Distinguish can't-scan, can't-look-up, and genuinely-not-found

Implemented by a Sonnet worker (tasks/G-diagnostics.txt) against the
three-way contract added to SPEC in 1969b74; independently re-verified by
the orchestrator rather than accepted on the worker's own report.

The app previously conflated three outcomes. A barcode that failed the
ISBN-13 checksum produced NOTHING — no sheet, no message — which is
indistinguishable from a dead camera. A lookup that failed at the
transport layer (Google Books answers HTTP 429 to keyless callers) was
reported as "No match found", telling the user a book does not exist when
the app never managed to ask. Only the third case was ever honest.

Now: each client returns SourceResult{Found,NotFound,Failed} from a pure
classify() so the status-code matrix is testable offline without a
MockWebServer; MetadataRepository.combine folds those into
LookupResult{Found,NotFound,Unavailable}, where NotFound requires EVERY
source to have answered authoritatively. A rejected barcode raises a
throttled banner on the camera screen, sharing ScanCodeFilter's existing
debounce so a non-book barcode sitting in frame shows the message once
instead of flickering per analyzed frame. A failed lookup gets its own
sheet with Retry / Enter by hand / Skip that never claims the book is
unknown. The metadata OkHttpClient finally has a call timeout.

Orchestrator's own addition: the manual-ISBN dialog silently discarded an
unparseable entry — the same silent failure on the same screen, missed
because it sat just outside the worker's brief. It now marks the field in
error and disables Look up until the checksum passes.

assembleDebug + verifyPaparazziDebug exit 0; 138 tests, 1 skipped, 0
failures (was 107). Boundary check clean: no build files, no data/local,
data/remote, data/repo, ui/settings, ui/locations or ui/detail.

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

app/app/src/main/java/org/modg/bookshelf/AppContainer.kt

@@ -1,6 +1,7 @@
 package org.modg.bookshelf
 
 import android.content.Context
+import java.util.concurrent.TimeUnit
 import kotlinx.coroutines.flow.first
 import kotlinx.coroutines.runBlocking
 import kotlinx.coroutines.sync.Mutex
@@ -86,11 +87,19 @@ class AppContainer(private val context: Context) {
 
     val authRepository by lazy { AuthRepository(apiProvider, settingsStore) }
 
-    // A bare client — deliberately NOT [okHttpClient] above, which carries our
+    // Deliberately NOT [okHttpClient] above, which carries our PocketBase bearer
-    // PocketBase bearer token via PbAuthInterceptor. Open Library/Google Books
+    // token via PbAuthInterceptor. Open Library/Google Books are third-party
-    // are third-party services; that token must never leave this device's
+    // services; that token must never leave this device's requests to our own
-    // requests to our own server.
+    // server. A call timeout is load-bearing here: with none, a stalled
-    private val metadataHttpClient: OkHttpClient by lazy { OkHttpClient() }
+    // connection hangs on OkHttp's default (unbounded) socket timeouts, and the
+    // user is standing at a bookshelf waiting on it.
+    private val metadataHttpClient: OkHttpClient by lazy {
+        OkHttpClient.Builder()
+            .callTimeout(12, TimeUnit.SECONDS)
+            .connectTimeout(10, TimeUnit.SECONDS)
+            .readTimeout(10, TimeUnit.SECONDS)
+            .build()
+    }
 
     val metadataRepository by lazy { MetadataRepository(metadataHttpClient, json) }
 

app/app/src/main/java/org/modg/bookshelf/data/metadata/GoogleBooksClient.kt

@@ -1,6 +1,7 @@
 package org.modg.bookshelf.data.metadata
 
 import java.io.IOException
+import java.net.SocketTimeoutException
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.withContext
 import kotlinx.serialization.SerializationException
@@ -10,7 +11,8 @@ import okhttp3.Request
 
 /**
  * Google Books lookup — SPEC.md "Book metadata lookup" fallback source. No API key.
- * Never throws: network/parse failures fail soft and return null.
+ * Never throws: every outcome, including transport failure, comes back as a
+ * [SourceResult] rather than a swallowed null.
  */
 class GoogleBooksClient(
     private val httpClient: OkHttpClient,
@@ -18,29 +20,42 @@ class GoogleBooksClient(
 ) {
     private val json = Json(from = json) { ignoreUnknownKeys = true }
 
-    suspend fun lookup(isbn13: String): BookMetadata? = withContext(Dispatchers.IO) {
+    suspend fun lookup(isbn13: String): SourceResult = withContext(Dispatchers.IO) { fetch(isbn13) }
-        val body = fetchBody(isbn13) ?: return@withContext null
-        parseResponse(body)
-    }
 
-    private fun fetchBody(isbn13: String): String? = try {
+    private fun fetch(isbn13: String): SourceResult = try {
         val request = Request.Builder()
             .url("https://www.googleapis.com/books/v1/volumes?q=isbn:$isbn13")
             .build()
         httpClient.newCall(request).execute().use { response ->
-            if (!response.isSuccessful) null else response.body?.string()
+            classify(response.code, response.body.string())
         }
+    } catch (e: SocketTimeoutException) {
+        SourceResult.Failed("timeout")
     } catch (e: IOException) {
-        null
+        SourceResult.Failed("network error")
+    }
+
+    /**
+     * Package-visible pure function — no socket involved — so it's exhaustively
+     * unit-testable offline (2xx-with-record, 2xx-without-record, 404, 429, 500,
+     * malformed body). [parseResponse] is defined in terms of this so the two
+     * can never disagree about what a body means.
+     */
+    internal fun classify(httpCode: Int, body: String?): SourceResult {
+        if (httpCode !in 200..299) return SourceResult.Failed("http $httpCode")
+        if (body.isNullOrBlank()) return SourceResult.Failed("empty body")
+        return try {
+            val dto = json.decodeFromString(GoogleBooksResponseDto.serializer(), body)
+            val metadata = dto.items.firstOrNull()?.volumeInfo?.toBookMetadata()
+            if (metadata != null) SourceResult.Found(metadata) else SourceResult.NotFound
+        } catch (e: SerializationException) {
+            SourceResult.Failed("malformed json")
+        } catch (e: IllegalArgumentException) {
+            SourceResult.Failed("malformed json")
+        }
     }
 
