diff --git a/docs/subagents/INDEX.md b/docs/subagents/INDEX.md
index 9569d6e..b5c9f8e 100644
--- a/docs/subagents/INDEX.md
+++ b/docs/subagents/INDEX.md
@@ -17,3 +17,4 @@ Read first. Keep concise.
 | `codex-preserve-linebreaks-20260220T063538Z-s4nd` | `codex-preserve-linebreaks` | `Add config option to preserve subtitle line breaks in visible overlay rendering` | `completed` | `docs/subagents/agents/codex-preserve-linebreaks-20260220T063538Z-s4nd.md` | `2026-02-20T06:42:51Z` |
 | `codex-release-mpv-plugin-20260220T035757Z-d4yf` | `codex-release-mpv-plugin` | `Package optional release assets bundle (mpv plugin + rofi theme), move theme to assets/themes, update install/docs` | `completed` | `docs/subagents/agents/codex-release-mpv-plugin-20260220T035757Z-d4yf.md` | `2026-02-20T04:02:26Z` |
 | `codex-bundle-config-example-20260220T092408Z-a1b2` | `codex-bundle-config-example` | `Bundle config.example.jsonc in release assets tarball and align install docs` | `completed` | `docs/subagents/agents/codex-bundle-config-example-20260220T092408Z-a1b2.md` | `2026-02-20T09:26:24Z` |
+| `codex-tsconfig-modernize-20260220T093035Z-68qb` | `codex-tsconfig-modernize` | `Enable noUncheckedIndexedAccess + isolatedModules in root tsconfig and fix resulting compile errors` | `completed` | `docs/subagents/agents/codex-tsconfig-modernize-20260220T093035Z-68qb.md` | `2026-02-20T09:46:26Z` |
diff --git a/docs/subagents/agents/codex-tsconfig-modernize-20260220T093035Z-68qb.md b/docs/subagents/agents/codex-tsconfig-modernize-20260220T093035Z-68qb.md
new file mode 100644
index 0000000..98aeeff
--- /dev/null
+++ b/docs/subagents/agents/codex-tsconfig-modernize-20260220T093035Z-68qb.md
@@ -0,0 +1,30 @@
+# Agent Log: codex-tsconfig-modernize-20260220T093035Z-68qb
+
+- alias: `codex-tsconfig-modernize`
+- mission: `Enable noUncheckedIndexedAccess + isolatedModules in root tsconfig and fix resulting compile errors`
+- status: `completed`
+- started_utc: `2026-02-20T09:31:11Z`
+- last_update_utc: `2026-02-20T09:46:26Z`
+- planned_files:
+  - `tsconfig.json`
+  - `src/**/*.ts` (targeted type-safety fixes)
+  - `docs/subagents/INDEX.md`
+  - `docs/subagents/agents/codex-tsconfig-modernize-20260220T093035Z-68qb.md`
+- assumptions:
+  - User wants requested config adopted for root project tsconfig.
+  - Keep root CJS build semantics unchanged; only add requested strict flags.
+- phase:
+  - `handoff`
+- notes:
+  - Read `docs/subagents/INDEX.md` and `docs/subagents/collaboration.md`.
+  - Root build uses `tsc` emit flow; avoid `noEmit` and conflicting module modes.
+  - Applied and kept: `lib` includes `DOM.Iterable`, `moduleDetection: force`, `noImplicitOverride: true`.
+  - Tried then reverted: `noUncheckedIndexedAccess`, `isolatedModules` (large compile breakage).
+  - Validation: `bun run tsc --noEmit` passed.
+  - New user directive: proceed with enabling both strict flags and remediate errors.
+  - Re-enabled `noUncheckedIndexedAccess` + `isolatedModules`; fixed all compile errors across source/tests.
+  - Final validation: `bun run tsc --noEmit` and `bun run test:fast` both pass.
+- touched_files:
+  - `tsconfig.json`
+  - `docs/subagents/INDEX.md`
+  - `docs/subagents/agents/codex-tsconfig-modernize-20260220T093035Z-68qb.md`
diff --git a/src/anki-integration.ts b/src/anki-integration.ts
index 2444b9d..297be01 100644
--- a/src/anki-integration.ts
+++ b/src/anki-integration.ts
@@ -411,7 +411,7 @@ export class AnkiIntegration {
         return;
       }
 
-      const noteInfo = notesInfo[0];
+      const noteInfo = notesInfo[0]!;
       this.appendKnownWordsFromNoteInfo(noteInfo);
       const fields = this.extractFields(noteInfo.fields);
 
@@ -1000,13 +1000,13 @@ export class AnkiIntegration {
   private extractLastSoundTag(value: string): string {
     const matches = value.match(/\[sound:[^\]]+\]/g);
     if (!matches || matches.length === 0) return '';
-    return matches[matches.length - 1];
+    return matches[matches.length - 1]!;
   }
 
   private extractLastImageTag(value: string): string {
     const matches = value.match(/<img\b[^>]*>/gi);
     if (!matches || matches.length === 0) return '';
-    return matches[matches.length - 1];
+    return matches[matches.length - 1]!;
   }
 
   private extractImageTags(value: string): string[] {
@@ -1472,7 +1472,7 @@ export class AnkiIntegration {
       log.warn('Keep note not found:', keepNoteId);
       return;
     }
-    const keepNoteInfo = keepNotesInfo[0];
+    const keepNoteInfo = keepNotesInfo[0]!;
     const mergedFields = await this.computeFieldGroupingMergedFields(
       keepNoteId,
       deleteNoteId,
@@ -1539,7 +1539,7 @@ export class AnkiIntegration {
       if (!originalNotesInfo || originalNotesInfo.length === 0) {
         return false;
       }
-      const originalNoteInfo = originalNotesInfo[0];
+      const originalNoteInfo = originalNotesInfo[0]!;
       const sentenceCardConfig = this.getEffectiveSentenceCardConfig();
 
       const originalFields = this.extractFields(originalNoteInfo.fields);
diff --git a/src/anki-integration/card-creation.ts b/src/anki-integration/card-creation.ts
index 03cc4bc..d81d9fd 100644
--- a/src/anki-integration/card-creation.ts
+++ b/src/anki-integration/card-creation.ts
@@ -199,7 +199,7 @@ export class CardCreationService {
           return;
         }
 
-        const noteInfo = notesInfoResult[0];
+        const noteInfo = notesInfoResult[0]!;
         const fields = this.deps.extractFields(noteInfo.fields);
         const expressionText = fields.expression || fields.word || '';
         const sentenceAudioField = this.getResolvedSentenceAudioFieldName(noteInfo);
@@ -366,7 +366,7 @@ export class CardCreationService {
           return;
         }
 
-        const noteInfo = notesInfoResult[0];
+        const noteInfo = notesInfoResult[0]!;
         const fields = this.deps.extractFields(noteInfo.fields);
         const expressionText = fields.expression || fields.word || '';
 
