diff --git a/app/app/src/main/java/org/modg/bookshelf/data/prefs/SettingsStore.kt b/app/app/src/main/java/org/modg/bookshelf/data/prefs/SettingsStore.kt
index 35ef865..9a04dfe 100644
--- a/app/app/src/main/java/org/modg/bookshelf/data/prefs/SettingsStore.kt
+++ b/app/app/src/main/java/org/modg/bookshelf/data/prefs/SettingsStore.kt
@@ -3,6 +3,7 @@ package org.modg.bookshelf.data.prefs
 import android.content.Context
 import androidx.datastore.core.DataStore
 import androidx.datastore.preferences.core.Preferences
+import androidx.datastore.preferences.core.booleanPreferencesKey
 import androidx.datastore.preferences.core.edit
 import androidx.datastore.preferences.core.longPreferencesKey
 import androidx.datastore.preferences.core.stringPreferencesKey
@@ -27,6 +28,10 @@ class SettingsStore(private val context: Context) {
         val USER_EMAIL = stringPreferencesKey("user_email")
         val LAST_SYNC_TIME = longPreferencesKey("last_sync_time")
         val LAST_SHELF_ID = stringPreferencesKey("last_shelf_id")
+        // Named for the incident, not the feature: a generic "did the full repull
+        // run" flag would invite reuse for some future, unrelated repull with
+        // different safety requirements. See SyncEngine.sync().
+        val FULL_REPULL_2026_09_17_DONE = booleanPreferencesKey("full_repull_2026_09_17_done")
         fun cursor(collection: String) = stringPreferencesKey("cursor_$collection")
     }
 
@@ -36,6 +41,9 @@ class SettingsStore(private val context: Context) {
     val userEmail: Flow<String?> = context.dataStore.data.map { it[Keys.USER_EMAIL] }
     val lastSyncTime: Flow<Long?> = context.dataStore.data.map { it[Keys.LAST_SYNC_TIME] }
 
+    /** See [Keys.FULL_REPULL_2026_09_17_DONE] and [setFullRepullDone]. */
+    val fullRepullDone: Flow<Boolean> = context.dataStore.data.map { it[Keys.FULL_REPULL_2026_09_17_DONE] ?: false }
+
     /**
      * The shelf most recently assigned to a book — SPEC "remember the most recently
      * used shelf" (shelving a box of books usually means one shelf, over and over).
@@ -66,10 +74,30 @@ class SettingsStore(private val context: Context) {
         context.dataStore.edit { it[Keys.cursor(collection)] = cursor }
     }
 
+    /** Forces the next sync's pull to fetch that collection from scratch. */
+    suspend fun clearCursor(collection: String) {
+        context.dataStore.edit { it.remove(Keys.cursor(collection)) }
+    }
+
     suspend fun setLastSyncTime(epochMillis: Long) {
         context.dataStore.edit { it[Keys.LAST_SYNC_TIME] = epochMillis }
     }
 
+    /** One-way: production code never needs to un-set this once a full re-pull has landed. */
+    suspend fun setFullRepullDone() {
+        context.dataStore.edit { it[Keys.FULL_REPULL_2026_09_17_DONE] = true }
+    }
+
+    /**
+     * Test-only seam (internal, not private) — DataStore's on-disk state
+     * otherwise leaks across Robolectric test methods within a class; see
+     * [org.modg.bookshelf.data.prefs.SettingsStoreTest]'s [clearLastShelfId]
+     * call in its own setUp for the same reason.
+     */
+    internal suspend fun resetFullRepullDoneForTesting() {
+        context.dataStore.edit { it.remove(Keys.FULL_REPULL_2026_09_17_DONE) }
+    }
+
     suspend fun setLastShelfId(shelfId: String) {
         context.dataStore.edit { it[Keys.LAST_SHELF_ID] = shelfId }
     }
@@ -92,6 +120,18 @@ class SettingsStore(private val context: Context) {
         }
     }
 
+    /**
+     * Auth EXPIRY, not sign-out: the 2026-09-17 incident (server logged
+     * `auth: ""` on every request — a 5-day-old token PocketBase silently
+     * treated as anonymous rather than rejecting) means the token can go bad
+     * on its own. Unlike [clearAuth], this keeps server URL, user id AND
+     * email so the setup screen can pre-fill everything but the password —
+     * re-signing-in after an expiry should be one field, not three.
+     */
+    suspend fun clearAuthToken() {
+        context.dataStore.edit { it.remove(Keys.AUTH_TOKEN) }
+    }
+
     companion object {
         const val COLLECTION_BOOKS = "books"
         const val COLLECTION_SHELVES = "shelves"
diff --git a/app/app/src/main/java/org/modg/bookshelf/data/remote/PocketBaseApi.kt b/app/app/src/main/java/org/modg/bookshelf/data/remote/PocketBaseApi.kt
index e2824d4..9ae3518 100644
--- a/app/app/src/main/java/org/modg/bookshelf/data/remote/PocketBaseApi.kt
+++ b/app/app/src/main/java/org/modg/bookshelf/data/remote/PocketBaseApi.kt
@@ -20,6 +20,19 @@ interface PocketBaseApi {
     @POST("api/collections/users/auth-with-password")
     suspend fun authWithPassword(@Body body: AuthWithPasswordRequest): AuthResponse
 
+    /**
+     * Re-issues a fresh token for the currently-attached one (see
+     * [PbAuthInterceptor]) — same response shape as [authWithPassword]. Called
+     * at the start of every [org.modg.bookshelf.data.repo.SyncEngine.sync]: a
+     * PocketBase user token that has quietly expired is NOT rejected by the
+     * server, it's just treated as anonymous, and an anonymous request 404s
+     * on records the rules hide instead of 401ing — see
+     * [org.modg.bookshelf.data.repo.SyncEngine]'s 404 hard-delete helpers for
+     * the 2026-09-17 incident that caused.
+     */
+    @POST("api/collections/users/auth-refresh")
+    suspend fun authRefresh(): AuthResponse
+
     // ---- books ----
 
     @GET("api/collections/books/records")
diff --git a/app/app/src/main/java/org/modg/bookshelf/data/repo/AuthRepository.kt b/app/app/src/main/java/org/modg/bookshelf/data/repo/AuthRepository.kt
index 366c33c..aed0b63 100644
--- a/app/app/src/main/java/org/modg/bookshelf/data/repo/AuthRepository.kt
+++ b/app/app/src/main/java/org/modg/bookshelf/data/repo/AuthRepository.kt
@@ -18,6 +18,9 @@ class AuthRepository(
     val isLoggedIn: Flow<Boolean> = settingsStore.authToken.map { !it.isNullOrBlank() }
     val serverUrl: Flow<String?> = settingsStore.serverUrl
 
+    /** Read by the setup screen to pre-fill re-sign-in after an auth expiry (SettingsStore.clearAuthToken keeps this). */
+    val userEmail: Flow<String?> = settingsStore.userEmail
+
     /** Normalizes per SPEC: requires https, strips a trailing slash. */
     suspend fun setServerUrl(rawUrl: String): Result<Unit> {
         val trimmed = rawUrl.trim().trimEnd('/')
@@ -36,6 +39,12 @@ class AuthRepository(
             settingsStore.setAuthToken(response.token)
             settingsStore.setUserId(response.record.id)
             settingsStore.setUserEmail(email)
+            // Every fresh sign-in reconciles fully, not just the one after the
+            // 2026-09-17 incident's stuck cursors — the same backstop for any
+            // future cause we haven't thought of yet.
+            settingsStore.clearCursor(SettingsStore.COLLECTION_BOOKCASES)
+            settingsStore.clearCursor(SettingsStore.COLLECTION_SHELVES)
+            settingsStore.clearCursor(SettingsStore.COLLECTION_BOOKS)
             Result.success(Unit)
         } catch (e: Exception) {
             Result.failure(e)
diff --git a/app/app/src/main/java/org/modg/bookshelf/data/repo/SyncEngine.kt b/app/app/src/main/java/org/modg/bookshelf/data/repo/SyncEngine.kt
index c71fa81..3c19d39 100644
--- a/app/app/src/main/java/org/modg/bookshelf/data/repo/SyncEngine.kt
+++ b/app/app/src/main/java/org/modg/bookshelf/data/repo/SyncEngine.kt
@@ -1,5 +1,6 @@
 package org.modg.bookshelf.data.repo
 