     /** Package-visible for offline fixture tests — parses a raw response body with no network involved. */
-    internal fun parseResponse(body: String): BookMetadata? = try {
+    internal fun parseResponse(body: String): BookMetadata? =
-        val dto = json.decodeFromString(GoogleBooksResponseDto.serializer(), body)
+        (classify(200, body) as? SourceResult.Found)?.metadata
-        dto.items.firstOrNull()?.volumeInfo?.toBookMetadata()
-    } catch (e: SerializationException) {
-        null
-    } catch (e: IllegalArgumentException) {
-        null
-    }
 }

app/app/src/main/java/org/modg/bookshelf/data/metadata/LookupResult.kt

@@ -0,0 +1,18 @@
+package org.modg.bookshelf.data.metadata
+
+/**
+ * The combined outcome of a metadata lookup across both sources (SPEC.md "Book
+ * metadata lookup"). Produced by [MetadataRepository.lookup] from the two
+ * [SourceResult]s per [MetadataRepository.combine]'s rules:
+ *   - any source [SourceResult.Found]                     -> [Found]
+ *   - every source [SourceResult.NotFound]                 -> [NotFound]
+ *   - otherwise (at least one [SourceResult.Failed], none Found) -> [Unavailable]
+ *
+ * The last rule is the whole point: one reachable source answering "no" is not
+ * authoritative while the other source couldn't be asked at all.
+ */
+sealed interface LookupResult {
+    data class Found(val metadata: BookMetadata) : LookupResult
+    data object NotFound : LookupResult
+    data class Unavailable(val reason: String) : LookupResult
+}

app/app/src/main/java/org/modg/bookshelf/data/metadata/MetadataRepository.kt

@@ -7,9 +7,10 @@ import okhttp3.OkHttpClient
 
 /**
  * Single entry point for book metadata lookup (SPEC.md "Book metadata lookup").
- * Queries both sources concurrently and merges per [MetadataMerger]. Returns null
+ * Queries both sources concurrently and combines their [SourceResult]s into one
- * if [isbn] doesn't checksum-validate or if both sources miss — callers (the scan
+ * [LookupResult] per [combine]. [isbn] must already be a checksum-valid ISBN-10/13
- * screen) must then fall back to manual entry pre-filled with the scanned ISBN.
+ * by the time it reaches here — the scan layer is responsible for that, and an
+ * invalid one is a programming error at this layer, not a lookup outcome.
  */
 class MetadataRepository(
     private val openLibraryClient: OpenLibraryClient,
@@ -20,21 +21,46 @@ class MetadataRepository(
         GoogleBooksClient(httpClient, json),
     )
 
-    suspend fun lookup(isbn: String): BookMetadata? {
+    suspend fun lookup(isbn: String): LookupResult {
-        val isbn13 = IsbnUtils.toIsbn13(isbn) ?: return null
+        val isbn13 = checkNotNull(IsbnUtils.toIsbn13(isbn)) {
-        val merged = coroutineScope {
+            "MetadataRepository.lookup requires an already-validated ISBN-10/13; got: $isbn"
+        }
+        return coroutineScope {
             val openLibrary = async { openLibraryClient.lookup(isbn13) }
             val googleBooks = async { googleBooksClient.lookup(isbn13) }
-            MetadataMerger.merge(openLibrary.await(), googleBooks.await())
+            combine(openLibrary.await(), googleBooks.await(), isbn13)
-        } ?: return null
-        return if (merged.coverUrl.isNullOrBlank()) {
-            merged.copy(coverUrl = byIsbnCoverUrl(isbn13))
-        } else {
-            merged
         }
     }
 
     companion object {
+        /**
+         * Combines the two sources' outcomes per SPEC.md's three-way rule: any Found
+         * wins (merged via [MetadataMerger] and given the last-resort cover), all
+         * NotFound is an honest miss, and anything else — at least one Failed and
+         * nothing Found — is Unavailable, because a source that couldn't be reached
+         * must never be reported as a book that doesn't exist. Package-visible and
+         * pure (no I/O) so the full 3x3 matrix is unit-testable offline.
+         */
+        internal fun combine(openLibrary: SourceResult, googleBooks: SourceResult, isbn13: String): LookupResult {
+            val merged = MetadataMerger.merge(
+                (openLibrary as? SourceResult.Found)?.metadata,
+                (googleBooks as? SourceResult.Found)?.metadata,
+            )
+            if (merged != null) {
+                val withCover = if (merged.coverUrl.isNullOrBlank()) {
+                    merged.copy(coverUrl = byIsbnCoverUrl(isbn13))
+                } else {
+                    merged
+                }
+                return LookupResult.Found(withCover)
+            }
+            val failures = listOfNotNull(
+                (openLibrary as? SourceResult.Failed)?.reason?.let { "open library: $it" },
+                (googleBooks as? SourceResult.Failed)?.reason?.let { "google books: $it" },
+            )
+            return if (failures.isNotEmpty()) LookupResult.Unavailable(failures.joinToString("; ")) else LookupResult.NotFound
+        }
+
         /**
          * Last resort when neither source reported cover art, per SPEC's cover chain.
          * `default=false` is load-bearing: without it this endpoint answers 200 with a

app/app/src/main/java/org/modg/bookshelf/data/metadata/OpenLibraryClient.kt

@@ -1,6 +1,7 @@
 package org.modg.bookshelf.data.metadata
 
 import java.io.IOException
+import java.net.SocketTimeoutException
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.withContext
 import kotlinx.serialization.SerializationException
@@ -12,7 +13,8 @@ import okhttp3.Request
 
 /**
  * Open Library lookup — SPEC.md "Book metadata lookup" primary source.
- * Never throws: network/parse failures fail soft and return null.
+ * Never throws: every outcome, including transport failure, comes back as a
+ * [SourceResult] rather than a swallowed null.
  */
 class OpenLibraryClient(
     private val httpClient: OkHttpClient,
@@ -21,30 +23,42 @@ class OpenLibraryClient(
     // Real responses carry fields this DTO doesn't model; never let an unknown key throw.
     private val json = Json(from = json) { ignoreUnknownKeys = true }
 
-    suspend fun lookup(isbn13: String): BookMetadata? = withContext(Dispatchers.IO) {
+    suspend fun lookup(isbn13: String): SourceResult = withContext(Dispatchers.IO) { fetch(isbn13) }
-        val body = fetchBody(isbn13) ?: return@withContext null
-        parseResponse(body, isbn13)
-    }
 
-    private fun fetchBody(isbn13: String): String? = try {
+    private fun fetch(isbn13: String): SourceResult = try {
         val request = Request.Builder()
             .url("https://openlibrary.org/api/books?bibkeys=ISBN:$isbn13&format=json&jscmd=data")
             .build()
         httpClient.newCall(request).execute().use { response ->
-            if (!response.isSuccessful) null else response.body?.string()
+            classify(response.code, response.body.string(), isbn13)
         }
+    } catch (e: SocketTimeoutException) {
+        SourceResult.Failed("timeout")
     } catch (e: IOException) {
-        null
+        SourceResult.Failed("network error")
+    }
+
+    /**
+     * Package-visible pure function — no socket involved — so it's exhaustively
+     * unit-testable offline (2xx-with-record, 2xx-without-record, 404, 429, 500,
+     * malformed body). [parseResponse] is defined in terms of this so the two
+     * can never disagree about what a body means.
+     */
+    internal fun classify(httpCode: Int, body: String?, isbn13: String): SourceResult {
+        if (httpCode !in 200..299) return SourceResult.Failed("http $httpCode")
+        if (body.isNullOrBlank()) return SourceResult.Failed("empty body")
+        return try {
+            val root = json.parseToJsonElement(body).jsonObject
+            val entry = root["ISBN:$isbn13"]?.jsonObject ?: return SourceResult.NotFound
+            SourceResult.Found(json.decodeFromJsonElement<OpenLibraryBookDto>(entry).toBookMetadata(isbn13))
+        } catch (e: SerializationException) {
+            SourceResult.Failed("malformed json")
+        } catch (e: IllegalArgumentException) {
+            SourceResult.Failed("malformed json")
+        }
     }
 