@@ -536,7 +536,7 @@ export class CardCreationService {
           const noteInfoResult = await this.deps.client.notesInfo([noteId]);
           const noteInfos = noteInfoResult as CardCreationNoteInfo[];
           if (noteInfos.length > 0) {
-            const createdNoteInfo = noteInfos[0];
+            const createdNoteInfo = noteInfos[0]!;
             this.deps.appendKnownWordsFromNoteInfo(createdNoteInfo);
             resolvedSentenceAudioField =
               this.deps.resolveNoteFieldName(createdNoteInfo, audioFieldName) || audioFieldName;
diff --git a/src/anki-integration/duplicate.ts b/src/anki-integration/duplicate.ts
index 1848dbd..23b33cb 100644
--- a/src/anki-integration/duplicate.ts
+++ b/src/anki-integration/duplicate.ts
@@ -23,7 +23,7 @@ export async function findDuplicateNote(
 ): Promise<number | null> {
   let fieldName = '';
   for (const name of Object.keys(noteInfo.fields)) {
-    if (['word', 'expression'].includes(name.toLowerCase()) && noteInfo.fields[name].value) {
+    if (['word', 'expression'].includes(name.toLowerCase()) && noteInfo.fields[name]?.value) {
       fieldName = name;
       break;
     }
diff --git a/src/anki-integration/field-grouping.ts b/src/anki-integration/field-grouping.ts
index 1d32f5e..becb2f2 100644
--- a/src/anki-integration/field-grouping.ts
+++ b/src/anki-integration/field-grouping.ts
@@ -100,7 +100,7 @@ export class FieldGroupingService {
           this.deps.showOsdNotification('Card not found');
           return;
         }
-        const noteInfoBeforeUpdate = notesInfo[0];
+        const noteInfoBeforeUpdate = notesInfo[0]!;
         const fields = this.deps.extractFields(noteInfoBeforeUpdate.fields);
         const expressionText = fields.expression || fields.word || '';
         if (!expressionText) {
@@ -135,7 +135,7 @@ export class FieldGroupingService {
           return;
         }
 
-        const noteInfo = refreshedInfo[0];
+        const noteInfo = refreshedInfo[0]!;
 
         if (sentenceCardConfig.kikuFieldGrouping === 'auto') {
           await this.deps.handleFieldGroupingAuto(
diff --git a/src/anki-integration/known-word-cache.ts b/src/anki-integration/known-word-cache.ts
index 323eece..b693fb8 100644
--- a/src/anki-integration/known-word-cache.ts
+++ b/src/anki-integration/known-word-cache.ts
@@ -236,7 +236,7 @@ export class KnownWordCacheManager {
     }
 
     if (decks.length === 1) {
-      return `deck:"${escapeAnkiSearchValue(decks[0])}"`;
+      return `deck:"${escapeAnkiSearchValue(decks[0]!)}"`;
     }
 
     const deckQueries = decks.map((deck) => `deck:"${escapeAnkiSearchValue(deck)}"`);
diff --git a/src/cli/args.ts b/src/cli/args.ts
index 5d5a548..a031d54 100644
--- a/src/cli/args.ts
+++ b/src/cli/args.ts
@@ -115,7 +115,7 @@ export function parseArgs(argv: string[]): CliArgs {
 
   for (let i = 0; i < argv.length; i += 1) {
     const arg = argv[i];
-    if (!arg.startsWith('--')) continue;
+    if (!arg || !arg.startsWith('--')) continue;
 
     if (arg === '--background') args.background = true;
     else if (arg === '--start') args.start = true;
diff --git a/src/config/path-resolution.ts b/src/config/path-resolution.ts
index 50cc9b7..62ba79e 100644
--- a/src/config/path-resolution.ts
+++ b/src/config/path-resolution.ts
@@ -54,7 +54,7 @@ export function resolveConfigDir(options: ConfigPathOptions): string {
     }
   }
 
-  return path.join(baseDirs[0], getDefaultAppName(options));
+  return path.join(baseDirs[0]!, getDefaultAppName(options));
 }
 
 export function resolveConfigFilePath(options: ConfigPathOptions): string {
@@ -72,5 +72,5 @@ export function resolveConfigFilePath(options: ConfigPathOptions): string {
     }
   }
 
-  return path.join(baseDirs[0], getDefaultAppName(options), DEFAULT_FILE_NAMES[0]);
+  return path.join(baseDirs[0]!, getDefaultAppName(options), DEFAULT_FILE_NAMES[0]!);
 }
diff --git a/src/core/services/anilist/anilist-updater.ts b/src/core/services/anilist/anilist-updater.ts
index 2dd1b13..4012651 100644
--- a/src/core/services/anilist/anilist-updater.ts
+++ b/src/core/services/anilist/anilist-updater.ts
@@ -168,7 +168,7 @@ function pickBestSearchResult(
     return titles.includes(normalizedTarget);
   });
 
-  const selected = exact ?? candidates[0];
+  const selected = exact ?? candidates[0]!;
   const selectedTitle =
     selected.title?.english || selected.title?.romaji || selected.title?.native || title;
   return { id: selected.id, title: selectedTitle };
diff --git a/src/core/services/anki-jimaku.test.ts b/src/core/services/anki-jimaku.test.ts
index 32ef750..8d6c143 100644
--- a/src/core/services/anki-jimaku.test.ts
+++ b/src/core/services/anki-jimaku.test.ts
@@ -129,7 +129,7 @@ test('refreshKnownWords throws when integration is unavailable', async () => {
 
   await assert.rejects(
     async () => {
-      await registered.refreshKnownWords();
+      await registered.refreshKnownWords!();
     },
     { message: 'AnkiConnect integration not enabled' },
   );
@@ -144,7 +144,7 @@ test('refreshKnownWords delegates to integration', async () => {
     },
   };
 
-  await registered.refreshKnownWords();
+  await registered.refreshKnownWords!();
 
   assert.equal(refreshed, 1);
 });
@@ -158,7 +158,7 @@ test('setAnkiConnectEnabled disables active integration and broadcasts changes',
     },
   };
 
-  registered.setAnkiConnectEnabled(false);
+  registered.setAnkiConnectEnabled!(false);
 
   assert.deepEqual(state.patches, [false]);
   assert.equal(destroyed, 1);
@@ -188,8 +188,8 @@ test('clearAnkiHistory and respondFieldGrouping execute runtime callbacks', () =
     deleteDuplicate: true,
     cancelled: false,
   };
-  registered.clearAnkiHistory();
-  registered.respondFieldGrouping(choice);
+  registered.clearAnkiHistory!();
+  registered.respondFieldGrouping!(choice);
 
   options.getSubtitleTimingTracker = originalGetTracker;
 
@@ -201,7 +201,7 @@ test('clearAnkiHistory and respondFieldGrouping execute runtime callbacks', () =
 test('buildKikuMergePreview returns guard error when integration is missing', async () => {
   const { registered } = createHarness();
 
-  const result = await registered.buildKikuMergePreview({
+  const result = await registered.buildKikuMergePreview!({
     keepNoteId: 1,
     deleteNoteId: 2,
     deleteDuplicate: false,
@@ -227,7 +227,7 @@ test('buildKikuMergePreview delegates to integration when available', async () =
     },
   };
 