+import kotlinx.coroutines.CancellationException
 import kotlinx.coroutines.flow.first
 import okhttp3.MultipartBody
 import okhttp3.RequestBody.Companion.asRequestBody
@@ -26,6 +27,13 @@ sealed class SyncResult {
     object Success : SyncResult()
     object Skipped : SyncResult()
     data class Failure(val error: Throwable) : SyncResult()
+
+    /**
+     * The session is gone (auth-refresh, or any push/pull mid-sync, came back
+     * 401/403). Distinct from [Failure]: the fix isn't "retry later", it's
+     * "sign in again" — see the 2026-09-17 incident in SyncEngine's KDoc.
+     */
+    object AuthExpired : SyncResult()
 }
 
 /**
@@ -35,6 +43,18 @@ sealed class SyncResult {
  * is bookcases -> shelves -> books both ways, so relations resolve cleanly
  * even though Room does not enforce foreign keys between them (sync would
  * otherwise have to special-case out-of-order pages).
+ *
+ * 2026-09-17 incident: user auth tokens lasted PocketBase's default 5 days,
+ * the app logged in once on the setup screen and never refreshed, and
+ * PocketBase does NOT reject an expired/invalid `Authorization` header — it
+ * silently treats the request as anonymous. The `@request.auth.id != ""`
+ * collection rules then turned every write anonymous-404/400, and this
+ * class's own 404-means-deleted handling (see the update* helpers below)
+ * hard-deleted five books that were still intact on the server. Server
+ * tokens now last 180 days (commit ab1d294) as a backstop, but the real fix
+ * is here: refresh the token at the top of every sync, before any push/pull,
+ * so a stale token surfaces as [SyncResult.AuthExpired] instead of silently
+ * degrading into anonymous requests that 404 on records the rules are hiding.
  */
 class SyncEngine(
     private val apiProvider: ApiProvider,
@@ -46,6 +66,25 @@ class SyncEngine(
     suspend fun sync(): SyncResult {
         val api = apiProvider.api() ?: return SyncResult.Skipped
         return try {
+            val refreshed = api.authRefresh()
+            settingsStore.setAuthToken(refreshed.token)
+            settingsStore.setUserId(refreshed.record.id)
+
+            // One-time recovery for the 2026-09-17 incident: rows hard-deleted
+            // locally by an anonymous-404 during the outage are still on the
+            // server, but the pull cursor has already moved past their
+            // `updated`, so an ordinary incremental pull will never see them
+            // again. Re-pulling everything once is safe — see
+            // applyIncomingBook/shouldApplyRemote below: a row missing
+            // locally is always inserted, and a locally-dirty row only ever
+            // loses to a strictly newer remote copy.
+            val recovering = !settingsStore.fullRepullDone.first()
+            if (recovering) {
+                settingsStore.clearCursor(SettingsStore.COLLECTION_BOOKCASES)
+                settingsStore.clearCursor(SettingsStore.COLLECTION_SHELVES)
+                settingsStore.clearCursor(SettingsStore.COLLECTION_BOOKS)
+            }
+
             pushBookcases(api)
             pushShelves(api)
             pushBooks(api)
@@ -53,7 +92,24 @@ class SyncEngine(
             pullShelves(api)
             pullBooks(api)
             settingsStore.setLastSyncTime(System.currentTimeMillis())
+            // Only after the whole sync succeeds — a sync that fails partway
+            // through (e.g. pull throws) must retry the full re-pull next time.
+            if (recovering) settingsStore.setFullRepullDone()
             SyncResult.Success
+        } catch (e: CancellationException) {
+            throw e
+        } catch (e: HttpException) {
+            if (e.code() == 401 || e.code() == 403) {
+                // Belt and braces: this catches both authRefresh's own 401/403
+                // and one from any push/pull call later in this same sync
+                // (e.g. the token was revoked mid-sync). The 404 handling in
+                // the update* helpers below only matches code 404, so it can
+                // never swallow this.
+                settingsStore.clearAuthToken()
+                SyncResult.AuthExpired
+            } else {
+                SyncResult.Failure(e)
+            }
         } catch (e: IOException) {
             SyncResult.Failure(e)
         } catch (e: Exception) {
@@ -85,6 +141,15 @@ class SyncEngine(
         }
     }
 
+    /**
+     * A 404 here is trusted as "deleted on the server" ONLY because [sync]
+     * proved the attached token valid, moments earlier, by refreshing it. On
+     * 2026-09-17 a request that carried a quietly-expired token 404'd on a
+     * bookcase that still existed — PocketBase's create/update rules hide a
+     * record from an unauthenticated caller rather than 401ing it, so an
+     * anonymous PATCH to an existing id looks identical to one to a deleted
+     * id. Do not call this without a fresh refresh in the same [sync] run.
+     */
     private suspend fun updateBookcase(api: PocketBaseApi, entity: BookcaseEntity) {
         try {
             val response = api.updateBookcase(entity.id, entity.toDto())
@@ -115,6 +180,7 @@ class SyncEngine(
         }
     }
 
+    /** Same trust argument as [updateBookcase]'s KDoc — see it for the 2026-09-17 incident. */
     private suspend fun updateShelf(api: PocketBaseApi, entity: ShelfEntity) {
         try {
             val response = api.updateShelf(entity.id, entity.toDto())
@@ -145,6 +211,13 @@ class SyncEngine(
         }
     }
 
+    /**
+     * Same trust argument as [updateBookcase]'s KDoc — see it for the
+     * 2026-09-17 incident. This is the helper that actually lost the user's
+     * books that day: five records hard-deleted locally while fully intact
+     * on the server, because the request that 404'd on them carried no valid
+     * auth and PocketBase's rules hid rather than rejected them.
+     */
     private suspend fun updateBook(api: PocketBaseApi, entity: BookEntity) {
         try {
             val response = api.updateBook(entity.id, entity.toDto())
diff --git a/app/app/src/main/java/org/modg/bookshelf/ui/components/SyncStatusBar.kt b/app/app/src/main/java/org/modg/bookshelf/ui/components/SyncStatusBar.kt
index e935b94..8060bb5 100644
--- a/app/app/src/main/java/org/modg/bookshelf/ui/components/SyncStatusBar.kt
+++ b/app/app/src/main/java/org/modg/bookshelf/ui/components/SyncStatusBar.kt
@@ -7,6 +7,7 @@ import androidx.compose.animation.core.infiniteRepeatable
 import androidx.compose.animation.core.rememberInfiniteTransition
 import androidx.compose.animation.core.tween
 import androidx.compose.foundation.background
+import androidx.compose.foundation.clickable
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Row
@@ -36,6 +37,9 @@ enum class SyncStatus {
     Syncing,
     Offline,
     Error,
+
+    /** The session expired (see SyncEngine's 2026-09-17 incident KDoc) — distinct from [Error] so the bar is tappable to re-sign-in. */
+    AuthExpired,
 }
 