     /** Package-visible for offline fixture tests — parses a raw response body with no network involved. */
-    internal fun parseResponse(body: String, isbn13: String): BookMetadata? = try {
+    internal fun parseResponse(body: String, isbn13: String): BookMetadata? =
-        val root = json.parseToJsonElement(body).jsonObject
+        (classify(200, body, isbn13) as? SourceResult.Found)?.metadata
-        val entry = root["ISBN:$isbn13"]?.jsonObject ?: return null
-        json.decodeFromJsonElement<OpenLibraryBookDto>(entry).toBookMetadata(isbn13)
-    } catch (e: SerializationException) {
-        null
-    } catch (e: IllegalArgumentException) {
-        null
-    }
 }

app/app/src/main/java/org/modg/bookshelf/data/metadata/SourceResult.kt

@@ -0,0 +1,23 @@
+package org.modg.bookshelf.data.metadata
+
+/**
+ * Per-source lookup outcome (SPEC.md "Book metadata lookup": "Lookup outcome is
+ * THREE-WAY, never a bare null"). [OpenLibraryClient] and [GoogleBooksClient] each
+ * produce one of these instead of collapsing "couldn't be reached" and "answered,
+ * doesn't have it" into the same `null`. [MetadataRepository] combines the two
+ * source results into a [LookupResult].
+ */
+sealed interface SourceResult {
+    data class Found(val metadata: BookMetadata) : SourceResult
+
+    /** The source answered (2xx) and, in good faith, has no record for this ISBN. */
+    data object NotFound : SourceResult
+
+    /**
+     * The source could not be asked, or its answer couldn't be trusted: non-2xx,
+     * timeout, transport error, or a body that didn't parse. [reason] is a short
+     * diagnostic ("http 429", "timeout", "malformed json") for logs — never shown
+     * to the user verbatim.
+     */
+    data class Failed(val reason: String) : SourceResult
+}

app/app/src/main/java/org/modg/bookshelf/ui/scan/ScanCodeFilter.kt

@@ -2,11 +2,31 @@ package org.modg.bookshelf.ui.scan
 
 import org.modg.bookshelf.data.metadata.IsbnUtils
 
+/** What a single decoded barcode read means, once debouncing has been applied. */
+sealed interface ScanOutcome {
+    /** A checksum-valid ISBN-13, ready for metadata lookup. */
+    data class Isbn(val isbn13: String) : ScanOutcome
+
     /**
- * Pure decode-to-ISBN pipeline shared by the analyzer and its tests: normalizes a raw
+     * Decoded fine, but its checksum rules it out as a book ISBN (SPEC.md
- * barcode value, validates the ISBN-13 checksum (this is what makes EAN_8/UPC_A reads
+     * "Barcode scanning": "ignore non-book barcodes" — but not silently, see
- * fall out as "non-book barcodes" per SPEC.md "Barcode scanning" — they can never be
+     * [rawValue]). Carries the raw value so the camera screen can echo back what
- * 13 digits), and debounces repeat reads of the same code.
+     * it read.
+     */
+    data class NotAnIsbn(val rawValue: String) : ScanOutcome
+
+    /** A debounced repeat of the last code (valid or not) — emit no UI at all. */
+    data object Ignored : ScanOutcome
+}
+
+/**
+ * Pure decode-to-outcome pipeline shared by the analyzer and its tests: normalizes
+ * a raw barcode value, classifies it as a valid ISBN-13 or not (this is what makes
+ * EAN_8/UPC_A reads fall out as "non-book barcodes" per SPEC.md "Barcode scanning"
+ * — they can never be 13 digits), and debounces repeats of the same code, whether
+ * or not it validated. The debounce is what keeps a non-book barcode sitting in
+ * frame — which decodes on nearly every analyzed frame — from flickering a
+ * rejection message instead of showing it once.
  */
 class ScanCodeFilter(
     private val debounceMillis: Long = 2000L,
@@ -15,15 +35,19 @@ class ScanCodeFilter(
     private var lastCode: String? = null
     private var lastEmitMillis: Long = Long.MIN_VALUE
 
-    /** Returns the normalized ISBN-13 if [rawValue] is a valid, non-debounced hit; null otherwise. */
+    /** Classifies [rawValue] per [ScanOutcome], applying the debounce window. */
-    fun accept(rawValue: String?): String? {
+    fun accept(rawValue: String?): ScanOutcome {
-        val normalized = IsbnUtils.normalize(rawValue ?: return null)
+        val raw = rawValue ?: return ScanOutcome.Ignored
-        if (!IsbnUtils.isValidIsbn13(normalized)) return null
+        val normalized = IsbnUtils.normalize(raw)
         val now = nowMillis()
-        if (normalized == lastCode && now - lastEmitMillis < debounceMillis) return null
+        if (normalized == lastCode && now - lastEmitMillis < debounceMillis) return ScanOutcome.Ignored
         lastCode = normalized
         lastEmitMillis = now
-        return normalized
+        return if (IsbnUtils.isValidIsbn13(normalized)) {
+            ScanOutcome.Isbn(normalized)
+        } else {
+            ScanOutcome.NotAnIsbn(raw)
+        }
     }
 
     /** Allows the next scan of any code (including a repeat) to emit immediately, e.g. after Skip. */

app/app/src/main/java/org/modg/bookshelf/ui/scan/ScanModels.kt

@@ -2,6 +2,7 @@ package org.modg.bookshelf.ui.scan
 
 import org.modg.bookshelf.data.local.BookEntity
 import org.modg.bookshelf.data.metadata.BookMetadata
+import org.modg.bookshelf.data.metadata.LookupResult
 
 /** SPEC.md scan screen: "Duplicate-ISBN warning if already owned." */
 sealed interface DuplicateStatus {
@@ -27,14 +28,30 @@ sealed interface ScanSheetState {
      */
     data class Loading(val isbn13: String) : ScanSheetState
     data class Found(val isbn13: String, val metadata: BookMetadata, val duplicate: DuplicateStatus) : ScanSheetState
-    data class NotFound(val isbn13: String) : ScanSheetState
+
+    /**
+     * Every source answered and none had the book (SPEC: an honest "no", not a
+     * cover story for a failed request). [viaLookupFailure] is true only when the
+     * user reached this form through [LookupFailed]'s "Enter by hand" escape hatch
+     * rather than a genuine miss, so the sheet's copy can stop short of claiming
+     * the book is unknown.
+     */
+    data class NotFound(val isbn13: String, val viaLookupFailure: Boolean = false) : ScanSheetState
+
+    /**
+     * At least one source couldn't be reached (non-2xx, timeout, transport error)
+     * and neither had the book — SPEC: "must NOT claim the book is unknown". Offers
+     * retry, manual entry, and skip instead of a manual-entry form captioned as a miss.
+     */
+    data class LookupFailed(val isbn13: String, val reason: String) : ScanSheetState
 }
 