-  const result = await registered.buildKikuMergePreview({
+  const result = await registered.buildKikuMergePreview!({
     keepNoteId: 3,
     deleteNoteId: 4,
     deleteDuplicate: true,
@@ -240,7 +240,7 @@ test('buildKikuMergePreview delegates to integration when available', async () =
 test('searchJimakuEntries caps results and onDownloadedSubtitle sends sub-add to mpv', async () => {
   const { registered, state } = createHarness();
 
-  const searchResult = await registered.searchJimakuEntries({ query: 'test' });
+  const searchResult = await registered.searchJimakuEntries!({ query: 'test' });
   assert.deepEqual(state.fetchCalls, [
     {
       endpoint: '/api/entries/search',
@@ -250,6 +250,6 @@ test('searchJimakuEntries caps results and onDownloadedSubtitle sends sub-add to
   assert.equal((searchResult as { ok: boolean }).ok, true);
   assert.equal((searchResult as { data: unknown[] }).data.length, 2);
 
-  registered.onDownloadedSubtitle('/tmp/subtitle.ass');
+  registered.onDownloadedSubtitle!('/tmp/subtitle.ass');
   assert.deepEqual(state.sentCommands, [{ command: ['sub-add', '/tmp/subtitle.ass', 'select'] }]);
 });
diff --git a/src/core/services/app-ready.test.ts b/src/core/services/app-ready.test.ts
index ffe723a..eee41cb 100644
--- a/src/core/services/app-ready.test.ts
+++ b/src/core/services/app-ready.test.ts
@@ -209,30 +209,31 @@ test('runAppReadyRuntime aggregates multiple critical anki mapping errors', asyn
 
   await runAppReadyRuntime(deps);
 
+  const firstErrorSet = capturedErrors[0]!;
   assert.equal(capturedErrors.length, 1);
-  assert.equal(capturedErrors[0].length, 5);
+  assert.equal(firstErrorSet.length, 5);
   assert.ok(
-    capturedErrors[0].includes(
+    firstErrorSet.includes(
       'ankiConnect.fields.audio must be a non-empty string when ankiConnect is enabled.',
     ),
   );
   assert.ok(
-    capturedErrors[0].includes(
+    firstErrorSet.includes(
       'ankiConnect.fields.image must be a non-empty string when ankiConnect is enabled.',
     ),
   );
   assert.ok(
-    capturedErrors[0].includes(
+    firstErrorSet.includes(
       'ankiConnect.fields.sentence must be a non-empty string when ankiConnect is enabled.',
     ),
   );
   assert.ok(
-    capturedErrors[0].includes(
+    firstErrorSet.includes(
       'ankiConnect.fields.miscInfo must be a non-empty string when ankiConnect is enabled.',
     ),
   );
   assert.ok(
-    capturedErrors[0].includes(
+    firstErrorSet.includes(
       'ankiConnect.fields.translation must be a non-empty string when ankiConnect is enabled.',
     ),
   );
diff --git a/src/core/services/immersion-tracker-service.test.ts b/src/core/services/immersion-tracker-service.test.ts
index 891f51c..13eba46 100644
--- a/src/core/services/immersion-tracker-service.test.ts
+++ b/src/core/services/immersion-tracker-service.test.ts
@@ -124,8 +124,8 @@ testIfSqlite('persists and retrieves minimum immersion tracking fields', async (
 
     const summaries = await tracker.getSessionSummaries(10);
     assert.ok(summaries.length >= 1);
-    assert.ok(summaries[0].linesSeen >= 1);
-    assert.ok(summaries[0].cardsMined >= 2);
+    assert.ok(summaries[0]!.linesSeen >= 1);
+    assert.ok(summaries[0]!.cardsMined >= 2);
 
     tracker.destroy();
 
@@ -376,8 +376,8 @@ testIfSqlite('monthly rollups are grouped by calendar month', async () => {
     const videoRows = rows.filter((row) => row.videoId === 1);
 
     assert.equal(videoRows.length, 2);
-    assert.equal(videoRows[0].rollupDayOrMonth, 202602);
-    assert.equal(videoRows[1].rollupDayOrMonth, 202601);
+    assert.equal(videoRows[0]!.rollupDayOrMonth, 202602);
+    assert.equal(videoRows[1]!.rollupDayOrMonth, 202601);
   } finally {
     tracker?.destroy();
     cleanupDbPath(dbPath);
diff --git a/src/core/services/immersion-tracker-service.ts b/src/core/services/immersion-tracker-service.ts
index efa3332..56b6e38 100644
--- a/src/core/services/immersion-tracker-service.ts
+++ b/src/core/services/immersion-tracker-service.ts
@@ -1443,7 +1443,7 @@ export class ImmersionTrackerService {
         const parsed = new URL(mediaPath);
         const parts = parsed.pathname.split('/').filter(Boolean);
         if (parts.length > 0) {
-          const leaf = decodeURIComponent(parts[parts.length - 1]);
+          const leaf = decodeURIComponent(parts[parts.length - 1]!);
           return this.normalizeText(leaf.replace(/\.[^/.]+$/, ''));
         }
         return this.normalizeText(parsed.hostname) || 'unknown';
diff --git a/src/core/services/index.ts b/src/core/services/index.ts
index 5dd66e0..5bcf1c7 100644
--- a/src/core/services/index.ts
+++ b/src/core/services/index.ts
@@ -35,7 +35,6 @@ export { createFrequencyDictionaryLookup } from './frequency-dictionary';
 export { createJlptVocabularyLookup } from './jlpt-vocab';
 export {
   getIgnoredPos1Entries,
-  JlptIgnoredPos1Entry,
   JLPT_EXCLUDED_TERMS,
   JLPT_IGNORED_MECAB_POS1,
   JLPT_IGNORED_MECAB_POS1_ENTRIES,
@@ -43,6 +42,7 @@ export {
   shouldIgnoreJlptByTerm,
   shouldIgnoreJlptForMecabPos1,
 } from './jlpt-token-filter';
+export type { JlptIgnoredPos1Entry } from './jlpt-token-filter';
 export { loadYomitanExtension } from './yomitan-extension-loader';
 export {
   getJimakuLanguagePreference,
@@ -72,8 +72,6 @@ export {
 export {
   MPV_REQUEST_ID_SECONDARY_SUB_VISIBILITY,
   MpvIpcClient,
-  MpvRuntimeClientLike,
-  MpvTrackProperty,
   playNextSubtitleRuntime,
   replayCurrentSubtitleRuntime,
   resolveCurrentAudioStreamIndex,
@@ -81,6 +79,7 @@ export {
   setMpvSubVisibilityRuntime,
   showMpvOsdRuntime,
 } from './mpv';
+export type { MpvRuntimeClientLike, MpvTrackProperty } from './mpv';
 export {
   applyMpvSubtitleRenderMetricsPatch,
   DEFAULT_MPV_SUBTITLE_RENDER_METRICS,
diff --git a/src/core/services/jellyfin-remote.test.ts b/src/core/services/jellyfin-remote.test.ts
index 4765644..071bada 100644
--- a/src/core/services/jellyfin-remote.test.ts
+++ b/src/core/services/jellyfin-remote.test.ts
@@ -70,11 +70,11 @@ test('start posts capabilities on socket connect', async () => {
   });
 
   service.start();
-  sockets[0].emit('open');
+  sockets[0]!.emit('open');
   await new Promise((resolve) => setTimeout(resolve, 0));
 
   assert.equal(fetchCalls.length, 1);
-  assert.equal(fetchCalls[0].input, 'http://jellyfin.local:8096/Sessions/Capabilities/Full');
+  assert.equal(fetchCalls[0]!.input, 'http://jellyfin.local:8096/Sessions/Capabilities/Full');
   assert.equal(service.isConnected(), true);
 });
 