 /**
@@ -52,11 +56,16 @@ fun SyncStatusBar(
     status: SyncStatus,
     label: String,
     modifier: Modifier = Modifier,
+    // Only [SyncStatus.AuthExpired] wires this up today (SPEC: sync failure is
+    // a quiet status line, never a dialog — but "sign in again" needs SOME
+    // affordance, and this bar is what the user is already looking at).
+    onClick: (() -> Unit)? = null,
 ) {
     Row(
         modifier = modifier
             .fillMaxWidth()
             .background(MaterialTheme.colorScheme.surfaceContainerLow)
+            .let { if (onClick != null) it.clickable(onClick = onClick) else it }
             .navigationBarsPadding()
             .padding(horizontal = 28.dp, vertical = 8.dp),
         verticalAlignment = Alignment.CenterVertically,
@@ -77,7 +86,7 @@ private fun StatusDot(status: SyncStatus) {
         SyncStatus.Synced -> MaterialTheme.colorScheme.secondary
         SyncStatus.Syncing -> MaterialTheme.colorScheme.secondary
         SyncStatus.Offline -> MaterialTheme.colorScheme.onSurfaceVariant
-        SyncStatus.Error -> MaterialTheme.colorScheme.error
+        SyncStatus.Error, SyncStatus.AuthExpired -> MaterialTheme.colorScheme.error
     }
     if (status == SyncStatus.Syncing) {
         val transition = rememberInfiniteTransition(label = "sync-pulse")
diff --git a/app/app/src/main/java/org/modg/bookshelf/ui/library/LibraryScreen.kt b/app/app/src/main/java/org/modg/bookshelf/ui/library/LibraryScreen.kt
index 2275236..c058589 100644
--- a/app/app/src/main/java/org/modg/bookshelf/ui/library/LibraryScreen.kt
+++ b/app/app/src/main/java/org/modg/bookshelf/ui/library/LibraryScreen.kt
@@ -105,6 +105,10 @@ fun LibraryScreen(
     container: AppContainer,
     onEnterByHandWithQuery: (String) -> Unit = { onAddByHandClick() },
     onEditOnlineResult: (OnlineBook, BookMetadata?) -> Unit = { _, _ -> },
+    // 2026-09-17 incident: the user is looking at THIS screen when the
+    // session expires, not the setup screen — MainActivity's start-destination
+    // check only runs on a fresh launch. Defaults to a no-op for previews/tests.
+    onAuthExpired: () -> Unit = {},
 ) {
     val viewModel: LibraryViewModel = viewModel(
         // A fresh VM per distinct shelf filter, so tapping a different shelf
@@ -161,7 +165,11 @@ fun LibraryScreen(
             }
         },
         syncStatusBar = {
-            SyncStatusBar(status = uiState.syncBar.status, label = uiState.syncBar.label)
+            SyncStatusBar(
+                status = uiState.syncBar.status,
+                label = uiState.syncBar.label,
+                onClick = if (uiState.syncBar.status == SyncStatus.AuthExpired) onAuthExpired else null,
+            )
         },
     ) { innerPadding ->
         PullToRefreshBox(
diff --git a/app/app/src/main/java/org/modg/bookshelf/ui/library/LibrarySyncPresenter.kt b/app/app/src/main/java/org/modg/bookshelf/ui/library/LibrarySyncPresenter.kt
index 532f06d..945e6b6 100644
--- a/app/app/src/main/java/org/modg/bookshelf/ui/library/LibrarySyncPresenter.kt
+++ b/app/app/src/main/java/org/modg/bookshelf/ui/library/LibrarySyncPresenter.kt
@@ -8,6 +8,9 @@ enum class SyncOutcome {
     SUCCESS,
     FAILURE,
     NO_SERVER,
+
+    /** See [org.modg.bookshelf.data.repo.SyncResult.AuthExpired] — the fix is "sign in again", not "retry". */
+    AUTH_EXPIRED,
 }
 
 data class SyncBarState(val status: SyncStatus, val label: String)
@@ -29,6 +32,7 @@ object LibrarySyncPresenter {
         return when (lastOutcome) {
             SyncOutcome.FAILURE -> SyncBarState(SyncStatus.Error, "Sync failed — showing local library")
             SyncOutcome.NO_SERVER -> SyncBarState(SyncStatus.Offline, "No server configured")
+            SyncOutcome.AUTH_EXPIRED -> SyncBarState(SyncStatus.AuthExpired, "Signed out — sign in again to sync")
             SyncOutcome.SUCCESS, SyncOutcome.NONE -> if (lastSyncMillis != null) {
                 SyncBarState(SyncStatus.Synced, "Synced ${elapsedPhrase(nowMillis - lastSyncMillis)}")
             } else {
diff --git a/app/app/src/main/java/org/modg/bookshelf/ui/library/LibraryViewModel.kt b/app/app/src/main/java/org/modg/bookshelf/ui/library/LibraryViewModel.kt
index 91e1dad..988089f 100644
--- a/app/app/src/main/java/org/modg/bookshelf/ui/library/LibraryViewModel.kt
+++ b/app/app/src/main/java/org/modg/bookshelf/ui/library/LibraryViewModel.kt
@@ -137,7 +137,9 @@ class LibraryViewModel(
     )
 
     private val _onlineSearchPhase = MutableStateFlow<OnlineSearchPhase>(OnlineSearchPhase.NotSearched)
-    private var onlineSearchJob: Job? = null
+
+    /** Internal (not private) so tests can join it to deterministically wait out a cancelled search. */
+    internal var onlineSearchJob: Job? = null
 
     /**
      * Recomputed from [allBooks] on every change, not frozen at search time — so a
@@ -425,6 +427,7 @@ class LibraryViewModel(
                 is SyncResult.Success -> _lastSyncOutcome.value = SyncOutcome.SUCCESS
                 is SyncResult.Failure -> _lastSyncOutcome.value = SyncOutcome.FAILURE
                 is SyncResult.Skipped -> _lastSyncOutcome.value = SyncOutcome.NO_SERVER
+                is SyncResult.AuthExpired -> _lastSyncOutcome.value = SyncOutcome.AUTH_EXPIRED
             }
             _isSyncing.value = false
         }
diff --git a/app/app/src/main/java/org/modg/bookshelf/ui/nav/BookshelfNavHost.kt b/app/app/src/main/java/org/modg/bookshelf/ui/nav/BookshelfNavHost.kt
index 6b64b56..b839a71 100644
--- a/app/app/src/main/java/org/modg/bookshelf/ui/nav/BookshelfNavHost.kt
+++ b/app/app/src/main/java/org/modg/bookshelf/ui/nav/BookshelfNavHost.kt
@@ -57,6 +57,11 @@ fun BookshelfNavHost(
                 onAddByHandClick = { navController.navigate(Routes.addBook()) },
                 onLocationsClick = { navController.navigate(Routes.LOCATIONS) },
                 onSettingsClick = { navController.navigate(Routes.SETTINGS) },
+                onAuthExpired = {
+                    navController.navigate(Routes.SETUP) {
+                        popUpTo(navController.graph.id) { inclusive = true }
+                    }
+                },
                 container = container,
                 // Wave 10: "Can't find it? Enter it by hand" under the online results —
                 // pre-fills the add-by-hand form's title with the search query.
diff --git a/app/app/src/main/java/org/modg/bookshelf/ui/settings/SettingsScreen.kt b/app/app/src/main/java/org/modg/bookshelf/ui/settings/SettingsScreen.kt
index 4330be9..5a7ba35 100644
--- a/app/app/src/main/java/org/modg/bookshelf/ui/settings/SettingsScreen.kt
+++ b/app/app/src/main/java/org/modg/bookshelf/ui/settings/SettingsScreen.kt
@@ -104,12 +104,30 @@ fun SettingsScreen(
 
                 SectionHeading("Sync")
                 InfoRow(label = "Last synced", value = formatLastSync(state.lastSyncTime))
-                PrimaryButton(
-                    text = if (state.isSyncing) "Syncing…" else "Sync now",
-                    onClick = viewModel::syncNow,
-                    enabled = !state.isSyncing,
-                    modifier = Modifier.padding(top = 8.dp),
-                )
+                if (state.syncAuthExpired) {
+                    // Not a "Sync failed" dead end: the fix is signing in again, so the
+                    // button goes straight to setup instead of retrying the same sync.
+                    // Goes through onSignedOut directly, NOT viewModel.signOut(onSignedOut)
+                    // — that also clears the stored email/user id, and setup pre-fills those.
+                    Text(
+                        text = "Your session expired. Sign in again.",
+                        style = MaterialTheme.typography.bodyMedium,
+                        color = MaterialTheme.colorScheme.error,
+                        modifier = Modifier.padding(top = 8.dp),
+                    )
+                    PrimaryButton(
+                        text = "Sign in",
+                        onClick = onSignedOut,
+                        modifier = Modifier.padding(top = 8.dp),
+                    )
+                } else {
+                    PrimaryButton(
+                        text = if (state.isSyncing) "Syncing…" else "Sync now",
+                        onClick = viewModel::syncNow,
+                        enabled = !state.isSyncing,
+                        modifier = Modifier.padding(top = 8.dp),
+                    )
+                }
 
                 GoldDivider(modifier = Modifier.padding(vertical = 16.dp))
 
