diff --git a/src/modules/ai-job/ai-job-lifecycle.repository.spec.ts b/src/modules/ai-job/ai-job-lifecycle.repository.spec.ts
index 6473c63..f275d07 100644
--- a/src/modules/ai-job/ai-job-lifecycle.repository.spec.ts
+++ b/src/modules/ai-job/ai-job-lifecycle.repository.spec.ts
@@ -369,22 +369,42 @@ describe('AiJobLifecycleRepository', () => {
       expect(result.status).toBe('cancelled');
     });
 
-    it('running Job 设置 cancelRequestedAt 信号', async () => {
+    it('running Job CAS 写入 cancelRequestedAt 信号', async () => {
       (prisma.aiJob.findUnique as jest.Mock).mockResolvedValue({ lifecycleStatus: 'running' });
-      (prisma.aiJob.update as jest.Mock).mockResolvedValue({
-        id: 'job-001',
-        lifecycleStatus: 'running',
-        cancelRequestedAt: new Date(),
-      });
+      // updateMany (CAS) succeeds
+      (prisma.aiJob.updateMany as jest.Mock).mockResolvedValue({ count: 1 });
 
       const result = await repo.requestCancellation('job-001');
       expect(result.status).toBe('cancel_requested');
-      expect(prisma.aiJob.update).toHaveBeenCalledWith(
+      expect(prisma.aiJob.updateMany).toHaveBeenCalledWith(
         expect.objectContaining({
           where: { id: 'job-001', lifecycleStatus: 'running' },
+          data: { cancelRequestedAt: expect.any(Date) },
         }),
       );
     });
+
+    it('running Job TOCTOU：CAS 失败且已变为 cancelled → 幂等返回', async () => {
+      (prisma.aiJob.findUnique as jest.Mock).mockResolvedValue({ lifecycleStatus: 'running' });
+      // CAS fails (job already cancelled between read and write)
+      (prisma.aiJob.updateMany as jest.Mock).mockResolvedValue({ count: 0 });
+      // Re-read: already cancelled
+      (prisma.aiJob.findUnique as jest.Mock).mockResolvedValueOnce({ lifecycleStatus: 'running' });
+      // The second findUnique call (after CAS failure)
+      (prisma.aiJob.findUnique as jest.Mock).mockResolvedValueOnce({ lifecycleStatus: 'cancelled' });
+
+      const result = await repo.requestCancellation('job-001');
+      expect(result.status).toBe('cancelled'); // 幂等返回
+    });
+
+    it('running Job TOCTOU：CAS 失败且已变为 succeeded → 抛出错误', async () => {
+      (prisma.aiJob.findUnique as jest.Mock).mockResolvedValue({ lifecycleStatus: 'running' });
+      (prisma.aiJob.updateMany as jest.Mock).mockResolvedValue({ count: 0 });
+      // Re-read: already succeeded (terminal)
+      (prisma.aiJob.findUnique as jest.Mock).mockResolvedValueOnce({ lifecycleStatus: 'succeeded' });
+
+      await expect(repo.requestCancellation('job-001')).rejects.toThrow(JobNotCancellableError);
+    });
   });
 
   describe('findByLifecycleStatus', () => {
diff --git a/src/modules/ai-job/ai-job-lifecycle.repository.ts b/src/modules/ai-job/ai-job-lifecycle.repository.ts
index 15fcaf8..97687ce 100644
--- a/src/modules/ai-job/ai-job-lifecycle.repository.ts
+++ b/src/modules/ai-job/ai-job-lifecycle.repository.ts
@@ -375,17 +375,38 @@ export class AiJobLifecycleRepository {
     const status = this.stateMachine.parse(job.lifecycleStatus);
 
     if (status === 'queued') {
-      // 直接取消
+      // 直接取消（markCancelled 内部已有 CAS + 错误分类）
       await this.markCancelled(jobId);
       return { status: 'cancelled' };
     }
 
     if (status === 'running') {
-      // 设置取消信号
-      await this.prisma.aiJob.update({
+      // CAS 写入 cancelRequestedAt — 只有 lifecycleStatus 仍为 running 时才成功。
+      // 使用 updateMany 避免 read→write 之间的 TOCTOU 竞态窗口。
+      const result = await this.prisma.aiJob.updateMany({
         where: { id: jobId, lifecycleStatus: 'running' },
         data: { cancelRequestedAt: new Date() },
       });
+
+      if (result.count === 0) {
+        // 在 findUnique 和 updateMany 之间 Job 已进入终态或被其他 Worker 修改。
+        // 重新读取以返回精确错误。
+        const current = await this.prisma.aiJob.findUnique({
+          where: { id: jobId },
+          select: { lifecycleStatus: true },
+        });
+        const currentStatus = current?.lifecycleStatus ?? 'unknown';
+        if (currentStatus === 'cancelled') {
+          return { status: 'cancelled' }; // 已被其他路径取消，幂等返回
+        }
+        if (this.stateMachine.isTerminal(
+          this.stateMachine.parse(currentStatus),
+        )) {
+          throw new JobNotCancellableError(jobId, currentStatus);
+        }
+        throw new JobNotCancellableError(jobId, currentStatus);
+      }
+
       return { status: 'cancel_requested' };
     }