@@ -97,9 +97,9 @@ test('socket headers include jellyfin authorization metadata', () => {
 
   service.start();
   assert.equal(seenHeaders.length, 1);
-  assert.ok(seenHeaders[0].Authorization.includes('Client="SubMiner"'));
-  assert.ok(seenHeaders[0].Authorization.includes('DeviceId="device-auth"'));
-  assert.ok(seenHeaders[0]['X-Emby-Authorization']);
+  assert.ok(seenHeaders[0]!['Authorization']!.includes('Client="SubMiner"'));
+  assert.ok(seenHeaders[0]!['Authorization']!.includes('DeviceId="device-auth"'));
+  assert.ok(seenHeaders[0]!['X-Emby-Authorization']);
 });
 
 test('dispatches inbound Play, Playstate, and GeneralCommand messages', () => {
@@ -124,7 +124,7 @@ test('dispatches inbound Play, Playstate, and GeneralCommand messages', () => {
   });
 
   service.start();
-  const socket = sockets[0];
+  const socket = sockets[0]!;
   socket.emit('message', JSON.stringify({ MessageType: 'Play', Data: { ItemId: 'movie-1' } }));
   socket.emit(
     'message',
@@ -174,13 +174,13 @@ test('schedules reconnect with bounded exponential backoff', () => {
   });
 
   service.start();
-  sockets[0].emit('close');
+  sockets[0]!.emit('close');
   pendingTimers.shift()?.();
-  sockets[1].emit('close');
+  sockets[1]!.emit('close');
   pendingTimers.shift()?.();
-  sockets[2].emit('close');
+  sockets[2]!.emit('close');
   pendingTimers.shift()?.();
-  sockets[3].emit('close');
+  sockets[3]!.emit('close');
 
   assert.deepEqual(delays, [100, 200, 400, 400]);
   assert.equal(sockets.length, 4);
@@ -216,7 +216,7 @@ test('Jellyfin remote stop prevents further reconnect/network activity', () => {
 
   service.start();
   assert.equal(sockets.length, 1);
-  sockets[0].emit('close');
+  sockets[0]!.emit('close');
   assert.equal(pendingTimers.length, 1);
 
   service.stop();
@@ -252,7 +252,7 @@ test('reportProgress posts timeline payload and treats failure as non-fatal', as
   });
 
   service.start();
-  sockets[0].emit('open');
+  sockets[0]!.emit('open');
   await new Promise((resolve) => setTimeout(resolve, 0));
 
   const expectedPayload = buildJellyfinTimelinePayload({
@@ -310,7 +310,7 @@ test('advertiseNow validates server registration using Sessions endpoint', async
   });
 
   service.start();
-  sockets[0].emit('open');
+  sockets[0]!.emit('open');
   const ok = await service.advertiseNow();
   assert.equal(ok, true);
   assert.ok(calls.some((url) => url.endsWith('/Sessions')));
diff --git a/src/core/services/jellyfin.test.ts b/src/core/services/jellyfin.test.ts
index 8252320..1c84bf9 100644
--- a/src/core/services/jellyfin.test.ts
+++ b/src/core/services/jellyfin.test.ts
@@ -119,7 +119,7 @@ test('listItems supports search and formats title', async () => {
         limit: 25,
       },
     );
-    assert.equal(items[0].title, 'Space Show S01E02 Pilot');
+    assert.equal(items[0]!.title, 'Space Show S01E02 Pilot');
   } finally {
     globalThis.fetch = originalFetch;
   }
@@ -338,14 +338,14 @@ test('listSubtitleTracks returns all subtitle streams with delivery urls', async
       [2, 3, 4],
     );
     assert.equal(
-      tracks[0].deliveryUrl,
+      tracks[0]!.deliveryUrl,
       'http://jellyfin.local/Videos/movie-1/ms-1/Subtitles/2/Stream.srt?api_key=token',
     );
     assert.equal(
-      tracks[1].deliveryUrl,
+      tracks[1]!.deliveryUrl,
       'http://jellyfin.local/Videos/movie-1/ms-1/Subtitles/3/Stream.srt?api_key=token',
     );
-    assert.equal(tracks[2].deliveryUrl, 'https://cdn.example.com/subs.srt');
+    assert.equal(tracks[2]!.deliveryUrl, 'https://cdn.example.com/subs.srt');
   } finally {
     globalThis.fetch = originalFetch;
   }
@@ -556,9 +556,9 @@ test('listSubtitleTracks falls back from PlaybackInfo to item media sources', as
     );
     assert.equal(requestCount, 2);
     assert.equal(tracks.length, 1);
-    assert.equal(tracks[0].index, 11);
+    assert.equal(tracks[0]!.index, 11);
     assert.equal(
-      tracks[0].deliveryUrl,
+      tracks[0]!.deliveryUrl,
       'http://jellyfin.local/Videos/movie-fallback/ms-fallback/Subtitles/11/Stream.srt?api_key=token',
     );
   } finally {
diff --git a/src/core/services/mpv-protocol.test.ts b/src/core/services/mpv-protocol.test.ts
index 14097f2..110685d 100644
--- a/src/core/services/mpv-protocol.test.ts
+++ b/src/core/services/mpv-protocol.test.ts
@@ -177,8 +177,8 @@ test('splitMpvMessagesFromBuffer parses complete lines and preserves partial buf
 
   assert.equal(parsed.messages.length, 2);
   assert.equal(parsed.nextBuffer, '{"partial"');
-  assert.equal(parsed.messages[0].event, 'shutdown');
-  assert.equal(parsed.messages[1].name, 'media-title');
+  assert.equal(parsed.messages[0]!.event, 'shutdown');
+  assert.equal(parsed.messages[1]!.name, 'media-title');
 });
 
 test('splitMpvMessagesFromBuffer reports invalid JSON lines', () => {
@@ -189,7 +189,7 @@ test('splitMpvMessagesFromBuffer reports invalid JSON lines', () => {
   });
 
   assert.equal(errors.length, 1);
-  assert.equal(errors[0].line, '{invalid}');
+  assert.equal(errors[0]!.line, '{invalid}');
 });
 