@@ -239,6 +257,7 @@ internal fun InfoRow(label: String, value: String) {
 
 private fun syncLabel(state: SettingsUiState): String = when {
     state.isSyncing -> "Syncing…"
+    state.syncAuthExpired -> "Signed out — sign in again to sync"
     state.syncError != null -> "Sync failed: ${state.syncError}"
     state.lastSyncTime != null -> "Synced • ${formatLastSync(state.lastSyncTime)}"
     else -> "Not synced yet"
diff --git a/app/app/src/main/java/org/modg/bookshelf/ui/settings/SettingsViewModel.kt b/app/app/src/main/java/org/modg/bookshelf/ui/settings/SettingsViewModel.kt
index 6c4edc7..177dbb0 100644
--- a/app/app/src/main/java/org/modg/bookshelf/ui/settings/SettingsViewModel.kt
+++ b/app/app/src/main/java/org/modg/bookshelf/ui/settings/SettingsViewModel.kt
@@ -26,11 +26,13 @@ data class SettingsUiState(
     val lastSyncTime: Long? = null,
     val isSyncing: Boolean = false,
     val syncError: String? = null,
+    val syncAuthExpired: Boolean = false,
     val latestCrash: CrashReport? = null,
 ) {
     val syncStatus: SyncStatus
         get() = when {
             isSyncing -> SyncStatus.Syncing
+            syncAuthExpired -> SyncStatus.AuthExpired
             syncError != null -> SyncStatus.Error
             lastSyncTime != null -> SyncStatus.Synced
             else -> SyncStatus.Offline
@@ -56,6 +58,7 @@ class SettingsViewModel(
 
     private val isSyncing = MutableStateFlow(false)
     private val syncError = MutableStateFlow<String?>(null)
+    private val syncAuthExpired = MutableStateFlow(false)
 
     // Read only when this screen is opened (never at app startup) and off the
     // main thread — CrashReporter.list() reads files. See its KDoc.
@@ -83,7 +86,7 @@ class SettingsViewModel(
     }
 
     val uiState: StateFlow<SettingsUiState> =
-        combine(baseInfo, isSyncing, syncError, latestCrash) { base, syncing, error, crash ->
+        combine(baseInfo, isSyncing, syncError, syncAuthExpired, latestCrash) { base, syncing, error, authExpired, crash ->
             SettingsUiState(
                 serverUrl = base.serverUrl,
                 userEmail = base.userEmail,
@@ -92,6 +95,7 @@ class SettingsViewModel(
                 lastSyncTime = base.lastSyncTime,
                 isSyncing = syncing,
                 syncError = error,
+                syncAuthExpired = authExpired,
                 latestCrash = crash,
             )
         }.stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), SettingsUiState())
@@ -101,6 +105,7 @@ class SettingsViewModel(
         viewModelScope.launch {
             isSyncing.value = true
             syncError.value = null
+            syncAuthExpired.value = false
             // SyncEngine never throws (SPEC: sync failure is a quiet status
             // line, never a crash or a blocking dialog) — surface its result
             // as that quiet line, nothing more.
@@ -108,6 +113,9 @@ class SettingsViewModel(
                 is SyncResult.Success -> Unit
                 is SyncResult.Skipped -> syncError.value = "No server configured yet."
                 is SyncResult.Failure -> syncError.value = result.error.message ?: "Sync failed."
+                // Not a generic failure: "Sync failed: HTTP 401" would tell the user
+                // to retry, when what actually fixes this is signing in again.
+                is SyncResult.AuthExpired -> syncAuthExpired.value = true
             }
             isSyncing.value = false
         }
diff --git a/app/app/src/main/java/org/modg/bookshelf/ui/setup/SetupViewModel.kt b/app/app/src/main/java/org/modg/bookshelf/ui/setup/SetupViewModel.kt
index eed0e5b..78cc8e9 100644
--- a/app/app/src/main/java/org/modg/bookshelf/ui/setup/SetupViewModel.kt
+++ b/app/app/src/main/java/org/modg/bookshelf/ui/setup/SetupViewModel.kt
@@ -5,6 +5,7 @@ import androidx.lifecycle.viewModelScope
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.first
 import kotlinx.coroutines.flow.update
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
@@ -46,6 +47,16 @@ class SetupViewModel(private val authRepository: AuthRepository) : ViewModel() {
         .readTimeout(6, TimeUnit.SECONDS)
         .build()
 
+    init {
+        // Re-signing-in after an auth expiry (SettingsStore.clearAuthToken keeps
+        // server URL, user id and email) should be typing the password only.
+        viewModelScope.launch {
+            val url = authRepository.serverUrl.first()
+            val email = authRepository.userEmail.first()
+            _uiState.update { it.copy(serverUrl = url ?: it.serverUrl, email = email ?: it.email) }
+        }
+    }
+
     fun onServerUrlChanged(value: String) {
         _uiState.update { it.copy(serverUrl = value, urlError = null) }
     }
diff --git a/app/app/src/test/java/org/modg/bookshelf/data/repo/AuthRepositoryTest.kt b/app/app/src/test/java/org/modg/bookshelf/data/repo/AuthRepositoryTest.kt
index b16eebd..69ca183 100644
--- a/app/app/src/test/java/org/modg/bookshelf/data/repo/AuthRepositoryTest.kt
+++ b/app/app/src/test/java/org/modg/bookshelf/data/repo/AuthRepositoryTest.kt
@@ -49,4 +49,22 @@ class AuthRepositoryTest {
 
         assertNull(settingsStore.userEmail.first())
     }
+
+    /**
+     * Backstop for causes of stuck cursors we haven't thought of yet (see the
+     * 2026-09-17 incident's one-time re-pull in SyncEngine.sync()): every fresh
+     * sign-in reconciles fully, not just the specific recovery path.
+     */
+    @Test
+    fun `login success clears all three pull cursors so a fresh sign-in reconciles fully`() = runTest {
+        settingsStore.setCursor(SettingsStore.COLLECTION_BOOKS, "2024-01-01 00:00:00.000Z")
+        settingsStore.setCursor(SettingsStore.COLLECTION_SHELVES, "2024-01-01 00:00:00.000Z")
+        settingsStore.setCursor(SettingsStore.COLLECTION_BOOKCASES, "2024-01-01 00:00:00.000Z")
+
+        repository.login(email = "reader@example.com", password = "hunter2")
+
+        assertNull(settingsStore.cursorFor(SettingsStore.COLLECTION_BOOKS).first())
+        assertNull(settingsStore.cursorFor(SettingsStore.COLLECTION_SHELVES).first())
+        assertNull(settingsStore.cursorFor(SettingsStore.COLLECTION_BOOKCASES).first())
+    }
 }
diff --git a/app/app/src/test/java/org/modg/bookshelf/data/repo/FakePocketBaseApi.kt b/app/app/src/test/java/org/modg/bookshelf/data/repo/FakePocketBaseApi.kt
index 1cf297b..c40ab92 100644
--- a/app/app/src/test/java/org/modg/bookshelf/data/repo/FakePocketBaseApi.kt
+++ b/app/app/src/test/java/org/modg/bookshelf/data/repo/FakePocketBaseApi.kt
@@ -1,5 +1,6 @@
 package org.modg.bookshelf.data.repo
 
+import kotlinx.coroutines.CancellationException
 import okhttp3.MediaType.Companion.toMediaType
 import okhttp3.MultipartBody
 import okhttp3.ResponseBody.Companion.toResponseBody
@@ -10,8 +11,10 @@ import org.modg.bookshelf.data.remote.BookcaseDto
 import org.modg.bookshelf.data.remote.PbListResponse
 import org.modg.bookshelf.data.remote.PocketBaseApi
 import org.modg.bookshelf.data.remote.ShelfDto
+import org.modg.bookshelf.data.remote.UserRecordDto
 import retrofit2.HttpException
 import retrofit2.Response
+import java.io.IOException
 
 fun httpError(code: Int): HttpException {
     val body = "{}".toResponseBody("application/json".toMediaType())
@@ -37,6 +40,16 @@ class FakePocketBaseApi : PocketBaseApi {
     val updateShelfErrors = mutableMapOf<String, HttpException>()
     val updateBookErrors = mutableMapOf<String, HttpException>()
 
+    /** Defaults to a normal, successful refresh — most tests aren't exercising the auth-expiry path. */
+    var authRefreshHttpError: HttpException? = null
+    var authRefreshIOException: IOException? = null
+    var authRefreshCancellation: CancellationException? = null
+    var refreshedToken: String = "refreshed-fake-token"
+    var refreshedUserId: String = "fake-user-id"
+
+    /** One-shot: exercises the "a sync whose pull fails leaves the one-time-repull flag unset" case. */
+    var nextListBooksError: HttpException? = null
+
     fun seedBook(dto: BookDto) {
         books[dto.id] = dto
     }
@@ -44,9 +57,19 @@ class FakePocketBaseApi : PocketBaseApi {
     override suspend fun authWithPassword(body: AuthWithPasswordRequest): AuthResponse =
         AuthResponse(token = "fake-token")
 