 /** Combines a metadata lookup result with duplicate status into the sheet state to show. */
 object ScanMetadataOutcome {
-    fun from(isbn13: String, metadata: BookMetadata?, duplicate: DuplicateStatus): ScanSheetState = when (metadata) {
+    fun from(isbn13: String, result: LookupResult, duplicate: DuplicateStatus): ScanSheetState = when (result) {
-        null -> ScanSheetState.NotFound(isbn13)
+        is LookupResult.Found -> ScanSheetState.Found(isbn13, result.metadata, duplicate)
-        else -> ScanSheetState.Found(isbn13, metadata, duplicate)
+        is LookupResult.NotFound -> ScanSheetState.NotFound(isbn13)
+        is LookupResult.Unavailable -> ScanSheetState.LookupFailed(isbn13, result.reason)
     }
 }
 

app/app/src/main/java/org/modg/bookshelf/ui/scan/ScanScreen.kt

@@ -18,6 +18,7 @@ import androidx.compose.foundation.layout.fillMaxWidth
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.size
 import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.foundation.text.KeyboardOptions
 import androidx.compose.material.icons.Icons
 import androidx.compose.material.icons.automirrored.outlined.ArrowBack
 import androidx.compose.material.icons.outlined.FlashOff
@@ -44,6 +45,7 @@ import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.platform.LocalContext
+import androidx.compose.ui.text.input.KeyboardType
 import androidx.compose.ui.platform.LocalLifecycleOwner
 import androidx.compose.ui.unit.dp
 import androidx.compose.ui.viewinterop.AndroidView
@@ -58,6 +60,7 @@ import org.modg.bookshelf.AppContainer
 import org.modg.bookshelf.data.local.BookcaseEntity
 import org.modg.bookshelf.data.local.ShelfEntity
 import org.modg.bookshelf.data.metadata.BookMetadata
+import org.modg.bookshelf.data.metadata.IsbnUtils
 import org.modg.bookshelf.ui.components.BookCover
 import org.modg.bookshelf.ui.components.BookshelfScaffold
 import org.modg.bookshelf.ui.components.EmptyState
@@ -89,6 +92,7 @@ fun ScanScreen(
 
     val sheetState by viewModel.sheetState.collectAsState()
     val sessionState by viewModel.sessionState.collectAsState()
+    val rejectedMessage by viewModel.rejectedMessage.collectAsState()
     val torchEnabled by viewModel.scannerController.torchEnabled.collectAsState()
     val bookcases by viewModel.bookcases.collectAsState()
     val shelves by viewModel.shelves.collectAsState()
@@ -128,6 +132,12 @@ fun ScanScreen(
                 CameraPreview(controller = viewModel.scannerController, torchEnabled = torchEnabled)
                 ScanReticle(modifier = Modifier.align(Alignment.Center))
                 SessionBadge(count = sessionState.savedCount, modifier = Modifier.align(Alignment.TopCenter).padding(16.dp))
+                rejectedMessage?.let { message ->
+                    RejectedBarcodeBanner(
+                        message = message,
+                        modifier = Modifier.align(Alignment.BottomCenter).padding(24.dp),
+                    )
+                }
             } else {
                 PermissionDeniedContent(onRequestAgain = { permissionState.launchPermissionRequest() })
             }
@@ -160,6 +170,7 @@ fun ScanScreen(
         ) {
             ManualEntrySheet(
                 isbn13 = state.isbn13,
+                authoritative = !state.viaLookupFailure,
                 bookcases = bookcases,
                 shelves = shelves,
                 selectedShelfId = selectedShelfId,
@@ -168,6 +179,17 @@ fun ScanScreen(
                 onSkip = { viewModel.skip() },
             )
         }
+        is ScanSheetState.LookupFailed -> ModalBottomSheet(
+            onDismissRequest = { viewModel.dismissSheet() },
+            sheetState = rememberModalBottomSheetState(),
+        ) {
+            LookupFailedSheet(
+                isbn13 = state.isbn13,
+                onRetry = { viewModel.retryLookup(state.isbn13) },
+                onEnterByHand = { viewModel.enterByHand(state.isbn13) },
+                onSkip = { viewModel.skip() },
+            )
+        }
     }
 
     if (showManualEntry) {
@@ -340,9 +362,16 @@ internal fun FoundBookSheet(
     }
 }
 
+/**
+ * Shown when every source answered and none had the book — SPEC: word this as the
+ * authoritative negative it is, not as a generic failure. [authoritative] is false
+ * only when this form was reached via [LookupFailedSheet]'s "Enter by hand", where
+ * the lookup never completed and the copy must not imply the book is unknown.
+ */
 @Composable
 internal fun ManualEntrySheet(
     isbn13: String,
+    authoritative: Boolean,
     bookcases: List<BookcaseEntity>,
     shelves: List<ShelfEntity>,
     selectedShelfId: String?,
@@ -354,9 +383,12 @@ internal fun ManualEntrySheet(
     var authors by remember { mutableStateOf("") }
 
     Column(modifier = Modifier.fillMaxWidth().padding(16.dp)) {
-        Text(text = "No match found", style = MaterialTheme.typography.titleLarge)
         Text(
-            text = "ISBN $isbn13 — enter the details by hand.",
+            text = if (authoritative) "Not in Open Library or Google Books" else "Enter the details by hand",
+            style = MaterialTheme.typography.titleLarge,
+        )
+        Text(
+            text = if (authoritative) "ISBN $isbn13 — enter the details by hand." else "ISBN $isbn13",
             style = MaterialTheme.typography.bodyMedium,
             color = MaterialTheme.colorScheme.onSurfaceVariant,
             modifier = Modifier.padding(bottom = 12.dp),
@@ -393,6 +425,54 @@ internal fun ManualEntrySheet(
     }
 }
 