 test('visibility and boolean parsers handle text values', () => {
diff --git a/src/core/services/mpv-transport.test.ts b/src/core/services/mpv-transport.test.ts
index 1f496c7..b602b61 100644
--- a/src/core/services/mpv-transport.test.ts
+++ b/src/core/services/mpv-transport.test.ts
@@ -84,11 +84,11 @@ test('scheduleMpvReconnect clears existing timer and increments attempt', () =>
 
   assert.equal(nextAttempt, 4);
   assert.equal(cleared.length, 1);
-  assert.equal(cleared[0], existing);
+  assert.equal(cleared[0]!, existing);
   assert.equal(setTimers.length, 1);
   assert.equal(calls.length, 1);
-  assert.equal(calls[0].attempt, 4);
-  assert.equal(calls[0].delay, getMpvReconnectDelay(3, true));
+  assert.equal(calls[0]!.attempt, 4);
+  assert.equal(calls[0]!.delay, getMpvReconnectDelay(3, true));
   assert.equal(connected, 1);
 });
 
diff --git a/src/core/services/mpv.test.ts b/src/core/services/mpv.test.ts
index feffc6c..98816ce 100644
--- a/src/core/services/mpv.test.ts
+++ b/src/core/services/mpv.test.ts
@@ -54,8 +54,8 @@ test('MpvIpcClient handles sub-text property change and broadcasts tokenized sub
   });
 
   assert.equal(events.length, 1);
-  assert.equal(events[0].text, '字幕');
-  assert.equal(events[0].isOverlayVisible, false);
+  assert.equal(events[0]!.text, '字幕');
+  assert.equal(events[0]!.isOverlayVisible, false);
 });
 
 test('MpvIpcClient parses JSON line protocol in processBuffer', () => {
@@ -70,8 +70,8 @@ test('MpvIpcClient parses JSON line protocol in processBuffer', () => {
   (client as any).processBuffer();
 
   assert.equal(seen.length, 2);
-  assert.equal(seen[0].name, 'path');
-  assert.equal(seen[1].request_id, 1);
+  assert.equal(seen[0]!.name, 'path');
+  assert.equal(seen[1]!.request_id, 1);
   assert.equal((client as any).buffer, '{"partial":');
 });
 
diff --git a/src/core/services/overlay-content-measurement.test.ts b/src/core/services/overlay-content-measurement.test.ts
index f01a27a..43e0fa4 100644
--- a/src/core/services/overlay-content-measurement.test.ts
+++ b/src/core/services/overlay-content-measurement.test.ts
@@ -83,5 +83,5 @@ test('overlay measurement store rate-limits invalid payload warnings', () => {
   now = 11_000;
   store.report({ layer: 'visible' });
   assert.equal(warnings.length, 1);
-  assert.match(warnings[0], /Dropped 3 invalid measurement payload/);
+  assert.match(warnings[0]!, /Dropped 3 invalid measurement payload/);
 });
diff --git a/src/core/services/overlay-drop.ts b/src/core/services/overlay-drop.ts
index 93b57c4..8698307 100644
--- a/src/core/services/overlay-drop.ts
+++ b/src/core/services/overlay-drop.ts
@@ -23,7 +23,7 @@ const VIDEO_EXTENSIONS = new Set([
 ]);
 
 function getPathExtension(pathValue: string): string {
-  const normalized = pathValue.split(/[?#]/, 1)[0];
+  const normalized = pathValue.split(/[?#]/, 1)[0] ?? '';
   const dot = normalized.lastIndexOf('.');
   return dot >= 0 ? normalized.slice(dot).toLowerCase() : '';
 }
diff --git a/src/core/services/shortcut-fallback.ts b/src/core/services/shortcut-fallback.ts
index 61fa002..d89a567 100644
--- a/src/core/services/shortcut-fallback.ts
+++ b/src/core/services/shortcut-fallback.ts
@@ -26,7 +26,7 @@ export function shortcutMatchesInputForLocalFallback(
   const parts = normalized.split('+').filter(Boolean);
   if (parts.length === 0) return false;
 
-  const keyToken = parts[parts.length - 1];
+  const keyToken = parts[parts.length - 1]!;
   const modifierTokens = new Set(parts.slice(0, -1));
   const allowedModifiers = new Set(['shift', 'alt', 'meta', 'control', 'commandorcontrol']);
   for (const token of modifierTokens) {
diff --git a/src/core/services/subsync.test.ts b/src/core/services/subsync.test.ts
index e8b1739..d33be08 100644
--- a/src/core/services/subsync.test.ts
+++ b/src/core/services/subsync.test.ts
@@ -339,6 +339,6 @@ test('runSubsyncManual resolves string sid values from mpv stream properties', a
   assert.equal(syncOutputIndex >= 0, true);
   const outputPath = ffArgs[syncOutputIndex + 1];
   assert.equal(typeof outputPath, 'string');
-  assert.ok(outputPath.length > 0);
-  assert.equal(fs.readFileSync(outputPath, 'utf8'), '');
+  assert.ok(outputPath!.length > 0);
+  assert.equal(fs.readFileSync(outputPath!, 'utf8'), '');
 });
diff --git a/src/core/services/subtitle-position.ts b/src/core/services/subtitle-position.ts
index 89d5a15..9974367 100644
--- a/src/core/services/subtitle-position.ts
+++ b/src/core/services/subtitle-position.ts
@@ -28,7 +28,8 @@ export function cycleSecondarySubMode(deps: CycleSecondarySubModeDeps): void {
 
   const currentMode = deps.getSecondarySubMode();
   const currentIndex = SECONDARY_SUB_CYCLE.indexOf(currentMode);
-  const nextMode = SECONDARY_SUB_CYCLE[(currentIndex + 1) % SECONDARY_SUB_CYCLE.length];
+  const safeIndex = currentIndex >= 0 ? currentIndex : 0;
+  const nextMode = SECONDARY_SUB_CYCLE[(safeIndex + 1) % SECONDARY_SUB_CYCLE.length]!;
   deps.setSecondarySubMode(nextMode);
   deps.broadcastSecondarySubMode(nextMode);
   deps.showMpvOsd(`Secondary subtitle: ${nextMode}`);
diff --git a/src/core/services/subtitle-ws.ts b/src/core/services/subtitle-ws.ts
index 869b59e..f416b4f 100644
--- a/src/core/services/subtitle-ws.ts
+++ b/src/core/services/subtitle-ws.ts
@@ -82,8 +82,9 @@ export function serializeSubtitleMarkup(
     const klass = computeWordClass(token, options);
     const parts = token.surface.split('\n');
     for (let index = 0; index < parts.length; index += 1) {
-      if (parts[index]) {
-        chunks.push(`<span class="${klass}">${escapeHtml(parts[index])}</span>`);
+      const part = parts[index];
+      if (part) {
+        chunks.push(`<span class="${klass}">${escapeHtml(part)}</span>`);
       }
       if (index < parts.length - 1) {
         chunks.push('<br>');
diff --git a/src/core/services/texthooker.ts b/src/core/services/texthooker.ts
index 19f78e0..6d9e6ff 100644
--- a/src/core/services/texthooker.ts
+++ b/src/core/services/texthooker.ts
@@ -20,7 +20,7 @@ export class Texthooker {
     }
 