+    override suspend fun authRefresh(): AuthResponse {
+        callLog += "authRefresh"
+        authRefreshCancellation?.let { throw it }
+        authRefreshIOException?.let { throw it }
+        authRefreshHttpError?.let { throw it }
+        return AuthResponse(token = refreshedToken, record = UserRecordDto(id = refreshedUserId))
+    }
+
     // ---- books ----
 
     override suspend fun listBooks(filter: String?, sort: String, perPage: Int, page: Int): PbListResponse<BookDto> {
+        callLog += "listBooks"
+        nextListBooksError?.let { nextListBooksError = null; throw it }
         val cursor = filter?.substringAfter("'")?.substringBefore("'")
         val items = books.values.filter { cursor == null || it.updated > cursor }.sortedBy { it.updated }
         return PbListResponse(page = 1, totalPages = 1, totalItems = items.size, items = items)
diff --git a/app/app/src/test/java/org/modg/bookshelf/data/repo/SyncEngineTest.kt b/app/app/src/test/java/org/modg/bookshelf/data/repo/SyncEngineTest.kt
index a048fc6..08ecd92 100644
--- a/app/app/src/test/java/org/modg/bookshelf/data/repo/SyncEngineTest.kt
+++ b/app/app/src/test/java/org/modg/bookshelf/data/repo/SyncEngineTest.kt
@@ -2,11 +2,16 @@ package org.modg.bookshelf.data.repo
 
 import androidx.room.Room
 import androidx.test.core.app.ApplicationProvider
+import kotlinx.coroutines.CancellationException
+import kotlinx.coroutines.flow.first
 import kotlinx.coroutines.test.runTest
 import org.junit.After
 import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNotNull
 import org.junit.Assert.assertNull
 import org.junit.Assert.assertTrue
+import org.junit.Assert.fail
 import org.junit.Before
 import org.junit.Test
 import org.junit.runner.RunWith
@@ -36,6 +41,11 @@ class SyncEngineTest {
             .build()
         settingsStore = SettingsStore(ApplicationProvider.getApplicationContext())
         settingsStore.setServerUrl("https://example.test")
+        // DataStore's on-disk state leaks across Robolectric test methods in
+        // this class (see SettingsStoreTest's setUp for the same issue) — the
+        // one-time-repull flag being true from an earlier test would silently
+        // change the repull tests' behavior.
+        settingsStore.resetFullRepullDoneForTesting()
         api = FakePocketBaseApi()
         engine = SyncEngine(
             apiProvider = { api },
@@ -198,4 +208,124 @@ class SyncEngineTest {
         assertEquals("My local edit", db.bookDao().getByIdIncludingDeleted("book1")?.title)
         assertEquals(SyncState.PENDING_UPDATE, db.bookDao().getByIdIncludingDeleted("book1")?.syncState)
     }
+
+    // ---------------------------------------------------------- auth refresh (2026-09-17)
+
+    @Test
+    fun `sync refreshes the token before any push or pull, and persists the refreshed token`() = runTest {
+        db.bookcaseDao().upsert(bookcase("case1"))
+        api.refreshedToken = "brand-new-token"
+        api.refreshedUserId = "user-42"
+
+        val result = engine.sync()
+
+        assertTrue(result is SyncResult.Success)
+        val refreshIdx = api.callLog.indexOf("authRefresh")
+        val pushIdx = api.callLog.indexOf("createBookcase:case1")
+        assertTrue("expected authRefresh before any push, got ${api.callLog}", refreshIdx == 0 && refreshIdx < pushIdx)
+        assertEquals("brand-new-token", settingsStore.authToken.first())
+        assertEquals("user-42", settingsStore.userId.first())
+    }
+
+    @Test
+    fun `a 401 on refresh is the 2026-09-17 incident's regression test - AuthExpired, token cleared, nothing else touched`() = runTest {
+        settingsStore.setAuthToken("stale-5-day-old-token")
+        settingsStore.setUserEmail("reader@example.com")
+        db.bookDao().upsert(book("book1", state = SyncState.PENDING_UPDATE).copy(title = "unsynced edit"))
+        api.authRefreshHttpError = httpError(401)
+
+        val result = engine.sync()
+
+        assertTrue(result is SyncResult.AuthExpired)
+        assertNull("token must be cleared so the app doesn't keep hammering the server with it", settingsStore.authToken.first())
+        assertEquals("https://example.test", settingsStore.serverUrl.first())
+        assertEquals("reader@example.com", settingsStore.userEmail.first())
+        assertEquals("no push/pull call may have been made", listOf("authRefresh"), api.callLog)
+        val stillLocal = db.bookDao().getByIdIncludingDeleted("book1")
+        assertEquals("the PENDING_UPDATE book must survive an unauthenticated sync attempt intact", "unsynced edit", stillLocal?.title)
+        assertEquals(SyncState.PENDING_UPDATE, stillLocal?.syncState)
+    }
+
+    @Test
+    fun `a 403 on refresh also reports AuthExpired`() = runTest {
+        api.authRefreshHttpError = httpError(403)
+
+        assertTrue(engine.sync() is SyncResult.AuthExpired)
+    }
+
+    @Test
+    fun `an IOException on refresh is a plain Failure and does not clear the token`() = runTest {
+        settingsStore.setAuthToken("still-good-token")
+        api.authRefreshIOException = java.io.IOException("no network")
+
+        val result = engine.sync()
+
+        assertTrue(result is SyncResult.Failure)
+        assertEquals("still-good-token", settingsStore.authToken.first())
+    }
+
+    @Test
+    fun `a 401 from a push call mid-sync reports AuthExpired and does not hard-delete the record`() = runTest {
+        db.bookDao().upsert(book("book1", state = SyncState.PENDING_UPDATE).copy(title = "keep me"))
+        api.updateBookErrors["book1"] = httpError(401)
+
+        val result = engine.sync()
+
+        assertTrue(result is SyncResult.AuthExpired)
+        val stillLocal = db.bookDao().getByIdIncludingDeleted("book1")
+        assertNotNull("a 401 must never be mistaken for the 404-means-deleted case", stillLocal)
+        assertEquals("keep me", stillLocal?.title)
+        assertEquals(SyncState.PENDING_UPDATE, stillLocal?.syncState)
+    }
+
+    @Test
+    fun `CancellationException thrown during sync propagates instead of becoming a Failure`() = runTest {
+        api.authRefreshCancellation = CancellationException("scope cancelled")
+
+        try {
+            engine.sync()
+            fail("expected CancellationException to propagate")
+        } catch (e: CancellationException) {
+            // expected
+        }
+    }
+
+    // ---------------------------------------------------------- one-time full re-pull (2026-09-17)
+
+    @Test
+    fun `first sync after the fix restores a book the incident hard-deleted, and marks the repull done`() = runTest {
+        // Mirrors the incident: the pull cursor is already past the lost row's
+        // `updated`, and the row is gone locally, but it is still intact on the server.
+        settingsStore.setCursor(SettingsStore.COLLECTION_BOOKS, "2024-06-01 00:00:00.000Z")
+        api.seedBook(BookDto(id = "lost-book", title = "Recovered", updated = "2024-01-01 00:00:00.000Z", created = "2024-01-01 00:00:00.000Z"))
+        assertNull(db.bookDao().getByIdIncludingDeleted("lost-book"))
+
+        val result = engine.sync()
+
+        assertTrue(result is SyncResult.Success)
+        assertEquals("Recovered", db.bookDao().getById("lost-book")?.title)
+        assertTrue(settingsStore.fullRepullDone.first())
+    }
+
+    @Test
+    fun `a sync whose pull fails leaves the one-time-repull flag unset so the next sync retries it`() = runTest {
+        api.nextListBooksError = httpError(500)
+
+        val result = engine.sync()
+
+        assertTrue(result is SyncResult.Failure)
+        assertFalse(settingsStore.fullRepullDone.first())
+    }
+
+    @Test
+    fun `once the repull flag is set, a normal sync no longer clears cursors`() = runTest {
+        settingsStore.setFullRepullDone()
+        settingsStore.setCursor(SettingsStore.COLLECTION_BOOKS, "2024-06-01 00:00:00.000Z")
+        // Older than the cursor: an incremental pull must NOT see this one.
+        api.seedBook(BookDto(id = "old-book", title = "Too old", updated = "2024-01-01 00:00:00.000Z", created = "2024-01-01 00:00:00.000Z"))
+
+        engine.sync()
+
+        assertNull(db.bookDao().getByIdIncludingDeleted("old-book"))
+    }
 }
diff --git a/app/app/src/test/java/org/modg/bookshelf/ui/library/LibrarySyncPresenterTest.kt b/app/app/src/test/java/org/modg/bookshelf/ui/library/LibrarySyncPresenterTest.kt
index 8c1729b..dce48ba 100644
--- a/app/app/src/test/java/org/modg/bookshelf/ui/library/LibrarySyncPresenterTest.kt
+++ b/app/app/src/test/java/org/modg/bookshelf/ui/library/LibrarySyncPresenterTest.kt
@@ -20,6 +20,13 @@ class LibrarySyncPresenterTest {
         assertEquals("Sync failed — showing local library", state.label)
     }
 
