diff --git a/src/find-issue.ts b/src/find-issue.ts
index 2b45474..88f19fc 100644
--- a/src/find-issue.ts
+++ b/src/find-issue.ts
@@ -1,13 +1,18 @@
 // src/find-issue.ts
-import { readFileSync } from 'node:fs';
 import { findMarkedIssue } from './issue.js';
+import { readJsonOrNull } from './finalize.js';
 import { ISSUE_MARKER } from './report-issue.js';
 import type { IssueRef } from './issue.js';
 
+// 畸形輸入（缺檔、壞 JSON、非陣列）一律降級為「找不到」→ 空字串
+export function findIssueNumber(input: unknown): string {
+  if (!Array.isArray(input)) return '';
+  const n = findMarkedIssue(input as IssueRef[], ISSUE_MARKER);
+  return n === null ? '' : String(n);
+}
+
 // CLI: find-issue.ts <issuesJsonFile>
 // 檔內容為 `gh issue list --json number,body` 的 JSON 陣列；印出帶 marker 的 number（無則印空字串）
 if (import.meta.url === `file://${process.argv[1]}`) {
-  const issues = JSON.parse(readFileSync(process.argv[2], 'utf8')) as IssueRef[];
-  const n = findMarkedIssue(issues, ISSUE_MARKER);
-  process.stdout.write(n === null ? '' : String(n));
+  process.stdout.write(findIssueNumber(readJsonOrNull(process.argv[2])));
 }
diff --git a/test/find-issue.test.ts b/test/find-issue.test.ts
new file mode 100644
index 0000000..538dd75
--- /dev/null
+++ b/test/find-issue.test.ts
@@ -0,0 +1,28 @@
+// test/find-issue.test.ts
+import { describe, it, expect } from 'vitest';
+import { writeFileSync, mkdtempSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { findIssueNumber } from '../src/find-issue.js';
+import { readJsonOrNull } from '../src/finalize.js';
+import { ISSUE_MARKER } from '../src/report-issue.js';
+
+describe('findIssueNumber', () => {
+  it('returns the marked issue number as a string', () => {
+    expect(findIssueNumber([{ number: 7, body: `x\n${ISSUE_MARKER}` }, { number: 1, body: 'y' }])).toBe('7');
+  });
+  it('returns empty string when no issue carries the marker', () => {
+    expect(findIssueNumber([{ number: 1, body: 'x' }])).toBe('');
+  });
+  it('degrades to empty string on null input (missing/malformed file)', () => {
+    expect(findIssueNumber(null)).toBe('');
+  });
+  it('degrades to empty string on non-array JSON', () => {
+    expect(findIssueNumber({ number: 7, body: ISSUE_MARKER })).toBe('');
+  });
+  it('degrades instead of throwing when fed a malformed JSON file via readJsonOrNull', () => {
+    const file = join(mkdtempSync(join(tmpdir(), 'find-issue-')), 'issues.json');
+    writeFileSync(file, '{not json');
+    expect(findIssueNumber(readJsonOrNull(file))).toBe('');
+  });
+});