     this.server = http.createServer((req, res) => {
-      const urlPath = (req.url || '/').split('?')[0];
+      const urlPath = (req.url || '/').split('?')[0] ?? '/';
       const filePath = path.join(texthookerPath, urlPath === '/' ? 'index.html' : urlPath);
 
       const ext = path.extname(filePath);
diff --git a/src/core/services/tokenizer.ts b/src/core/services/tokenizer.ts
index b88a0d2..c1d0644 100644
--- a/src/core/services/tokenizer.ts
+++ b/src/core/services/tokenizer.ts
@@ -377,7 +377,7 @@ function isRepeatedKanaSfx(text: string): boolean {
   let hasAdjacentRepeat = false;
 
   for (let i = 0; i < chars.length; i += 1) {
-    const char = chars[i];
+    const char = chars[i]!;
     counts.set(char, (counts.get(char) ?? 0) + 1);
     if (i > 0 && chars[i] === chars[i - 1]) {
       hasAdjacentRepeat = true;
diff --git a/src/core/services/yomitan-settings.ts b/src/core/services/yomitan-settings.ts
index a01da2e..ae86c1b 100644
--- a/src/core/services/yomitan-settings.ts
+++ b/src/core/services/yomitan-settings.ts
@@ -73,7 +73,8 @@ export function openYomitanSettingsWindow(options: OpenYomitanSettingsWindowOpti
 
   setTimeout(() => {
     if (!settingsWindow.isDestroyed()) {
-      settingsWindow.setSize(settingsWindow.getSize()[0], settingsWindow.getSize()[1]);
+      const [width = 0, height = 0] = settingsWindow.getSize();
+      settingsWindow.setSize(width, height);
       settingsWindow.webContents.invalidate();
       settingsWindow.show();
     }
diff --git a/src/jimaku/utils.ts b/src/jimaku/utils.ts
index 67ec7a5..ac1b382 100644
--- a/src/jimaku/utils.ts
+++ b/src/jimaku/utils.ts
@@ -51,7 +51,7 @@ function getRetryAfter(headers: http.IncomingHttpHeaders): number | undefined {
   const value = headers['x-ratelimit-reset-after'];
   if (!value) return undefined;
   const raw = Array.isArray(value) ? value[0] : value;
-  const parsed = Number.parseFloat(raw);
+  const parsed = Number.parseFloat(raw!);
   if (!Number.isFinite(parsed)) return undefined;
   return parsed;
 }
@@ -146,8 +146,8 @@ function matchEpisodeFromName(name: string): {
   const seasonEpisode = name.match(/S(\d{1,2})E(\d{1,3})/i);
   if (seasonEpisode && seasonEpisode.index !== undefined) {
     return {
-      season: Number.parseInt(seasonEpisode[1], 10),
-      episode: Number.parseInt(seasonEpisode[2], 10),
+      season: Number.parseInt(seasonEpisode[1]!, 10),
+      episode: Number.parseInt(seasonEpisode[2]!, 10),
       index: seasonEpisode.index,
       confidence: 'high',
     };
@@ -156,8 +156,8 @@ function matchEpisodeFromName(name: string): {
   const alt = name.match(/(\d{1,2})x(\d{1,3})/i);
   if (alt && alt.index !== undefined) {
     return {
-      season: Number.parseInt(alt[1], 10),
-      episode: Number.parseInt(alt[2], 10),
+      season: Number.parseInt(alt[1]!, 10),
+      episode: Number.parseInt(alt[2]!, 10),
       index: alt.index,
       confidence: 'high',
     };
@@ -167,7 +167,7 @@ function matchEpisodeFromName(name: string): {
   if (epOnly && epOnly.index !== undefined) {
     return {
       season: null,
-      episode: Number.parseInt(epOnly[1], 10),
+      episode: Number.parseInt(epOnly[1]!, 10),
       index: epOnly.index,
       confidence: 'medium',
     };
@@ -177,7 +177,7 @@ function matchEpisodeFromName(name: string): {
   if (numeric && numeric.index !== undefined) {
     return {
       season: null,
-      episode: Number.parseInt(numeric[1], 10),
+      episode: Number.parseInt(numeric[1]!, 10),
       index: numeric.index,
       confidence: 'medium',
     };
@@ -190,7 +190,7 @@ function detectSeasonFromDir(mediaPath: string): number | null {
   const parent = path.basename(path.dirname(mediaPath));
   const match = parent.match(/(?:Season|S)\s*(\d{1,2})/i);
   if (!match) return null;
-  const parsed = Number.parseInt(match[1], 10);
+  const parsed = Number.parseInt(match[1]!, 10);
   return Number.isFinite(parsed) ? parsed : null;
 }
 
diff --git a/src/main/runtime/anilist-setup-window.test.ts b/src/main/runtime/anilist-setup-window.test.ts
index 127d550..96b37d3 100644
--- a/src/main/runtime/anilist-setup-window.test.ts
+++ b/src/main/runtime/anilist-setup-window.test.ts
@@ -29,7 +29,7 @@ test('manual anilist setup submission forwards access token to callback consumer
   const handled = handleSubmission('subminer://anilist-setup?access_token=abc123');
   assert.equal(handled, true);
   assert.equal(consumed.length, 1);
-  assert.ok(consumed[0].includes('https://anilist.subminer.moe/#access_token=abc123'));
+  assert.ok(consumed[0]!.includes('https://anilist.subminer.moe/#access_token=abc123'));
 });
 
 test('maybe focus anilist setup window focuses existing window', () => {
@@ -179,7 +179,7 @@ test('anilist setup did-fail-load handler forwards details', () => {
   });
 
   assert.equal(seen.length, 1);
-  assert.equal(seen[0].errorCode, -3);
+  assert.equal(seen[0]!.errorCode, -3);
 });
 
 test('anilist setup did-finish-load handler triggers fallback on blank page', () => {
diff --git a/src/main/runtime/jellyfin-remote-connection.test.ts b/src/main/runtime/jellyfin-remote-connection.test.ts
index 3fe4502..f51cf6c 100644
--- a/src/main/runtime/jellyfin-remote-connection.test.ts
+++ b/src/main/runtime/jellyfin-remote-connection.test.ts
@@ -49,8 +49,8 @@ test('createLaunchMpvIdleForJellyfinPlaybackHandler builds expected mpv args', (
 
   launch();
   assert.equal(spawnedArgs.length, 1);
-  assert.ok(spawnedArgs[0].includes('--idle=yes'));
-  assert.ok(spawnedArgs[0].some((arg) => arg.includes('--input-ipc-server=/tmp/subminer.sock')));
+  assert.ok(spawnedArgs[0]!.includes('--idle=yes'));
+  assert.ok(spawnedArgs[0]!.some((arg) => arg.includes('--input-ipc-server=/tmp/subminer.sock')));
   assert.ok(logs.some((entry) => entry.includes('Launched mpv for Jellyfin playback')));
 });
 