+    @Test
+    fun `an expired session is a distinct status from a plain failure, not "Sync failed"`() {
+        val state = LibrarySyncPresenter.present(nowMillis = 1_000L, lastSyncMillis = null, isSyncing = false, lastOutcome = SyncOutcome.AUTH_EXPIRED)
+        assertEquals(SyncStatus.AuthExpired, state.status)
+        assertEquals("Signed out — sign in again to sync", state.label)
+    }
+
     @Test
     fun `no server configured is offline, not an error`() {
         val state = LibrarySyncPresenter.present(nowMillis = 1_000L, lastSyncMillis = null, isSyncing = false, lastOutcome = SyncOutcome.NO_SERVER)
diff --git a/app/app/src/test/java/org/modg/bookshelf/ui/library/LibraryViewModelTest.kt b/app/app/src/test/java/org/modg/bookshelf/ui/library/LibraryViewModelTest.kt
index c41df80..09bbb4a 100644
--- a/app/app/src/test/java/org/modg/bookshelf/ui/library/LibraryViewModelTest.kt
+++ b/app/app/src/test/java/org/modg/bookshelf/ui/library/LibraryViewModelTest.kt
@@ -97,6 +97,31 @@ class LibraryViewModelTest {
         initialShelfId = null,
     )
 
+    // --- sync (2026-09-17 auth-expiry incident) ---
+
+    @Test
+    fun `an expired session maps to the AuthExpired sync bar state, not a generic failure`() = runTest {
+        settingsStore.setServerUrl("https://example.test")
+        val api = org.modg.bookshelf.data.repo.FakePocketBaseApi()
+        api.authRefreshHttpError = org.modg.bookshelf.data.repo.httpError(401)
+        val vm = LibraryViewModel(
+            coverDownloader = coverHost.downloader(java.io.File(context.cacheDir, "cover-preload")),
+            computeDispatcher = Dispatchers.Main,
+            bookRepository = bookRepository,
+            locationRepository = locationRepository,
+            syncEngine = SyncEngine(ApiProvider { api }, db.bookDao(), db.bookcaseDao(), db.shelfDao(), settingsStore),
+            settingsStore = settingsStore,
+            bookSearchRepository = BookSearchRepository(OkHttpClient(), Json { ignoreUnknownKeys = true }),
+            metadataRepository = MetadataRepository(OkHttpClient(), Json { ignoreUnknownKeys = true }),
+            initialShelfId = null,
+        )
+
+        val bar = vm.uiState.first { it.syncBar.status == org.modg.bookshelf.ui.components.SyncStatus.AuthExpired }.syncBar
+
+        assertEquals(org.modg.bookshelf.ui.components.SyncStatus.AuthExpired, bar.status)
+        assertEquals("Signed out — sign in again to sync", bar.label)
+    }
+
     // --- typing vs submit ---
 
     @Test
@@ -150,7 +175,12 @@ class LibraryViewModelTest {
         vm.onQueryChange("something else")
 
         assertEquals(OnlineSearchState.NotSearched, vm.onlineSearch.first())
-        gate.countDown() // release the blocked background call so its thread isn't leaked
+        gate.countDown() // release the blocked background call
+        // Wait for the cancelled job to actually finish unwinding on its own thread before
+        // this test returns -- otherwise it can still be dispatching onto Dispatchers.Main
+        // after tearDown() (or even the next test's setUp()) touches it, which fails with
+        // "Dispatchers.Main is used concurrently with setting it" in whatever test runs next.
+        vm.onlineSearchJob?.join()
     }
 
     // --- partial and total source failure ---
diff --git a/app/app/src/test/java/org/modg/bookshelf/ui/screens/LibraryScreenPaparazziTest.kt b/app/app/src/test/java/org/modg/bookshelf/ui/screens/LibraryScreenPaparazziTest.kt
index e52d144..680a7d2 100644
--- a/app/app/src/test/java/org/modg/bookshelf/ui/screens/LibraryScreenPaparazziTest.kt
+++ b/app/app/src/test/java/org/modg/bookshelf/ui/screens/LibraryScreenPaparazziTest.kt
@@ -71,6 +71,10 @@ class LibraryScreenPaparazziTest {
     @Test
     fun libraryEmptyLight() = snapshotBoth("library-empty") { Empty() }
 
+    /** 2026-09-17 incident: the sync bar's distinct "session expired" state — never "Sync failed". */
+    @Test
+    fun libraryAuthExpiredLight() = snapshotBoth("library-auth-expired") { AuthExpired() }
+
     @Test
     fun librarySearchBothSectionsLight() = snapshotBoth("library-search-both-sections") {
         SearchShell(
@@ -170,6 +174,13 @@ class LibraryScreenPaparazziTest {
     @Composable
     private fun Empty() = Shell(books = emptyList(), syncStatus = SyncStatus.Offline, syncLabel = "Not synced yet")
 
+    @Composable
+    private fun AuthExpired() = Shell(
+        books = ScreenFixtures.books,
+        syncStatus = SyncStatus.AuthExpired,
+        syncLabel = "Signed out — sign in again to sync",
+    )
+
     /** Same chrome as [Shell] but with a non-blank query, so [LibraryScreen] shows the sectioned search-results view instead of the grid. */
     @Composable
     private fun SearchShell(localMatches: List<BookEntity>, onlineSearch: OnlineSearchState) {
diff --git a/app/app/src/test/java/org/modg/bookshelf/ui/screens/SettingsScreenPaparazziTest.kt b/app/app/src/test/java/org/modg/bookshelf/ui/screens/SettingsScreenPaparazziTest.kt
index 633c3f1..36b92ec 100644
--- a/app/app/src/test/java/org/modg/bookshelf/ui/screens/SettingsScreenPaparazziTest.kt
+++ b/app/app/src/test/java/org/modg/bookshelf/ui/screens/SettingsScreenPaparazziTest.kt
@@ -7,6 +7,8 @@ import androidx.compose.material.icons.Icons
 import androidx.compose.material.icons.filled.ArrowBack
 import androidx.compose.material3.Icon
 import androidx.compose.material3.IconButton
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.unit.dp
@@ -49,14 +51,21 @@ class SettingsScreenPaparazziTest {
     @Test
     fun settingsDiagnosticsEmptyLight() = snapshotBoth("settings-diagnostics-empty") { DiagnosticsEmpty() }
 
-    @Composable
-    private fun Populated() = Shell(crash = sampleCrash)
+    /** 2026-09-17 incident: "Your session expired. Sign in again." — never "Sync failed: HTTP 401". */
+    @Test
+    fun settingsAuthExpiredLight() = snapshotBoth("settings-auth-expired") { AuthExpired() }
 
     @Composable
-    private fun DiagnosticsEmpty() = Shell(crash = null)
+    private fun Populated() = Shell(crash = sampleCrash, authExpired = false)
 
     @Composable
-    private fun Shell(crash: CrashReport?) {
+    private fun DiagnosticsEmpty() = Shell(crash = null, authExpired = false)
+
+    @Composable
+    private fun AuthExpired() = Shell(crash = null, authExpired = true)
+
+    @Composable
+    private fun Shell(crash: CrashReport?, authExpired: Boolean) {
         val lastSync = System.currentTimeMillis() - 2 * 60 * 1000L
         BookshelfScaffold(
             title = "Settings",
@@ -64,7 +73,11 @@ class SettingsScreenPaparazziTest {
                 IconButton(onClick = {}) { Icon(Icons.Filled.ArrowBack, contentDescription = "Back") }
             },
             syncStatusBar = {
-                SyncStatusBar(status = SyncStatus.Synced, label = "Synced • ${formatLastSync(lastSync)}")
+                if (authExpired) {
+                    SyncStatusBar(status = SyncStatus.AuthExpired, label = "Signed out — sign in again to sync")
+                } else {
+                    SyncStatusBar(status = SyncStatus.Synced, label = "Synced • ${formatLastSync(lastSync)}")
+                }
             },
         ) { innerPadding ->
             PaperSurface(modifier = Modifier.fillMaxWidth()) {
@@ -82,7 +95,17 @@ class SettingsScreenPaparazziTest {
 
