diff --git a/src/anki-integration/known-word-cache-maturity.test.ts b/src/anki-integration/known-word-cache-maturity.test.ts
index f3af5b40..6128af49 100644
--- a/src/anki-integration/known-word-cache-maturity.test.ts
+++ b/src/anki-integration/known-word-cache-maturity.test.ts
@@ -20,6 +20,7 @@ function createMaturityHarness(config: AnkiConnectConfig): {
     findNotesResult: number[];
     notesInfoResult: HarnessNoteInfo[];
     findNotesByQuery: Map<string, number[]>;
+    failedQueries: Set<string>;
   };
   createSiblingManager: () => KnownWordCacheManager;
   cleanup: () => void;
@@ -31,12 +32,16 @@ function createMaturityHarness(config: AnkiConnectConfig): {
     findNotesResult: [] as number[],
     notesInfoResult: [] as HarnessNoteInfo[],
     findNotesByQuery: new Map<string, number[]>(),
+    failedQueries: new Set<string>(),
   };
   const deps = {
     client: {
       findNotes: async (query: string) => {
         calls.findNotes += 1;
         calls.queries.push(query);
+        if (clientState.failedQueries.has(query)) {
+          throw new Error(`Anki unavailable for query: ${query}`);
+        }
         if (clientState.findNotesByQuery.has(query)) {
           return clientState.findNotesByQuery.get(query) ?? [];
         }
@@ -220,7 +225,10 @@ test('reading-only fallback resolves tiers unless opted out', async () => {
     await manager.refresh(true);
 
     assert.equal(manager.getKnownWordTier('けいこく'), 'mature');
-    assert.equal(manager.getKnownWordTier('けいこく', undefined, { allowReadingOnlyMatch: false }), null);
+    assert.equal(
+      manager.getKnownWordTier('けいこく', undefined, { allowReadingOnlyMatch: false }),
+      null,
+    );
   } finally {
     cleanup();
   }
@@ -245,6 +253,29 @@ test('getKnownWordTier returns null and skips tier queries when maturity is disa
   }
 });
 
+test('refresh preserves known-word cache when maturity lookup fails', async () => {
+  const { manager, statePath, clientState, cleanup } = createMaturityHarness(maturityConfig());
+
+  try {
+    clientState.findNotesByQuery.set('deck:"Mining"', [1]);
+    clientState.failedQueries.add('deck:"Mining" prop:ivl>=21');
+    clientState.notesInfoResult = [{ noteId: 1, fields: { Word: { value: '猫' } } }];
+
+    await manager.refresh(true);
+
+    assert.equal(manager.isKnownWord('猫'), true);
+    assert.equal(manager.getKnownWordTier('猫'), null);
+    const persisted = JSON.parse(fs.readFileSync(statePath, 'utf-8')) as {
+      version: number;
+      tiers: Record<string, string>;
+    };
+    assert.equal(persisted.version, 4);
+    assert.deepEqual(persisted.tiers, {});
+  } finally {
+    cleanup();
+  }
+});
+
 test('tiers persist to v4 state and reload without refetching', async () => {
   const { manager, calls, statePath, clientState, createSiblingManager, cleanup } =
     createMaturityHarness(maturityConfig());
diff --git a/src/anki-integration/known-word-cache.ts b/src/anki-integration/known-word-cache.ts
index 15db5a21..6c130850 100644
--- a/src/anki-integration/known-word-cache.ts
+++ b/src/anki-integration/known-word-cache.ts
@@ -389,13 +389,18 @@ export class KnownWordCacheManager {
     this.isRefreshingKnownWords = true;
     try {
       const noteFieldsById = await this.fetchKnownWordNoteFieldsById();
-      const tierSets = this.isMaturityTrackingEnabled()
-        ? await fetchKnownWordMaturityTierSets(
+      let tierSets = null;
+      if (this.isMaturityTrackingEnabled()) {
+        try {
+          tierSets = await fetchKnownWordMaturityTierSets(
             (query, options) => this.deps.client.findNotes(query, options),
             this.getKnownWordQueryScopes().map((scope) => scope.query),
             getMatureIntervalThresholdDays(this.deps.getConfig()),
-          )
-        : null;
+          );
+        } catch (error) {
+          log.warn('Failed to fetch known-word maturity tiers:', (error as Error).message);
+        }
+      }
       const currentNoteIds = Array.from(noteFieldsById.keys()).sort((a, b) => a - b);
 
       if (this.noteEntriesById.size === 0) {