+/**
+ * Shown when a lookup couldn't be completed — SPEC: "a retry affordance, with
+ * manual entry as the escape hatch; it must NOT claim the book is unknown." No
+ * shelf picker here: saving isn't offered from this sheet, only a path onward to
+ * one that does (Retry, or the manual-entry form via "Enter by hand").
+ */
+@Composable
+internal fun LookupFailedSheet(
+    isbn13: String,
+    onRetry: () -> Unit,
+    onEnterByHand: () -> Unit,
+    onSkip: () -> Unit,
+) {
+    Column(modifier = Modifier.fillMaxWidth().padding(16.dp)) {
+        Text(text = "Couldn't complete the lookup", style = MaterialTheme.typography.titleLarge)
+        Text(
+            text = "ISBN $isbn13 — one or more sources couldn't be reached. " +
+                "This doesn't mean the book is unknown.",
+            style = MaterialTheme.typography.bodyMedium,
+            color = MaterialTheme.colorScheme.onSurfaceVariant,
+            modifier = Modifier.padding(top = 8.dp, bottom = 16.dp),
+        )
+        PrimaryButton(text = "Retry", onClick = onRetry, modifier = Modifier.fillMaxWidth())
+        Row(modifier = Modifier.fillMaxWidth().padding(top = 12.dp), horizontalArrangement = Arrangement.spacedBy(12.dp)) {
+            SecondaryButton(text = "Enter by hand", onClick = onEnterByHand, modifier = Modifier.weight(1f))
+            SecondaryButton(text = "Skip", onClick = onSkip, modifier = Modifier.weight(1f))
+        }
+    }
+}
+
+/**
+ * Transient banner for a decoded-but-rejected barcode (SPEC.md "Barcode scanning":
+ * "A rejected barcode is NOT silent"). [ScanViewModel] owns the debounce (via
+ * [ScanCodeFilter]) and the auto-clear timer behind [message] — this composable
+ * just renders whatever it's handed.
+ */
+@Composable
+internal fun RejectedBarcodeBanner(message: String, modifier: Modifier = Modifier) {
+    Text(
+        text = message,
+        style = MaterialTheme.typography.labelLarge,
+        color = MaterialTheme.colorScheme.onErrorContainer,
+        modifier = modifier
+            .background(MaterialTheme.colorScheme.errorContainer, RoundedCornerShape(20.dp))
+            .padding(horizontal = 16.dp, vertical = 8.dp),
+    )
+}
+
 @Composable
 internal fun ShelfPicker(
     bookcases: List<BookcaseEntity>,
@@ -431,19 +511,37 @@ internal fun ShelfPicker(
 @Composable
 private fun ManualIsbnDialog(onDismiss: () -> Unit, onSubmit: (String) -> Unit) {
     var text by remember { mutableStateOf("") }
+    // Validate here rather than letting ScanViewModel.manualIsbnEntered drop an
+    // unparseable ISBN on the floor. A typed-in check digit is easy to get wrong,
+    // and a dialog whose button does nothing is the same silent failure this whole
+    // screen was just rebuilt to eliminate.
+    val isbn13 = remember(text) { IsbnUtils.toIsbn13(text) }
+    val malformed = text.isNotBlank() && isbn13 == null
     AlertDialog(
         onDismissRequest = onDismiss,
         title = { Text("Enter ISBN") },
         text = {
+            Column {
                 OutlinedTextField(
                     value = text,
                     onValueChange = { text = it },
                     label = { Text("ISBN-10 or ISBN-13") },
                     singleLine = true,
+                    isError = malformed,
+                    keyboardOptions = KeyboardOptions(keyboardType = KeyboardType.Number),
                 )
+                if (malformed) {
+                    Text(
+                        text = "That isn't a valid ISBN — check the digits.",
+                        style = MaterialTheme.typography.bodySmall,
+                        color = MaterialTheme.colorScheme.error,
+                        modifier = Modifier.padding(top = 4.dp),
+                    )
+                }
+            }
         },
         confirmButton = {
-            PrimaryButton(text = "Look up", onClick = { onSubmit(text) }, enabled = text.isNotBlank())
+            PrimaryButton(text = "Look up", onClick = { onSubmit(text) }, enabled = isbn13 != null)
         },
         dismissButton = {
             SecondaryButton(text = "Cancel", onClick = onDismiss)

app/app/src/main/java/org/modg/bookshelf/ui/scan/ScanViewModel.kt

@@ -2,6 +2,8 @@ package org.modg.bookshelf.ui.scan
 
 import androidx.lifecycle.ViewModel
 import androidx.lifecycle.viewModelScope
+import kotlinx.coroutines.Job
+import kotlinx.coroutines.delay
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.SharingStarted
 import kotlinx.coroutines.flow.StateFlow
@@ -35,6 +37,11 @@ class ScanViewModel(
     private val _sessionState = MutableStateFlow(ScanSessionState())
     val sessionState: StateFlow<ScanSessionState> = _sessionState.asStateFlow()
 
+    private val _rejectedMessage = MutableStateFlow<String?>(null)
+    /** Transient "read but not a book barcode" message for the camera overlay; auto-clears. */
+    val rejectedMessage: StateFlow<String?> = _rejectedMessage.asStateFlow()
+    private var rejectedMessageClearJob: Job? = null
+
     val bookcases: StateFlow<List<BookcaseEntity>> = locationRepository.observeBookcases()
         .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), emptyList())
     val shelves: StateFlow<List<ShelfEntity>> = locationRepository.observeShelves()
@@ -48,14 +55,41 @@ class ScanViewModel(
         viewModelScope.launch {
             scannerController.scanResults.collect { isbn13 -> onScanned(isbn13) }
         }
+        viewModelScope.launch {
+            scannerController.rejectedCodes.collect { rawValue -> showRejectedMessage(rawValue) }
+        }
     }
 
     private suspend fun onScanned(isbn13: String) {
         if (_sheetState.value !is ScanSheetState.Hidden) return // a sheet is already up for a previous hit
+        runLookup(isbn13)
+    }
+
+    private suspend fun runLookup(isbn13: String) {
         _sheetState.value = ScanSheetState.Loading(isbn13)
         val duplicate = DuplicateCheck.check(bookRepository.findByIsbn13(isbn13))
-        val metadata = metadataRepository.lookup(isbn13)
+        val result = metadataRepository.lookup(isbn13)
-        _sheetState.value = ScanMetadataOutcome.from(isbn13, metadata, duplicate)
+        _sheetState.value = ScanMetadataOutcome.from(isbn13, result, duplicate)
+    }
+
+    /** [ScanSheetState.LookupFailed]'s Retry — re-enters loading and re-runs the same lookup. */
+    fun retryLookup(isbn13: String) {
+        viewModelScope.launch { runLookup(isbn13) }
+    }
+
+    /** [ScanSheetState.LookupFailed]'s "Enter by hand" — falls through to the manual-entry form. */
+    fun enterByHand(isbn13: String) {
+        _sheetState.value = ScanSheetState.NotFound(isbn13, viaLookupFailure = true)
+    }
+
+    /** Throttled per [ScanCodeFilter]'s debounce; this just owns the auto-clear timer on top. */
+    private fun showRejectedMessage(rawValue: String) {
+        rejectedMessageClearJob?.cancel()
+        _rejectedMessage.value = "Read $rawValue — not a book barcode"
+        rejectedMessageClearJob = viewModelScope.launch {
+            delay(REJECTED_MESSAGE_MILLIS)
+            _rejectedMessage.value = null
+        }
     }
 
     /** The manual-ISBN-entry escape hatch (SPEC: for when a barcode won't scan). */
@@ -114,4 +148,9 @@ class ScanViewModel(
         _sheetState.value = ScanSheetState.Hidden
         scannerController.resetDebounce()
     }
+
+    private companion object {
+        /** How long a rejected-barcode message stays on screen before it auto-clears. */
+        const val REJECTED_MESSAGE_MILLIS = 3000L
+    }
 }

app/app/src/main/java/org/modg/bookshelf/ui/scan/ScannerController.kt

@@ -19,6 +19,14 @@ class ScannerController(
     /** Emits a debounced, checksum-valid ISBN-13 each time [IsbnBarcodeAnalyzer] sees a new book barcode. */
     val scanResults: SharedFlow<String> = _scanResults.asSharedFlow()
 
+    private val _rejectedCodes = MutableSharedFlow<String>(extraBufferCapacity = 1)
+    /**
+     * Emits the raw value of a debounced, non-book barcode read (SPEC.md "Barcode
+     * scanning": "A rejected barcode is NOT silent"). [ScanOutcome.Ignored] reads
+     * (debounced repeats) never reach here.
+     */
+    val rejectedCodes: SharedFlow<String> = _rejectedCodes.asSharedFlow()
+
     private val _torchEnabled = MutableStateFlow(false)
     val torchEnabled: StateFlow<Boolean> = _torchEnabled.asStateFlow()
 
