
       - "**/SyncWorker.kt"
       - "**/ReminderReceiver.kt"
+      - "**/AppUpdate.kt"

android/ffi/src/lib.rs

@@ -43,8 +43,8 @@ use thoughtsync_core::sync::blobs::BlobStore;
 use thoughtsync_core::sync::{client, compat, engine, push, state};
 
 use models::{
-    patch_from, Identity, Label, Note, NoteDraft, NoteEdit, NoteQuery, ProbeResult, RevokeOutcome,
-    SyncOutcome, SyncStatus,
+    patch_from, ClientUpdate, Identity, Label, Note, NoteDraft, NoteEdit, NoteQuery, ProbeResult,
+    RevokeOutcome, SyncOutcome, SyncStatus,
 };
 
 uniffi::setup_scaffolding!();
@@ -442,6 +442,54 @@ impl ThoughtSync {
     /// The only sync entry point, on purpose. Push and pull exist separately inside
     /// the core, but offering a bare "pull" would let the UI overwrite unsent local
     /// edits — the ordering isn't a suggestion, it's what keeps them.
+    /// The Android client the linked server is offering, if any.
+    ///
+    /// `None` covers two different-looking situations that are one answer to the
+    /// app: this server has no client, or it has one and it is not newer than what
+    /// is already installed. Comparing here rather than in Kotlin keeps the rule —
+    /// version CODE decides, never the name — in the layer that also has to get it
+    /// right for the desktop.
+    pub async fn client_update(
+        &self,
+        installed_version_code: i64,
+    ) -> Result<Option<ClientUpdate>, CoreError> {
+        let (base_url, token) = self.credentials()?;
+        let release = client::fetch_client_release(&base_url, &token)
+            .await
+            .map_err(CoreError::network)?;
+        Ok(release
+            .filter(|r| r.version_code > installed_version_code)
+            .map(ClientUpdate::from))
+    }
+
+    /// Download that client to `dest_path`, verified.
+    ///
+    /// Takes the destination rather than choosing one: only Android knows a
+    /// directory its own package installer can read from, and the core has no
+    /// business guessing at platform paths — the same reason `ThoughtSync::new`
+    /// takes a data dir.
+    pub async fn download_client_update(&self, dest_path: String) -> Result<(), CoreError> {
+        let (base_url, token) = self.credentials()?;
+        let release = client::fetch_client_release(&base_url, &token)
+            .await
+            .map_err(CoreError::network)?
+            // Re-read rather than trusting what the caller was shown: the server
+            // may have published a new build between the check and the tap, and
+            // downloading against a stale digest would fail verification on bytes
+            // that are perfectly good.
+            .ok_or_else(|| {
+                CoreError::network("This server no longer has an Android client.".to_string())
+            })?;
+        client::download_client(
+            &base_url,
+            &token,
+            &release,
+            std::path::Path::new(&dest_path),
+        )
+        .await
+        .map_err(CoreError::network)
+    }
+
     pub async fn sync_now(&self) -> Result<SyncOutcome, CoreError> {
         let (base_url, token) = self.credentials()?;
         engine::run_cycle(&self.db, &self.blobs, &base_url, &token)

android/ffi/src/models.rs

@@ -51,6 +51,42 @@ pub struct Note {
     pub updated_at: Option<String>,
 }
 
+/// An Android build the linked server is offering, already judged to be newer.
+///
+/// A mirror rather than a re-export of `client::ClientRelease`, for the same
+/// reason every other record here is one: the core's shapes are contracted with
+/// other consumers, and `url` in particular is an implementation detail of how
+/// the download is fetched — the app never needs it, because it asks the core to
+/// do the downloading.
+#[derive(Debug, Clone, uniffi::Record)]
+pub struct ClientUpdate {
+    /// For people to read.
+    pub version: String,
+    /// For machines to compare.
+    pub version_code: i64,
+    pub size: i64,
+}
+
+impl From<thoughtsync_core::sync::client::ClientRelease> for ClientUpdate {
+    fn from(r: thoughtsync_core::sync::client::ClientRelease) -> Self {
+        // Destructured exhaustively, like every other conversion in this file: a
+        // field added upstream stops this compiling until Android is told what to
+        // do with it, which turns silent drift into a build error.
+        let thoughtsync_core::sync::client::ClientRelease {
+            version,
+            version_code,
+            size,
+            sha256: _,
+            url: _,
+        } = r;
+        ClientUpdate {
+            version,
+            version_code,
+            size,
+        }
+    }
+}
+
 #[derive(Debug, Clone, uniffi::Record)]
 pub struct NoteLabel {
     pub id: String,

android/tools/check-symbols.py

@@ -104,9 +104,14 @@ def object_members(src: str) -> dict:
         depth = 0
         for line in body.splitlines():
             if depth == 0:
+                # Nested TYPES count as members too: `Foo.Bar` where Bar is a
+                # data class inside object Foo is an ordinary reference, and
+                # leaving them out made the checker report four false positives
+                # the first time an object held one.
                 decl = re.match(
                     r"\s*(?:@\w+\s+)*(?:public |private |internal |protected )?"
-                    r"(?:const |lateinit |inline |suspend )*(?:fun|val|var)\s+"
+                    r"(?:const |lateinit |inline |suspend |data |sealed |enum |value |abstract |open )*"
+                    r"(?:fun|val|var|class|object|interface)\s+"
                     r"(?:<[^>]*>\s*)?(\w+)",
                     line,
                 )

core/src/sync/client.rs

@@ -8,6 +8,7 @@
 //! Nothing here runs unless the user has linked a server; the app is local-first and
 //! fully usable with no network at all.
 
