diff --git a/src/__tests__/summarize.test.ts b/src/__tests__/summarize.test.ts
index e4107d1..3313897 100644
--- a/src/__tests__/summarize.test.ts
+++ b/src/__tests__/summarize.test.ts
@@ -239,17 +239,27 @@ describe('summarizeTaskName', () => {
     expect(result.length).toBeLessThanOrEqual(30);
   });
 
-  it('should handle mixed Japanese/English with romanization', async () => {
-    // When
-    const result = await summarizeTaskName('Add romanization', { cwd: '/project', useLLM: false });
-
-    // Then
-    expect(result).toMatch(/^[a-z0-9-]+$/);
-    expect(result).not.toMatch(/^-|-$/); // No leading/trailing hyphens
-  });
-
-  it('should use romaji by default', async () => {
-    // Given: branchNameStrategy is not set (undefined)
+  it('should handle mixed Japanese/English with romanization', async () => {
+    // When
+    const result = await summarizeTaskName('Add romanization', { cwd: '/project', useLLM: false });
+
+    // Then
+    expect(result).toMatch(/^[a-z0-9-]+$/);
+    expect(result).not.toMatch(/^-|-$/); // No leading/trailing hyphens
+  });
+
+  it('should handle very long names in romanization mode without stack overflow', async () => {
+    const result = await summarizeTaskName('a'.repeat(12000), {
+      cwd: '/project',
+      useLLM: false,
+    });
+
+    expect(result).toBe('a'.repeat(30));
+    expect(mockProviderCall).not.toHaveBeenCalled();
+  });
+
+  it('should use romaji by default', async () => {
+    // Given: branchNameStrategy is not set (undefined)
     mockResolveConfigValues.mockReturnValue({
       provider: 'claude',
       model: undefined,
diff --git a/src/infra/task/summarize.ts b/src/infra/task/summarize.ts
index 662b2fb..01f53c9 100644
--- a/src/infra/task/summarize.ts
+++ b/src/infra/task/summarize.ts
@@ -14,12 +14,33 @@ import type { SummarizeOptions } from './types.js';
 export type { SummarizeOptions };
 
 const log = createLogger('summarize');
+const MAX_ROMAJI_CHUNK_SIZE = 1024;
+
+function toRomajiSafely(text: string): string {
+  const romajiOptions = { customRomajiMapping: {} };
+  try {
+    if (text.length <= MAX_ROMAJI_CHUNK_SIZE) {
+      return wanakana.toRomaji(text, romajiOptions);
+    }
+    const convertedChunks: string[] = [];
+    for (let i = 0; i < text.length; i += MAX_ROMAJI_CHUNK_SIZE) {
+      convertedChunks.push(
+        wanakana.toRomaji(text.slice(i, i + MAX_ROMAJI_CHUNK_SIZE), romajiOptions),
+      );
+    }
+    return convertedChunks.join('');
+  } catch {
+    // Avoid blocking branch/task creation on rare parser edge cases or deep recursion
+    // with very long mixed/ASCII inputs.
+    return text;
+  }
+}
 
 /**
  * Convert Japanese text to romaji slug.
  */
 function toRomajiSlug(text: string): string {
-  const romaji = wanakana.toRomaji(text, { customRomajiMapping: {} });
+  const romaji = toRomajiSafely(text);
   return slugify(romaji);
 }