diff --git a/src/app/api/audiobook/route.ts b/src/app/api/audiobook/route.ts
index 9d6d0f3..15aa4cb 100644
--- a/src/app/api/audiobook/route.ts
+++ b/src/app/api/audiobook/route.ts
@@ -5,7 +5,7 @@ import { existsSync, createReadStream } from 'fs';
 import { basename, join, resolve } from 'path';
 import { randomUUID } from 'crypto';
 import { AUDIOBOOKS_V1_DIR, isAudiobooksV1Ready } from '@/lib/server/docstore';
-import { encodeChapterFileName, encodeChapterTitleTag, listStoredChapters, ffprobeAudio } from '@/lib/server/audiobook';
+import { encodeChapterFileName, encodeChapterTitleTag, listStoredChapters, ffprobeAudio, escapeFFMetadata } from '@/lib/server/audiobook';
 import type { TTSAudioBytes, TTSAudiobookFormat } from '@/types/tts';
 import type { AudiobookGenerationSettings } from '@/types/client';
 
@@ -178,6 +178,9 @@ export async function POST(request: NextRequest) {
     // Create intermediate directory
     await mkdir(intermediateDir, { recursive: true });
 
+    const existingChapters = await listStoredChapters(intermediateDir, request.signal);
+    const hasChapters = existingChapters.length > 0;
+
     const metaPath = join(intermediateDir, 'audiobook.meta.json');
     const incomingSettings = data.settings;
     let existingSettings: AudiobookGenerationSettings | null = null;
@@ -187,7 +190,9 @@ export async function POST(request: NextRequest) {
       existingSettings = null;
     }
 
-    if (existingSettings) {
+    // Only enforce mismatch check if we already have generated chapters.
+    // If no chapters exist, we can overwrite/ignore "existing" settings (which might be stale or partial).
+    if (existingSettings && hasChapters) {
       if (incomingSettings) {
         const mismatch =
           existingSettings.ttsProvider !== incomingSettings.ttsProvider ||
@@ -203,11 +208,9 @@ export async function POST(request: NextRequest) {
           );
         }
       }
-    } else if (incomingSettings) {
-      await writeFile(metaPath, JSON.stringify(incomingSettings, null, 2));
     }
-
-    const existingChapters = await listStoredChapters(intermediateDir, request.signal);
+    // Note: We deliberately do NOT write the meta file here yet.
+    // We wait until a chapter is successfully generated/saved below.
     const existingFormats = new Set(existingChapters.map((c) => c.format));
     if (existingFormats.size > 1) {
       return NextResponse.json(
@@ -224,10 +227,15 @@ export async function POST(request: NextRequest) {
     const rawPostSpeed = incomingSettings?.postSpeed ?? existingSettings?.postSpeed ?? 1;
     const postSpeed = Number.isFinite(Number(rawPostSpeed)) ? Number(rawPostSpeed) : 1;
 
+    // Use provided chapter index or find the next available index robustly (handles gaps)
     // Use provided chapter index or find the next available index robustly (handles gaps)
     let chapterIndex: number;
     if (data.chapterIndex !== undefined) {
-      chapterIndex = data.chapterIndex;
+      const normalized = Number(data.chapterIndex);
+      if (!Number.isInteger(normalized) || normalized < 0) {
+        return NextResponse.json({ error: 'Invalid chapterIndex parameter' }, { status: 400 });
+      }
+      chapterIndex = normalized;
     } else {
       const indices = existingChapters.map((c) => c.index);
       // Find smallest non-negative integer not present
@@ -435,7 +443,7 @@ export async function GET(request: NextRequest) {
       currentTime += chapter.duration;
       const endMs = Math.floor(currentTime * 1000);
 
-      metadata.push('[CHAPTER]', 'TIMEBASE=1/1000', `START=${startMs}`, `END=${endMs}`, `title=${chapter.title}`);
+      metadata.push('[CHAPTER]', 'TIMEBASE=1/1000', `START=${startMs}`, `END=${endMs}`, `title=${escapeFFMetadata(chapter.title)}`);
     }
     
     await writeFile(metadataPath, ';FFMETADATA1\n' + metadata.join('\n'));
diff --git a/src/lib/server/audiobook.ts b/src/lib/server/audiobook.ts
index 2ef5c7a..f32a2a5 100644
--- a/src/lib/server/audiobook.ts
+++ b/src/lib/server/audiobook.ts
@@ -23,6 +23,15 @@ function sanitizeFileStem(value: string): string {
     .slice(0, 180);
 }
 
+export function escapeFFMetadata(value: string): string {
+  return value
+    .replace(/\\/g, '\\\\')
+    .replace(/=/g, '\\=')
+    .replace(/;/g, '\\;')
+    .replace(/#/g, '\\#')
+    .replace(/\r|\n/g, ' ');
+}
+
 export function encodeChapterTitleTag(index: number, title: string): string {
   const safeTitle = sanitizeTagValue(title) || `Chapter ${index + 1}`;
   const prefix = String(index + 1).padStart(4, '0');
diff --git a/tests/unit/audiobook.spec.ts b/tests/unit/audiobook.spec.ts
new file mode 100644
index 0000000..1d990ad
--- /dev/null
+++ b/tests/unit/audiobook.spec.ts
@@ -0,0 +1,32 @@
+import { test, expect } from '@playwright/test';
+import { escapeFFMetadata } from '../../src/lib/server/audiobook';
+
+test.describe('escapeFFMetadata', () => {
+  test('escapes special characters correctly', () => {
+    const input = 'Title with = ; # and backslash \\';
+    // Expected: Equal -> \=, Semicolon -> \;, Hash -> \#, Backslash -> \\
+    const expected = 'Title with \\= \\; \\# and backslash \\\\';
+    expect(escapeFFMetadata(input)).toBe(expected);
+  });
+
+  test('normalizes newlines to spaces', () => {
+    const input = 'Title with\nnewline and\rreturn';
+    const expected = 'Title with newline and return';
+    expect(escapeFFMetadata(input)).toBe(expected);
+  });
+
+  test('handles mixed special characters and newlines', () => {
+    const input = 'Line1\nLine2=Value;Comment#';
+    const expected = 'Line1 Line2\\=Value\\;Comment\\#';
+    expect(escapeFFMetadata(input)).toBe(expected);
+  });
+
+  test('returns empty string as-is', () => {
+    expect(escapeFFMetadata('')).toBe('');
+  });
+
+  test('returns safe string as-is', () => {
+    const input = 'Safe Title 123';
+    expect(escapeFFMetadata(input)).toBe(input);
+  });
+});
diff --git a/tests/docstore_unit.spec.ts b/tests/unit/docstore.spec.ts
similarity index 96%
rename from tests/docstore_unit.spec.ts
rename to tests/unit/docstore.spec.ts
index ccd4879..55a57e7 100644
--- a/tests/docstore_unit.spec.ts
+++ b/tests/unit/docstore.spec.ts
@@ -1,5 +1,5 @@
 import { test, expect } from '@playwright/test';
-import { getMigratedDocumentFileName } from '@/lib/server/docstore';
+import { getMigratedDocumentFileName } from '../../src/lib/server/docstore';
 
 test.describe('Docstore Filename Safety', () => {
   const id = 'a'.repeat(64); // Simulate sha256 hex ID