                     SectionHeading("Sync")
                     InfoRow(label = "Last synced", value = formatLastSync(lastSync))
-                    PrimaryButton(text = "Sync now", onClick = {}, modifier = Modifier.padding(top = 8.dp))
+                    if (authExpired) {
+                        Text(
+                            text = "Your session expired. Sign in again.",
+                            style = MaterialTheme.typography.bodyMedium,
+                            color = MaterialTheme.colorScheme.error,
+                            modifier = Modifier.padding(top = 8.dp),
+                        )
+                        PrimaryButton(text = "Sign in", onClick = {}, modifier = Modifier.padding(top = 8.dp))
+                    } else {
+                        PrimaryButton(text = "Sync now", onClick = {}, modifier = Modifier.padding(top = 8.dp))
+                    }
 
                     GoldDivider(modifier = Modifier.padding(vertical = 16.dp))
 
diff --git a/app/app/src/test/java/org/modg/bookshelf/ui/settings/SettingsViewModelTest.kt b/app/app/src/test/java/org/modg/bookshelf/ui/settings/SettingsViewModelTest.kt
new file mode 100644
index 0000000..a64c3de
--- /dev/null
+++ b/app/app/src/test/java/org/modg/bookshelf/ui/settings/SettingsViewModelTest.kt
@@ -0,0 +1,116 @@
+package org.modg.bookshelf.ui.settings
+
+import androidx.room.Room
+import androidx.test.core.app.ApplicationProvider
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.flow.first
+import kotlinx.coroutines.test.UnconfinedTestDispatcher
+import kotlinx.coroutines.test.resetMain
+import kotlinx.coroutines.test.runTest
+import kotlinx.coroutines.test.setMain
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.modg.bookshelf.data.local.BookshelfDatabase
+import org.modg.bookshelf.data.prefs.SettingsStore
+import org.modg.bookshelf.data.remote.ApiProvider
+import org.modg.bookshelf.data.repo.AuthRepository
+import org.modg.bookshelf.data.repo.BookRepository
+import org.modg.bookshelf.data.repo.FakePocketBaseApi
+import org.modg.bookshelf.data.repo.SyncEngine
+import org.modg.bookshelf.data.repo.httpError
+import org.modg.bookshelf.diagnostics.CrashReporter
+import org.modg.bookshelf.ui.components.SyncStatus
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.annotation.Config
+
+/**
+ * 2026-09-17 incident: [SettingsViewModel.syncNow] used to surface an expired
+ * session as "Sync failed: HTTP 401", which reads as "try again later" rather
+ * than "you must sign in again". Covers the distinct [SyncResult.AuthExpired]
+ * mapping this class's SPEC section adds.
+ */
+@OptIn(ExperimentalCoroutinesApi::class)
+@RunWith(RobolectricTestRunner::class)
+@Config(sdk = [34])
+class SettingsViewModelTest {
+
+    private lateinit var db: BookshelfDatabase
+    private lateinit var settingsStore: SettingsStore
+    private lateinit var api: FakePocketBaseApi
+    private lateinit var context: android.content.Context
+
+    @Before
+    fun setUp() {
+        Dispatchers.setMain(UnconfinedTestDispatcher())
+        context = ApplicationProvider.getApplicationContext()
+        db = Room.inMemoryDatabaseBuilder(context, BookshelfDatabase::class.java)
+            .allowMainThreadQueries()
+            .build()
+        settingsStore = SettingsStore(context)
+        api = FakePocketBaseApi()
+    }
+
+    @After
+    fun tearDown() {
+        db.close()
+        Dispatchers.resetMain()
+    }
+
+    private fun viewModel(): SettingsViewModel = SettingsViewModel(
+        authRepository = AuthRepository(ApiProvider { api }, settingsStore),
+        settingsStore = settingsStore,
+        bookRepository = BookRepository(db.bookDao(), context),
+        syncEngine = SyncEngine(ApiProvider { api }, db.bookDao(), db.bookcaseDao(), db.shelfDao(), settingsStore),
+        crashReporter = CrashReporter(context),
+    )
+
+    @Test
+    fun `a plain sync failure keeps the generic error message`() = runTest {
+        settingsStore.setServerUrl("https://example.test")
+        api.authRefreshIOException = java.io.IOException("no network")
+        val vm = viewModel()
+
+        vm.syncNow()
+
+        // Not "!isSyncing" — that's also (trivially) true of the very first
+        // combined emission, before this sync's failure has landed.
+        val state = vm.uiState.first { it.syncError != null }
+        assertFalse(state.syncAuthExpired)
+        assertEquals(SyncStatus.Error, state.syncStatus)
+    }
+
+    @Test
+    fun `an expired session maps to syncAuthExpired, not a generic sync error`() = runTest {
+        settingsStore.setServerUrl("https://example.test")
+        api.authRefreshHttpError = httpError(401)
+        val vm = viewModel()
+
+        vm.syncNow()
+
+        val state = vm.uiState.first { it.syncAuthExpired }
+        assertTrue(state.syncAuthExpired)
+        assertEquals(SyncStatus.AuthExpired, state.syncStatus)
+        assertEquals(null, state.syncError)
+    }
+
+    @Test
+    fun `a later successful sync clears a previous auth-expired state`() = runTest {
+        settingsStore.setServerUrl("https://example.test")
+        api.authRefreshHttpError = httpError(401)
+        val vm = viewModel()
+        vm.syncNow()
+        vm.uiState.first { it.syncAuthExpired }
+
+        api.authRefreshHttpError = null
+        vm.syncNow()
+
+        val state = vm.uiState.first { !it.isSyncing && !it.syncAuthExpired }
+        assertFalse(state.syncAuthExpired)
+    }
+}
diff --git a/app/app/src/test/java/org/modg/bookshelf/ui/setup/SetupViewModelTest.kt b/app/app/src/test/java/org/modg/bookshelf/ui/setup/SetupViewModelTest.kt
new file mode 100644
index 0000000..63cd4a3
--- /dev/null
+++ b/app/app/src/test/java/org/modg/bookshelf/ui/setup/SetupViewModelTest.kt
@@ -0,0 +1,63 @@
+package org.modg.bookshelf.ui.setup
+
+import androidx.test.core.app.ApplicationProvider
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.flow.first
+import kotlinx.coroutines.test.UnconfinedTestDispatcher
+import kotlinx.coroutines.test.resetMain
+import kotlinx.coroutines.test.runTest
+import kotlinx.coroutines.test.setMain
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.modg.bookshelf.data.prefs.SettingsStore
+import org.modg.bookshelf.data.remote.ApiProvider
+import org.modg.bookshelf.data.repo.AuthRepository
+import org.modg.bookshelf.data.repo.FakePocketBaseApi
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.annotation.Config
+
+/**
+ * 2026-09-17 incident: after an auth expiry clears only the token (see
+ * SettingsStore.clearAuthToken), re-signing-in should be typing the password
+ * only. Covers that the setup screen's ViewModel pre-fills what's already
+ * stored, whether that's from a first-run-in-progress or a post-expiry return trip.
+ */
+@OptIn(ExperimentalCoroutinesApi::class)
+@RunWith(RobolectricTestRunner::class)
+@Config(sdk = [34])
+class SetupViewModelTest {
+
+    private lateinit var settingsStore: SettingsStore
+    private lateinit var authRepository: AuthRepository
+
+    @Before
+    fun setUp() {
+        Dispatchers.setMain(UnconfinedTestDispatcher())
+        settingsStore = SettingsStore(ApplicationProvider.getApplicationContext())
+        authRepository = AuthRepository(ApiProvider { FakePocketBaseApi() }, settingsStore)
+    }
+
+    @After
+    fun tearDown() {
+        Dispatchers.resetMain()
+    }
+
+    @Test
+    fun `pre-fills the stored server URL and email when present`() = runTest {
+        settingsStore.setServerUrl("https://library.montanaro.home")
+        settingsStore.setUserEmail("reader@example.com")
+
+        val viewModel = SetupViewModel(authRepository)
+
+        val state = viewModel.uiState.first {