@@ -27,7 +35,11 @@ class ScannerController(
 
     /** Called by [IsbnBarcodeAnalyzer] for every decoded barcode's raw value. */
     fun onBarcodeScanned(rawValue: String?) {
-        codeFilter.accept(rawValue)?.let { _scanResults.tryEmit(it) }
+        when (val outcome = codeFilter.accept(rawValue)) {
+            is ScanOutcome.Isbn -> _scanResults.tryEmit(outcome.isbn13)
+            is ScanOutcome.NotAnIsbn -> _rejectedCodes.tryEmit(outcome.rawValue)
+            ScanOutcome.Ignored -> Unit
+        }
     }
 
     fun toggleTorch() {

app/app/src/test/java/org/modg/bookshelf/data/metadata/GoogleBooksClientTest.kt

@@ -46,6 +46,47 @@ class GoogleBooksClientTest {
         assertNull(client.parseResponse(body))
     }
 
+    // --- classify(): the three-way per-source outcome (SPEC.md "Book metadata lookup"). ---
+
+    @Test
+    fun `classify reports Found for a 2xx response with a record`() {
+        val result = client.classify(200, fixture("googlebooks_success.json"))
+
+        val found = result as? SourceResult.Found
+        checkNotNull(found) { "expected Found, got $result" }
+        assertEquals("Effective Java", found.metadata.title)
+    }
+
+    @Test
+    fun `classify reports NotFound for a 2xx response with no items`() {
+        val result = client.classify(200, fixture("googlebooks_no_items.json"))
+        assertEquals(SourceResult.NotFound, result)
+    }
+
+    @Test
+    fun `classify reports Failed for a 404`() {
+        val result = client.classify(404, null)
+        assertEquals(SourceResult.Failed("http 404"), result)
+    }
+
+    @Test
+    fun `classify reports Failed for a 429 keyless-quota response, distinctly from NotFound`() {
+        val result = client.classify(429, null)
+        assertEquals(SourceResult.Failed("http 429"), result)
+    }
+
+    @Test
+    fun `classify reports Failed for a 500`() {
+        val result = client.classify(500, "Internal Server Error")
+        assertEquals(SourceResult.Failed("http 500"), result)
+    }
+
+    @Test
+    fun `classify reports Failed for a malformed body even on a 2xx status`() {
+        val result = client.classify(200, fixture("malformed.json"))
+        assertEquals(SourceResult.Failed("malformed json"), result)
+    }
+
     private fun fixture(name: String): String =
         checkNotNull(javaClass.classLoader.getResourceAsStream("fixtures/$name")) { "missing fixture $name" }
             .bufferedReader()

app/app/src/test/java/org/modg/bookshelf/data/metadata/MetadataRepositoryTest.kt

@@ -6,6 +6,80 @@ import org.junit.Test
 
 class MetadataRepositoryTest {
 
+    private val isbn13 = "9780201558029"
+    private val openLibraryHit = SourceResult.Found(BookMetadata(title = "Open Library Title", isbn13 = isbn13))
+    private val googleBooksHit = SourceResult.Found(BookMetadata(title = "Google Books Title", isbn13 = isbn13))
+
+    // --- The full 3x3 (Open Library outcome x Google Books outcome) combination matrix
+    // from SPEC.md "Book metadata lookup": any Found wins, all-NotFound is an honest
+    // miss, anything else with a Failed in it is Unavailable — never a false negative. ---
+
+    @Test
+    fun `Found + Found merges and returns Found`() {
+        val result = MetadataRepository.combine(openLibraryHit, googleBooksHit, isbn13)
+        assertTrue(result is LookupResult.Found)
+    }
+
+    @Test
+    fun `Found + NotFound returns Found from the one source that had it`() {
+        val result = MetadataRepository.combine(openLibraryHit, SourceResult.NotFound, isbn13)
+        assertEquals("Open Library Title", (result as LookupResult.Found).metadata.title)
+    }
+
+    @Test
+    fun `Found + Failed returns Found -- a reachable hit is not overruled by the other failing`() {
+        val result = MetadataRepository.combine(openLibraryHit, SourceResult.Failed("http 429"), isbn13)
+        assertEquals("Open Library Title", (result as LookupResult.Found).metadata.title)
+    }
+
+    @Test
+    fun `NotFound + Found returns Found from the one source that had it`() {
+        val result = MetadataRepository.combine(SourceResult.NotFound, googleBooksHit, isbn13)
+        assertEquals("Google Books Title", (result as LookupResult.Found).metadata.title)
+    }
+
+    @Test
+    fun `NotFound + NotFound returns NotFound -- both sources answered and neither had it`() {
+        val result = MetadataRepository.combine(SourceResult.NotFound, SourceResult.NotFound, isbn13)
+        assertEquals(LookupResult.NotFound, result)
+    }
+
+    @Test
+    fun `NotFound + Failed returns Unavailable -- one honest miss is not authoritative alone`() {
+        val result = MetadataRepository.combine(SourceResult.NotFound, SourceResult.Failed("timeout"), isbn13)
+        assertTrue(result is LookupResult.Unavailable)
+    }
+
+    @Test
+    fun `Failed + Found returns Found from the one source that had it`() {
+        val result = MetadataRepository.combine(SourceResult.Failed("http 500"), googleBooksHit, isbn13)
+        assertEquals("Google Books Title", (result as LookupResult.Found).metadata.title)
+    }
+
+    @Test
+    fun `Failed + NotFound returns Unavailable -- one honest miss is not authoritative alone`() {
+        val result = MetadataRepository.combine(SourceResult.Failed("timeout"), SourceResult.NotFound, isbn13)
+        assertTrue(result is LookupResult.Unavailable)
+    }
+
+    @Test
+    fun `Failed + Failed returns Unavailable and preserves both reasons`() {
+        val result = MetadataRepository.combine(SourceResult.Failed("http 429"), SourceResult.Failed("timeout"), isbn13)
+
+        val unavailable = result as? LookupResult.Unavailable
+        checkNotNull(unavailable) { "expected Unavailable, got $result" }
+        assertTrue(unavailable.reason.contains("http 429"))
+        assertTrue(unavailable.reason.contains("timeout"))
+    }
+
+    @Test
+    fun `a Found result without cover art falls back to the by-isbn cover url`() {
+        val bare = SourceResult.Found(BookMetadata(title = "No Cover", isbn13 = isbn13))
+        val result = MetadataRepository.combine(bare, SourceResult.NotFound, isbn13)
+
+        assertEquals(MetadataRepository.byIsbnCoverUrl(isbn13), (result as LookupResult.Found).metadata.coverUrl)
+    }
+
     /**
      * `default=false` is the whole point of this URL. Without it covers.openlibrary.org
      * answers 200 with a 43-byte 1x1 transparent GIF for an edition it holds no art for

app/app/src/test/java/org/modg/bookshelf/data/metadata/OpenLibraryClientTest.kt

@@ -105,6 +105,47 @@ class OpenLibraryClientTest {
         assertNull(client.parseResponse(body, "9780201558029"))
     }
 