diff --git a/src/main/runtime/tray-runtime.test.ts b/src/main/runtime/tray-runtime.test.ts
index 2e87262..6d9821c 100644
--- a/src/main/runtime/tray-runtime.test.ts
+++ b/src/main/runtime/tray-runtime.test.ts
@@ -38,8 +38,8 @@ test('tray menu template contains expected entries and handlers', () => {
   });
 
   assert.equal(template.length, 7);
-  template[0].click?.();
-  template[5].type === 'separator' ? calls.push('separator') : calls.push('bad');
-  template[6].click?.();
+  template[0]!.click?.();
+  template[5]!.type === 'separator' ? calls.push('separator') : calls.push('bad');
+  template[6]!.click?.();
   assert.deepEqual(calls, ['overlay', 'separator', 'quit']);
 });
diff --git a/src/media-generator.ts b/src/media-generator.ts
index ef5dab6..8268b27 100644
--- a/src/media-generator.ts
+++ b/src/media-generator.ts
@@ -180,7 +180,7 @@ export class MediaGenerator {
   ): Promise<Buffer> {
     const { format, quality = 92, maxWidth, maxHeight } = options;
     const ext = format === 'webp' ? 'webp' : format === 'png' ? 'png' : 'jpg';
-    const codecMap: Record<string, string> = {
+    const codecMap: Record<'jpg' | 'png' | 'webp', string> = {
       jpg: 'mjpeg',
       png: 'png',
       webp: 'webp',
diff --git a/src/renderer/error-recovery.test.ts b/src/renderer/error-recovery.test.ts
index c8b86cb..30bcae0 100644
--- a/src/renderer/error-recovery.test.ts
+++ b/src/renderer/error-recovery.test.ts
@@ -41,7 +41,7 @@ test('handleError logs context and recovers overlay state', () => {
   assert.equal(dismissed, 1);
   assert.equal(restored, 1);
   assert.equal(shown.length, 1);
-  assert.match(shown[0], /recovered/i);
+  assert.match(shown[0]!, /recovered/i);
   assert.equal(payloads.length, 1);
 
   const payload = payloads[0] as {
diff --git a/src/renderer/handlers/mouse.ts b/src/renderer/handlers/mouse.ts
index f6c79e6..e5d7d50 100644
--- a/src/renderer/handlers/mouse.ts
+++ b/src/renderer/handlers/mouse.ts
@@ -126,12 +126,12 @@ export function createMouseHandlers(
     if (!probeChar || isBoundary(probeChar)) return null;
 
     let start = probeIndex;
-    while (start > 0 && !isBoundary(text[start - 1])) {
+    while (start > 0 && !isBoundary(text[start - 1] ?? '')) {
       start -= 1;
     }
 
     let end = probeIndex + 1;
-    while (end < text.length && !isBoundary(text[end])) {
+    while (end < text.length && !isBoundary(text[end] ?? '')) {
       end += 1;
     }
 
diff --git a/src/renderer/modals/jimaku.ts b/src/renderer/modals/jimaku.ts
index a28d71d..f105eed 100644
--- a/src/renderer/modals/jimaku.ts
+++ b/src/renderer/modals/jimaku.ts
@@ -204,7 +204,7 @@ export function createJimakuModal(
     if (index < 0 || index >= ctx.state.jimakuEntries.length) return;
 
     ctx.state.selectedEntryIndex = index;
-    ctx.state.currentEntryId = ctx.state.jimakuEntries[index].id;
+    ctx.state.currentEntryId = ctx.state.jimakuEntries[index]!.id;
     renderEntries();
 
     if (ctx.state.currentEntryId !== null) {
@@ -223,7 +223,7 @@ export function createJimakuModal(
       return;
     }
 
-    const file = ctx.state.jimakuFiles[index];
+    const file = ctx.state.jimakuFiles[index]!;
     setJimakuStatus('Downloading subtitle...');
 
     const result: JimakuDownloadResult = await window.electronAPI.jimakuDownloadFile({
diff --git a/src/renderer/modals/runtime-options.ts b/src/renderer/modals/runtime-options.ts
index a7a2564..b069674 100644
--- a/src/renderer/modals/runtime-options.ts
+++ b/src/renderer/modals/runtime-options.ts
@@ -30,7 +30,7 @@ export function createRuntimeOptionsModal(
     if (ctx.state.runtimeOptionSelectedIndex >= ctx.state.runtimeOptions.length) {
       return null;
     }
-    return ctx.state.runtimeOptions[ctx.state.runtimeOptionSelectedIndex];
+    return ctx.state.runtimeOptions[ctx.state.runtimeOptionSelectedIndex] ?? null;
   }
 
   function renderRuntimeOptionsList(): void {
@@ -114,11 +114,11 @@ export function createRuntimeOptionsModal(
         ? (safeIndex + 1) % option.allowedValues.length
         : (safeIndex - 1 + option.allowedValues.length) % option.allowedValues.length;
 
-    ctx.state.runtimeOptionDraftValues.set(option.id, option.allowedValues[nextIndex]);
+    const nextValue = option.allowedValues[nextIndex];
+    if (nextValue === undefined) return;
+    ctx.state.runtimeOptionDraftValues.set(option.id, nextValue);
     renderRuntimeOptionsList();
-    setRuntimeOptionsStatus(
-      `Selected ${option.label}: ${formatRuntimeOptionValue(option.allowedValues[nextIndex])}`,
-    );
+    setRuntimeOptionsStatus(`Selected ${option.label}: ${formatRuntimeOptionValue(nextValue)}`);
   }
 
   async function applySelectedRuntimeOption(): Promise<void> {
diff --git a/src/renderer/modals/session-help.ts b/src/renderer/modals/session-help.ts
index b517a81..3df690a 100644
--- a/src/renderer/modals/session-help.ts
+++ b/src/renderer/modals/session-help.ts
@@ -357,7 +357,7 @@ function createSectionNode(
   list.className = 'session-help-item-list';
 
   section.rows.forEach((row, rowIndex) => {
-    const globalIndex = globalIndexMap[sectionIndex] + rowIndex;
+    const globalIndex = (globalIndexMap[sectionIndex] ?? 0) + rowIndex;
     const button = createShortcutRow(row, globalIndex);
     list.appendChild(button);
   });
@@ -406,6 +406,7 @@ export function createSessionHelpModal(
       item.tabIndex = idx === next ? 0 : -1;
     });
     const activeItem = items[next];
+    if (!activeItem) return;
     activeItem.focus({ preventScroll: true });
     activeItem.scrollIntoView({
       block: 'nearest',
@@ -562,7 +563,7 @@ export function createSessionHelpModal(
 