+            it.serverUrl == "https://library.montanaro.home" && it.email == "reader@example.com"
+        }
+        assertEquals("https://library.montanaro.home", state.serverUrl)
+        assertEquals("reader@example.com", state.email)
+        assertEquals("", state.password) // never pre-filled — SPEC "typing the password only"
+    }
+}
diff --git a/app/app/src/test/snapshots/images/org.modg.bookshelf.ui.screens_LibraryScreenPaparazziTest_libraryAuthExpiredLight_library-auth-expired-dark.png b/app/app/src/test/snapshots/images/org.modg.bookshelf.ui.screens_LibraryScreenPaparazziTest_libraryAuthExpiredLight_library-auth-expired-dark.png
new file mode 100644
index 0000000..eda2570
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diff --git a/app/app/src/test/snapshots/images/org.modg.bookshelf.ui.screens_LibraryScreenPaparazziTest_libraryAuthExpiredLight_library-auth-expired-light.png b/app/app/src/test/snapshots/images/org.modg.bookshelf.ui.screens_LibraryScreenPaparazziTest_libraryAuthExpiredLight_library-auth-expired-light.png
new file mode 100644
index 0000000..606f8df
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diff --git a/app/app/src/test/snapshots/images/org.modg.bookshelf.ui.screens_SettingsScreenPaparazziTest_settingsAuthExpiredLight_settings-auth-expired-dark.png b/app/app/src/test/snapshots/images/org.modg.bookshelf.ui.screens_SettingsScreenPaparazziTest_settingsAuthExpiredLight_settings-auth-expired-dark.png
new file mode 100644
index 0000000..b6d90cb
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diff --git a/app/app/src/test/snapshots/images/org.modg.bookshelf.ui.screens_SettingsScreenPaparazziTest_settingsAuthExpiredLight_settings-auth-expired-light.png b/app/app/src/test/snapshots/images/org.modg.bookshelf.ui.screens_SettingsScreenPaparazziTest_settingsAuthExpiredLight_settings-auth-expired-light.png
new file mode 100644
index 0000000..2d70c38
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diff --git a/logs/M-authexpiry-fix.sid b/logs/M-authexpiry-fix.sid
new file mode 100644
index 0000000..5003bfb
--- /dev/null
+++ b/logs/M-authexpiry-fix.sid
@@ -0,0 +1 @@
+d109abe5-9e54-4496-912d-b200c51d373e
diff --git a/logs/M-authexpiry-fix.summary b/logs/M-authexpiry-fix.summary
new file mode 100644
index 0000000..234eec6
--- /dev/null
+++ b/logs/M-authexpiry-fix.summary
@@ -0,0 +1,18 @@
+=== M-authexpiry-fix  attempts=1 quota_waits=0 hard_fails=0 ===
+cost=$1.3351339999999998 turns=44 err=?
+--- result (tail) ---
+ar onlineSearchJob` to `internal var onlineSearchJob`, matching the file's existing convention (same reasoning already documented on `performOnlineSearch`) of loosening visibility so tests can synchronize on it directly.
+- `app/app/src/test/java/org/modg/bookshelf/ui/library/LibraryViewModelTest.kt`: after `gate.countDown()`, added `vm.onlineSearchJob?.join()` so the test genuinely waits for the cancelled job to finish before returning, with a comment explaining why.
+
+No changes to the feature logic, SPEC, build files, or anything else — nothing else in the gate's failure pointed at the auth-expiry implementation itself.
+
+**Verification:**
+- `assembleDebug` → exit 0.
+- `testDebugUnitTest` → exit 0, reran 4 more times with `--rerun` after the fix, all clean (previously failed on run 2 of 2 before the fix).
+- Test count: 355 passed, 2 skipped, 0 failures (baseline was 337) — via summed XML.
+- `verifyPaparazziDebug` → exit 0.
+- Recorded the 4 new Paparazzi snapshots (`library-auth-expired-{light,dark}`, `settings-auth-expired-{light,dark}`) and viewed all four PNGs: library grid shows the "Signed out — sign in again to sync" sync bar; settings shows "Your session expired. Sign in again." with a "Sign in" button. Both match spec's design language in light and dark, and `git status` confirmed no other snapshots were touched by the record run.
+- Grepped a forced recompile for Kotlin warnings in touched files: none.
+- Final full sequence run in order (assembleDebug → testDebugUnitTest → verifyPaparazziDebug): all exit 0.
+
+I did not re-review the original worker's auth-expiry design/logic (token refresh, full re-pull, AuthExpired propagation) since none of that was implicated by the gate failure — per instructions, I only touched what was making the gate red.
diff --git a/logs/M-authexpiry.sid b/logs/M-authexpiry.sid
new file mode 100644
index 0000000..c22419b
--- /dev/null
+++ b/logs/M-authexpiry.sid
@@ -0,0 +1 @@
+6bcc6e7a-a2a5-449e-b75a-a62eea1cc657
diff --git a/logs/M-authexpiry.summary b/logs/M-authexpiry.summary
new file mode 100644
index 0000000..97893ef
--- /dev/null
+++ b/logs/M-authexpiry.summary
@@ -0,0 +1,13 @@
+=== M-authexpiry  attempts=1 quota_waits=0 hard_fails=0 ===
+cost=$4.886013000000001 turns=123 err=?
+--- result (tail) ---
+ption` handler before it can reach a `code() == 404` branch, so it can never be misread as "deleted." A local dirty (`PENDING_*`) record is never touched by an `AuthExpired` result — the exception aborts the whole `sync()` before the offending DAO write and before any subsequent push/pull calls run.
+
+**Full re-pull safety:** relied on `SyncEngine.shouldApplyRemote` (SyncEngine.kt, in the `applyIncomingBook`/`Shelf`/`Bookcase` helpers) — no local record → always inserted; a `SYNCED` local record only loses to a remote that's `>=` its own `updated`; a dirty (`PENDING_*`) local record only loses to a remote strictly newer than its own `updated`. So a full re-pull can only ever add back rows or apply genuinely newer server state — it can't clobber an unsynced local edit.
+
+**Deviations:**
+- I did not re-run baseline `testDebugUnitTest` before touching code — I trusted the prompt's stated 337/2-skipped baseline. I did do the required post-change runs.
+- Added a small `internal` test-only method (`SettingsStore.resetFullRepullDoneForTesting`) purely to prevent DataStore's known cross-test-method state leak (the same issue `SettingsStoreTest` already documents) from corrupting the new repull tests. No production caller.
+- Dropped one planned SetupViewModel test ("starts blank on first run") because it wasn't required and was fragile against the same cross-test DataStore leak; kept only the required pre-fill test, which is self-contained.
+
+**Out of scope but noticed:** several pre-existing Kotlin deprecation warnings (`Icons.Filled.ArrowBack`, `Icons.Outlined.Sort`, `centerAlignedTopAppBarColors`, `Modifier.menuAnchor()`, `LocalLifecycleOwner`) and a nullable-`ClassLoader` warning in some metadata client tests — none in files I touched, all pre-existing, left alone.
diff --git a/tasks/M-authexpiry-fix.txt b/tasks/M-authexpiry-fix.txt
new file mode 100644
index 0000000..659fe40
--- /dev/null
+++ b/tasks/M-authexpiry-fix.txt
@@ -0,0 +1,14 @@
+A previous worker implemented the task in ~/bookshelf/tasks/M-authexpiry.txt and left the
+working tree FAILING its acceptance gate. Read, in order: ~/bookshelf/docs/SPEC.md,
+~/bookshelf/tasks/M-authexpiry.txt (the original instructions — every constraint in it binds
+you too), ~/bookshelf/logs/M-authexpiry.summary (that worker's report), then the gate output at
+~/bookshelf/logs/gate-M-authexpiry.log. Lines starting "GATE:" and non-zero exits are the failures.
+
+Fix ONLY what makes the gate red. Do not redesign or re-implement the feature. The
+test count must end ABOVE 337 with zero failures; do not delete or @Ignore tests to
+get there. If a Paparazzi verify failed because snapshots were never recorded, record
+them and LOOK at the PNGs before accepting them.
+
+Run in the FOREGROUND (never background a build): ./tasks/gw assembleDebug,
+./tasks/gw testDebugUnitTest, ./tasks/gw verifyPaparazziDebug. Do not git commit/add/push.
+Report what was wrong, what you changed, and the verbatim output tails.