+    // --- classify(): the three-way per-source outcome (SPEC.md "Book metadata lookup"). ---
+
+    @Test
+    fun `classify reports Found for a 2xx response with a record`() {
+        val result = client.classify(200, fixture("openlibrary_success.json"), "9780201558029")
+
+        val found = result as? SourceResult.Found
+        checkNotNull(found) { "expected Found, got $result" }
+        assertEquals("Concrete Mathematics", found.metadata.title)
+    }
+
+    @Test
+    fun `classify reports NotFound for a 2xx response with no record for that isbn`() {
+        val result = client.classify(200, fixture("openlibrary_not_found.json"), "9780201558029")
+        assertEquals(SourceResult.NotFound, result)
+    }
+
+    @Test
+    fun `classify reports Failed for a 404`() {
+        val result = client.classify(404, null, "9780201558029")
+        assertEquals(SourceResult.Failed("http 404"), result)
+    }
+
+    @Test
+    fun `classify reports Failed for a 429, distinctly from NotFound`() {
+        val result = client.classify(429, null, "9780201558029")
+        assertEquals(SourceResult.Failed("http 429"), result)
+    }
+
+    @Test
+    fun `classify reports Failed for a 500`() {
+        val result = client.classify(500, "Internal Server Error", "9780201558029")
+        assertEquals(SourceResult.Failed("http 500"), result)
+    }
+
+    @Test
+    fun `classify reports Failed for a malformed body even on a 2xx status`() {
+        val result = client.classify(200, fixture("malformed.json"), "9780201558029")
+        assertEquals(SourceResult.Failed("malformed json"), result)
+    }
+
     private fun fixture(name: String): String =
         checkNotNull(javaClass.classLoader.getResourceAsStream("fixtures/$name")) { "missing fixture $name" }
             .bufferedReader()

app/app/src/test/java/org/modg/bookshelf/ui/scan/ScanCodeFilterTest.kt

@@ -1,7 +1,6 @@
 package org.modg.bookshelf.ui.scan
 
 import org.junit.Assert.assertEquals
-import org.junit.Assert.assertNull
 import org.junit.Test
 
 class ScanCodeFilterTest {
@@ -9,45 +8,45 @@ class ScanCodeFilterTest {
     @Test
     fun `accepts a checksum-valid isbn13`() {
         val filter = ScanCodeFilter()
-        assertEquals("9780201558029", filter.accept("9780201558029"))
+        assertEquals(ScanOutcome.Isbn("9780201558029"), filter.accept("9780201558029"))
     }
 
     @Test
-    fun `rejects a checksum-invalid isbn13-shaped code`() {
+    fun `rejects a checksum-invalid isbn13-shaped code as not-an-isbn`() {
         val filter = ScanCodeFilter()
-        assertNull(filter.accept("9780201558020"))
+        assertEquals(ScanOutcome.NotAnIsbn("9780201558020"), filter.accept("9780201558020"))
     }
 
     @Test
-    fun `rejects non-book barcode lengths such as EAN-8 or UPC-A`() {
+    fun `rejects non-book barcode lengths such as EAN-8 or UPC-A as not-an-isbn`() {
         val filter = ScanCodeFilter()
-        assertNull(filter.accept("12345670")) // EAN-8 shaped
+        assertEquals(ScanOutcome.NotAnIsbn("12345670"), filter.accept("12345670")) // EAN-8 shaped
-        assertNull(filter.accept("012345678905")) // UPC-A shaped, 12 digits
+        assertEquals(ScanOutcome.NotAnIsbn("012345678905"), filter.accept("012345678905")) // UPC-A shaped, 12 digits
     }
 
     @Test
-    fun `rejects null raw value`() {
+    fun `ignores null raw value`() {
-        assertNull(ScanCodeFilter().accept(null))
+        assertEquals(ScanOutcome.Ignored, ScanCodeFilter().accept(null))
     }
 
     @Test
-    fun `debounces a repeat of the same code within the window`() {
+    fun `debounces a repeat of the same valid code within the window`() {
         var now = 0L
         val filter = ScanCodeFilter(debounceMillis = 2000L, nowMillis = { now })
 
-        assertEquals("9780201558029", filter.accept("9780201558029"))
+        assertEquals(ScanOutcome.Isbn("9780201558029"), filter.accept("9780201558029"))
         now += 500
-        assertNull(filter.accept("9780201558029"))
+        assertEquals(ScanOutcome.Ignored, filter.accept("9780201558029"))
     }
 
     @Test
-    fun `re-emits the same code once the debounce window elapses`() {
+    fun `re-emits the same valid code once the debounce window elapses`() {
         var now = 0L
         val filter = ScanCodeFilter(debounceMillis = 2000L, nowMillis = { now })
 
-        assertEquals("9780201558029", filter.accept("9780201558029"))
+        assertEquals(ScanOutcome.Isbn("9780201558029"), filter.accept("9780201558029"))
         now += 2001
-        assertEquals("9780201558029", filter.accept("9780201558029"))
+        assertEquals(ScanOutcome.Isbn("9780201558029"), filter.accept("9780201558029"))
     }
 
     @Test
@@ -55,9 +54,9 @@ class ScanCodeFilterTest {
         var now = 0L
         val filter = ScanCodeFilter(debounceMillis = 2000L, nowMillis = { now })
 
-        assertEquals("9780201558029", filter.accept("9780201558029"))
+        assertEquals(ScanOutcome.Isbn("9780201558029"), filter.accept("9780201558029"))
         now += 10
-        assertEquals("9780134685991", filter.accept("9780134685991"))
+        assertEquals(ScanOutcome.Isbn("9780134685991"), filter.accept("9780134685991"))
     }
 
     @Test
@@ -65,8 +64,56 @@ class ScanCodeFilterTest {
         var now = 0L
         val filter = ScanCodeFilter(debounceMillis = 2000L, nowMillis = { now })
 
-        assertEquals("9780201558029", filter.accept("9780201558029"))
+        assertEquals(ScanOutcome.Isbn("9780201558029"), filter.accept("9780201558029"))
         filter.reset()
-        assertEquals("9780201558029", filter.accept("9780201558029"))
+        assertEquals(ScanOutcome.Isbn("9780201558029"), filter.accept("9780201558029"))
+    }
+
+    // --- Rejected-code throttle (SPEC.md "Barcode scanning": a non-book barcode sitting
+    // in frame decodes on almost every analyzed frame, so this must not flicker). ---
+
+    @Test
+    fun `a rejected code emits once, then is throttled on every subsequent frame within the window`() {
+        var now = 0L
+        val filter = ScanCodeFilter(debounceMillis = 2000L, nowMillis = { now })
+
+        assertEquals(ScanOutcome.NotAnIsbn("12345670"), filter.accept("12345670"))
+        // Simulate several more analyzed frames, all still within the debounce window.
+        now += 50
+        assertEquals(ScanOutcome.Ignored, filter.accept("12345670"))
+        now += 50
+        assertEquals(ScanOutcome.Ignored, filter.accept("12345670"))
+        now += 1000
+        assertEquals(ScanOutcome.Ignored, filter.accept("12345670"))
+    }
+
+    @Test
+    fun `a rejected code re-emits once the debounce window elapses`() {
+        var now = 0L
+        val filter = ScanCodeFilter(debounceMillis = 2000L, nowMillis = { now })
+
+        assertEquals(ScanOutcome.NotAnIsbn("12345670"), filter.accept("12345670"))
+        now += 2001
+        assertEquals(ScanOutcome.NotAnIsbn("12345670"), filter.accept("12345670"))
+    }
+
+    @Test
+    fun `a rejected code does not suppress a genuinely different valid isbn read right after it`() {
+        var now = 0L
+        val filter = ScanCodeFilter(debounceMillis = 2000L, nowMillis = { now })
+
+        assertEquals(ScanOutcome.NotAnIsbn("12345670"), filter.accept("12345670"))
+        now += 10
+        assertEquals(ScanOutcome.Isbn("9780201558029"), filter.accept("9780201558029"))
+    }
+
+    @Test
+    fun `switching between two different rejected codes does not throttle either`() {
+        var now = 0L
+        val filter = ScanCodeFilter(debounceMillis = 2000L, nowMillis = { now })
+
+        assertEquals(ScanOutcome.NotAnIsbn("12345670"), filter.accept("12345670"))
+        now += 10
+        assertEquals(ScanOutcome.NotAnIsbn("012345678905"), filter.accept("012345678905"))
     }
 }