+use std::path::Path;
 use std::time::Duration;
 
 use reqwest::{RequestBuilder, StatusCode};
@@ -431,3 +432,138 @@ mod tests {
         );
     }
 }
+
+/// The Android client a linked server can hand out.
+///
+/// Mirrors `/api/client/android` (see the server's `client_dist.py`). Absent there
+/// means the server has no client to offer, which is an ordinary state and not an
+/// error — a self-hoster who never touches Android has one.
+#[derive(Debug, Clone, Deserialize)]
+pub struct ClientRelease {
+    pub version: String,
+    /// What decides "is this newer". The name is for people and sorts like a string.
+    pub version_code: i64,
+    pub size: i64,
+    pub sha256: String,
+    /// Path on the same server, not an absolute URL — the client joins it to the
+    /// base it is already linked to, so a compromised or misconfigured server
+    /// cannot redirect the download somewhere else.
+    pub url: String,
+}
+
+/// What Android client the linked server has, if any.
+///
+/// `Ok(None)` for a server that simply has none — that is the answer to the
+/// question, not a failure to answer it.
+pub async fn fetch_client_release(
+    base_url: &str,
+    token: &str,
+) -> Result<Option<ClientRelease>, String> {
+    let url = format!("{base_url}/api/client/android");
+    let response = prepare(http()?.get(url), Some(token))
+        .send()
+        .await
+        .map_err(|e| describe_transport_error(base_url, &e))?;
+
+    let status = response.status();
+    if status == StatusCode::NOT_FOUND {
+        return Ok(None);
+    }
+    if status == StatusCode::UNAUTHORIZED {
+        return Err(TOKEN_REJECTED.to_string());
+    }
+    if !status.is_success() {
+        return Err(unexpected_status(base_url, status));
+    }
+
+    response
+        .json::<ClientRelease>()
+        .await
+        .map(Some)
+        .map_err(|e| {
+            format!("{base_url} described its Android client in a way this app could not read: {e}")
+        })
+}
+
+/// Download the client to `dest`, verifying it on the way in.
+///
+/// Streamed rather than buffered: the APK is ~55 MiB and holding that in memory on
+/// a phone, on top of whatever the app is already using, is how an update gets
+/// killed by the low-memory killer half way through.
+///
+/// Written to `dest.part` and renamed only once the digest matches, so an
+/// interrupted download can never be mistaken for a finished one. The digest is
+/// not a trust anchor — the APK signature is, and Android checks that at install —
+/// but it catches a truncated or corrupted transfer before the installer is
+/// bothered with it.
+pub async fn download_client(
+    base_url: &str,
+    token: &str,
+    release: &ClientRelease,
+    dest: &Path,
+) -> Result<(), String> {
+    use sha2::{Digest, Sha256};
+    use std::io::Write;
+
+    // The advertised path is joined to the base we are LINKED to. Taking an
+    // absolute URL from the response would let a server point the download at a
+    // host the user never agreed to.
+    let path = release.url.trim_start_matches('/');
+    let url = format!("{base_url}/{path}");
+
+    let mut response = prepare(http_with(SYNC_TIMEOUT)?.get(url), Some(token))
+        .send()
+        .await
+        .map_err(|e| describe_transport_error(base_url, &e))?;
+
+    let status = response.status();
+    if status == StatusCode::UNAUTHORIZED {
+        return Err(TOKEN_REJECTED.to_string());
+    }
+    if !status.is_success() {
+        return Err(unexpected_status(base_url, status));
+    }
+
+    let partial = dest.with_extension("part");
+    if let Some(parent) = partial.parent() {
+        std::fs::create_dir_all(parent)
+            .map_err(|e| format!("Couldn't prepare a place to download to: {e}"))?;
+    }
+    let mut file = std::fs::File::create(&partial)
+        .map_err(|e| format!("Couldn't open the download file: {e}"))?;
+
+    let mut hasher = Sha256::new();
+    let mut written: i64 = 0;
+    loop {
+        let chunk = response
+            .chunk()
+            .await
+            .map_err(|e| describe_transport_error(base_url, &e))?;
+        let Some(chunk) = chunk else { break };
+        hasher.update(&chunk);
+        written += chunk.len() as i64;
+        file.write_all(&chunk)
+            .map_err(|e| format!("Couldn't write the download: {e}"))?;
+    }
+    file.flush()
+        .map_err(|e| format!("Couldn't finish writing the download: {e}"))?;
+    drop(file);
+
+    let digest = format!("{:x}", hasher.finalize());
+    let mismatch = if written != release.size {
+        Some(format!("expected {} bytes, got {written}", release.size))
+    } else if !digest.eq_ignore_ascii_case(&release.sha256) {
+        Some("the contents did not match the checksum the server published".to_string())
+    } else {
+        None
+    };
+    if let Some(why) = mismatch {
+        // The half-file is removed rather than left: a later run finding it would
+        // have no way to tell it from a good one.
+        let _ = std::fs::remove_file(&partial);
+        return Err(format!("The download from {base_url} was damaged — {why}."));
+    }
+
+    std::fs::rename(&partial, dest)
+        .map_err(|e| format!("Couldn't put the downloaded update in place: {e}"))
+}