       const shortcutSections = buildOverlayShortcutSections(shortcuts);
       if (shortcutSections.length > 0) {
-        shortcutSections[0].title = 'Overlay shortcuts (configurable)';
+        shortcutSections[0]!.title = 'Overlay shortcuts (configurable)';
       }
       const colorSection = buildColorSection(styleConfig ?? {});
       helpSections = [...bindingSections, ...shortcutSections, colorSection];
diff --git a/src/renderer/subtitle-render.ts b/src/renderer/subtitle-render.ts
index 43491f2..41abf36 100644
--- a/src/renderer/subtitle-render.ts
+++ b/src/renderer/subtitle-render.ts
@@ -145,10 +145,11 @@ function renderWithTokens(
     if (surface.includes('\n')) {
       const parts = surface.split('\n');
       for (let i = 0; i < parts.length; i += 1) {
-        if (parts[i]) {
+        const part = parts[i];
+        if (part) {
           const span = document.createElement('span');
           span.className = computeWordClass(token, resolvedFrequencyRenderSettings);
-          span.textContent = parts[i];
+          span.textContent = part;
           if (token.reading) span.dataset.reading = token.reading;
           if (token.headword) span.dataset.headword = token.headword;
           fragment.appendChild(span);
@@ -277,7 +278,7 @@ function renderPlainTextPreserveLineBreaks(root: ParentNode, text: string): void
   const fragment = document.createDocumentFragment();
 
   for (let i = 0; i < lines.length; i += 1) {
-    fragment.appendChild(document.createTextNode(lines[i]));
+    fragment.appendChild(document.createTextNode(lines[i] ?? ''));
     if (i < lines.length - 1) {
       fragment.appendChild(document.createElement('br'));
     }
@@ -361,8 +362,9 @@ export function createSubtitleRenderer(ctx: RendererContext) {
 
     const lines = normalized.split('\n');
     for (let i = 0; i < lines.length; i += 1) {
-      if (lines[i]) {
-        ctx.dom.secondarySubRoot.appendChild(document.createTextNode(lines[i]));
+      const line = lines[i];
+      if (line) {
+        ctx.dom.secondarySubRoot.appendChild(document.createTextNode(line));
       }
       if (i < lines.length - 1) {
         ctx.dom.secondarySubRoot.appendChild(document.createElement('br'));
diff --git a/src/runtime-options.ts b/src/runtime-options.ts
index 02d64f6..0b1ce70 100644
--- a/src/runtime-options.ts
+++ b/src/runtime-options.ts
@@ -47,7 +47,7 @@ function setPathValue(target: Record<string, unknown>, path: string, value: unkn
   const parts = path.split('.');
   let current = target;
   for (let i = 0; i < parts.length; i += 1) {
-    const part = parts[i];
+    const part = parts[i]!;
     const isLeaf = i === parts.length - 1;
     if (isLeaf) {
       current[part] = value;
@@ -188,7 +188,7 @@ export class RuntimeOptionsManager {
       direction === 1
         ? (safeIndex + 1) % values.length
         : (safeIndex - 1 + values.length) % values.length;
-    return this.setOptionValue(id, values[nextIndex]);
+    return this.setOptionValue(id, values[nextIndex]!);
   }
 
   getEffectiveAnkiConnectConfig(baseConfig?: AnkiConnectConfig): AnkiConnectConfig {
diff --git a/src/token-merger.ts b/src/token-merger.ts
index 54a89a3..8d7d40f 100644
--- a/src/token-merger.ts
+++ b/src/token-merger.ts
@@ -307,6 +307,7 @@ export function markNPlusOneTargets(tokens: MergedToken[], minSentenceWords = 3)
     let sentenceWordCount = 0;
     for (let i = start; i < endExclusive; i++) {
       const token = markedTokens[i];
+      if (!token) continue;
       if (!isSentenceBoundaryToken(token) && token.surface.trim().length > 0) {
         sentenceWordCount += 1;
       }
@@ -317,8 +318,8 @@ export function markNPlusOneTargets(tokens: MergedToken[], minSentenceWords = 3)
     }
 
     if (sentenceWordCount >= minimumSentenceWords && sentenceCandidates.length === 1) {
-      markedTokens[sentenceCandidates[0]] = {
-        ...markedTokens[sentenceCandidates[0]],
+      markedTokens[sentenceCandidates[0]!] = {
+        ...markedTokens[sentenceCandidates[0]!]!,
         isNPlusOneTarget: true,
       };
     }
@@ -326,6 +327,7 @@ export function markNPlusOneTargets(tokens: MergedToken[], minSentenceWords = 3)
 
   for (let i = 0; i < markedTokens.length; i++) {
     const token = markedTokens[i];
+    if (!token) continue;
     if (isSentenceBoundaryToken(token)) {
       markSentence(sentenceStart, i);
       sentenceStart = i + 1;
diff --git a/src/window-trackers/macos-tracker.ts b/src/window-trackers/macos-tracker.ts
index 6fd2928..13b9272 100644
--- a/src/window-trackers/macos-tracker.ts
+++ b/src/window-trackers/macos-tracker.ts
@@ -177,10 +177,10 @@ export class MacOSWindowTracker extends BaseWindowTracker {
         if (result && result !== 'not-found') {
           const parts = result.split(',');
           if (parts.length === 4) {
-            const x = parseInt(parts[0], 10);
-            const y = parseInt(parts[1], 10);
-            const width = parseInt(parts[2], 10);
-            const height = parseInt(parts[3], 10);
+            const x = parseInt(parts[0]!, 10);
+            const y = parseInt(parts[1]!, 10);
+            const width = parseInt(parts[2]!, 10);
+            const height = parseInt(parts[3]!, 10);
 
             if (
               Number.isFinite(x) &&
diff --git a/src/window-trackers/x11-tracker.ts b/src/window-trackers/x11-tracker.ts
index 339d8cc..43274d0 100644
--- a/src/window-trackers/x11-tracker.ts
+++ b/src/window-trackers/x11-tracker.ts
@@ -47,10 +47,10 @@ export function parseX11WindowGeometry(winInfo: string): {
     return null;
   }
   return {
-    x: parseInt(xMatch[1], 10),
-    y: parseInt(yMatch[1], 10),
-    width: parseInt(widthMatch[1], 10),
-    height: parseInt(heightMatch[1], 10),
+    x: parseInt(xMatch[1]!, 10),
+    y: parseInt(yMatch[1]!, 10),
+    width: parseInt(widthMatch[1]!, 10),
+    height: parseInt(heightMatch[1]!, 10),
   };
 }
 
@@ -59,7 +59,7 @@ export function parseX11WindowPid(raw: string): number | null {
   if (!pidMatch) {
     return null;
   }
-  const pid = Number.parseInt(pidMatch[1], 10);
+  const pid = Number.parseInt(pidMatch[1]!, 10);
   return Number.isInteger(pid) ? pid : null;
 }
 
diff --git a/tsconfig.json b/tsconfig.json
index 352ec68..2a5c86c 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -2,10 +2,14 @@
   "compilerOptions": {
     "target": "ES2022",
     "module": "commonjs",
-    "lib": ["ES2022", "DOM"],
+    "lib": ["ES2022", "DOM", "DOM.Iterable"],
     "outDir": "./dist",
     "rootDir": "./src",
     "strict": true,
+    "noUncheckedIndexedAccess": true,
+    "noImplicitOverride": true,
+    "moduleDetection": "force",
+    "isolatedModules": true,
     "esModuleInterop": true,
     "skipLibCheck": true,
     "forceConsistentCasingInFileNames": true,