app/app/src/test/java/org/modg/bookshelf/ui/scan/ScanModelsTest.kt

@@ -5,6 +5,7 @@ import org.junit.Assert.assertTrue
 import org.junit.Test
 import org.modg.bookshelf.data.local.BookEntity
 import org.modg.bookshelf.data.metadata.BookMetadata
+import org.modg.bookshelf.data.metadata.LookupResult
 
 class ScanModelsTest {
 
@@ -25,21 +26,27 @@ class ScanModelsTest {
     }
 
     @Test
-    fun `metadata miss maps to NotFound regardless of duplicate status`() {
+    fun `an authoritative NotFound result maps to NotFound regardless of duplicate status`() {
-        val outcome = ScanMetadataOutcome.from("9780201558029", null, DuplicateStatus.New)
+        val outcome = ScanMetadataOutcome.from("9780201558029", LookupResult.NotFound, DuplicateStatus.New)
         assertEquals(ScanSheetState.NotFound("9780201558029"), outcome)
     }
 
     @Test
-    fun `metadata hit maps to Found carrying the duplicate status through`() {
+    fun `a Found result maps to Found carrying the duplicate status through`() {
         val metadata = BookMetadata(title = "Dune", isbn13 = "9780201558029")
         val duplicate = DuplicateStatus.AlreadyOwned("abc123", "Dune")
 
-        val outcome = ScanMetadataOutcome.from("9780201558029", metadata, duplicate)
+        val outcome = ScanMetadataOutcome.from("9780201558029", LookupResult.Found(metadata), duplicate)
 
         assertEquals(ScanSheetState.Found("9780201558029", metadata, duplicate), outcome)
     }
 
+    @Test
+    fun `an Unavailable result maps to LookupFailed and never claims the book is unknown`() {
+        val outcome = ScanMetadataOutcome.from("9780201558029", LookupResult.Unavailable("http 429"), DuplicateStatus.New)
+        assertEquals(ScanSheetState.LookupFailed("9780201558029", "http 429"), outcome)
+    }
+
     @Test
     fun `session count starts at zero and increments per save, not per skip`() {
         var session = ScanSessionState()

app/app/src/test/java/org/modg/bookshelf/ui/scan/ScannerControllerTest.kt

@@ -46,6 +46,39 @@ class ScannerControllerTest {
         assertEquals(listOf("9780201558029"), received)
     }
 
+    @Test
+    fun `non-book barcode emits on rejectedCodes, not scanResults`() = runTest {
+        val controller = ScannerController()
+        val scanned = mutableListOf<String>()
+        val rejected = mutableListOf<String>()
+        backgroundScope.launch { controller.scanResults.toList(scanned) }
+        backgroundScope.launch { controller.rejectedCodes.toList(rejected) }
+        runCurrent()
+
+        controller.onBarcodeScanned("012345678905") // UPC-A shaped, fails the ISBN-13 checksum
+        runCurrent()
+
+        assertTrue(scanned.isEmpty())
+        assertEquals(listOf("012345678905"), rejected)
+    }
+
+    @Test
+    fun `a debounced repeat reaches neither scanResults nor rejectedCodes`() = runTest {
+        val controller = ScannerController()
+        val scanned = mutableListOf<String>()
+        val rejected = mutableListOf<String>()
+        backgroundScope.launch { controller.scanResults.toList(scanned) }
+        backgroundScope.launch { controller.rejectedCodes.toList(rejected) }
+        runCurrent()
+
+        controller.onBarcodeScanned("012345678905")
+        controller.onBarcodeScanned("012345678905") // same frame's worth of repeat reads
+        runCurrent()
+
+        assertEquals(listOf("012345678905"), rejected)
+        assertTrue(scanned.isEmpty())
+    }
+
     @Test
     fun `torch toggles from its default off state`() {
         val controller = ScannerController()

app/app/src/test/java/org/modg/bookshelf/ui/screens/ScanScreenPaparazziTest.kt

@@ -27,6 +27,8 @@ import org.modg.bookshelf.data.metadata.BookMetadata
 import org.modg.bookshelf.ui.components.BookshelfScaffold
 import org.modg.bookshelf.ui.scan.DuplicateStatus
 import org.modg.bookshelf.ui.scan.FoundBookSheet
+import org.modg.bookshelf.ui.scan.LookupFailedSheet
+import org.modg.bookshelf.ui.scan.RejectedBarcodeBanner
 import org.modg.bookshelf.ui.scan.ScanReticle
 import org.modg.bookshelf.ui.scan.SearchingSheet
 import org.modg.bookshelf.ui.scan.SessionBadge
@@ -57,6 +59,14 @@ class ScanScreenPaparazziTest {
     @Test
     fun scanSearchingSheetLight() = snapshotBoth("scan-searching-sheet") { SearchingSheetOverlay() }
 
+    /** Neither source could be reached — see [LookupFailedSheet]. Must not read as "not found". */
+    @Test
+    fun scanLookupFailedSheetLight() = snapshotBoth("scan-lookup-failed-sheet") { LookupFailedSheetOverlay() }
+
+    /** A decoded barcode that failed the ISBN-13 checksum — see [RejectedBarcodeBanner]. */
+    @Test
+    fun scanRejectedBarcodeLight() = snapshotBoth("scan-rejected-barcode") { RejectedBarcodeOverlay() }
+
     @Composable
     private fun ReticleOverlay() = Shell {
         Box(modifier = Modifier.fillMaxSize(), contentAlignment = Alignment.Center) {
@@ -107,6 +117,35 @@ class ScanScreenPaparazziTest {
         }
     }
 
+    @Composable
+    private fun LookupFailedSheetOverlay() = Shell {
+        Box(modifier = Modifier.fillMaxSize()) {
+            Surface(
+                modifier = Modifier.align(Alignment.BottomCenter).fillMaxWidth(),
+                shape = RoundedCornerShape(topStart = 16.dp, topEnd = 16.dp),
+                color = MaterialTheme.colorScheme.surface,
+            ) {
+                LookupFailedSheet(
+                    isbn13 = "9780765326355",
+                    onRetry = {},
+                    onEnterByHand = {},
+                    onSkip = {},
+                )
+            }
+        }
+    }
+
+    @Composable
+    private fun RejectedBarcodeOverlay() = Shell {
+        Box(modifier = Modifier.fillMaxSize(), contentAlignment = Alignment.Center) {
+            ScanReticle(modifier = Modifier.align(Alignment.Center))
+            RejectedBarcodeBanner(
+                message = "Read 012345678905 — not a book barcode",
+                modifier = Modifier.align(Alignment.BottomCenter).padding(24.dp),
+            )
+        }
+    }
+
     @Composable
     private fun Shell(overlay: @Composable () -> Unit) {
         BookshelfScaffold(