fix(uploads): send an explicit Cache-Control on served files

backend/__tests__/unit/routes/agentsRuntime.fileContentType.test.js

@@ -192,3 +192,29 @@ describe('pod file read — content type classification', () => {
     },
   );
 });
+
+// Both byte sources return the bytes in this JSON body, so both must send the
+// same Cache-Control header.
+describe('pod file read — Cache-Control', () => {
+  beforeEach(() => jest.clearAllMocks());
+
+  test('object-store bytes are sent private, no-store', async () => {
+    serveFile({ contentType: 'text/plain', bytes: Buffer.from('STORE_SENTINEL_DELTA\n') });
+    const res = await read();
+
+    expect(res.status).toBe(200);
+    expect(res.body.content).toBe('STORE_SENTINEL_DELTA\n');
+    expect(res.headers['cache-control']).toBe('private, no-store');
+  });
+
+  test('legacy inline bytes are sent private, no-store', async () => {
+    serveFile({ contentType: 'text/plain', bytes: Buffer.from('unused') });
+    getObjectStore().get.mockResolvedValue(null);
+    File.findByFileName.mockResolvedValue({ data: Buffer.from('LEGACY_SENTINEL_EPSILON\n') });
+    const res = await read();
+
+    expect(res.status).toBe(200);
+    expect(res.body.content).toBe('LEGACY_SENTINEL_EPSILON\n');
+    expect(res.headers['cache-control']).toBe('private, no-store');
+  });
+});

backend/__tests__/unit/routes/pods.external-links.test.js

@@ -291,3 +291,31 @@ describe('pods routes - external links', () => {
     await request(app).get('/api/pods/p1/external-links').expect(404);
   });
 });
+
+describe('pods routes - WeChat QR image', () => {
+  let app;
+  let headerAtSend;
+  beforeEach(() => {
+    headerAtSend = undefined;
+    app = express();
+    app.use(express.json());
+    app.use((req, res, next) => {
+      // send() only adds its public default when no header is set, so record
+      // what the route had set at the moment it handed the file over.
+      res.sendFile = jest.fn(() => {
+        headerAtSend = res.getHeader('Cache-Control');
+        res.status(200).end();
+      });
+      next();
+    });
+    app.use('/api/pods', routes);
+    jest.clearAllMocks();
+  });
+
+  it('is sent private, no-store', async () => {
+    ExternalLink.findById.mockResolvedValue({ type: 'wechat', qrCodePath: '/uploads/qr.png', podId: 'p1' });
+    Pod.findById.mockResolvedValue({ members: [{ toString: () => 'user1' }] });
+    await request(app).get('/api/pods/external-link/l1/qrcode').expect(200);
+    expect(headerAtSend).toBe('private, no-store');
+  });
+});

backend/__tests__/unit/routes/uploads.test.js

@@ -85,6 +85,7 @@ describe('uploads GET /:fileName (ADR-002 Phase 1)', () => {
       })
       .expect(200);
     expect(res.headers['content-type']).toMatch(/image\/png/);
+    expect(res.headers['cache-control']).toBe('private, no-store');
     expect(Buffer.isBuffer(res.body) ? res.body : Buffer.from(res.body)).toEqual(
       Buffer.from('hello'),
     );
@@ -195,6 +196,7 @@ describe('uploads GET /:fileName (ADR-002 Phase 1)', () => {
 
     const res = await request(app).get('/api/uploads/legacy.jpg').expect(200);
     expect(res.headers['content-type']).toMatch(/image\/jpeg/);
+    expect(res.headers['cache-control']).toBe('private, no-store');
     expect(res.body.toString()).toBe('legacy-bytes');
     expect(File.findByFileName).toHaveBeenCalledWith('legacy.jpg');
   });

backend/routes/agentsRuntime.ts

@@ -2104,6 +2104,8 @@ router.get('/pods/:podId/files/:fileName/content', phase4RateLimit, agentRuntime
     if (!buffer) {
       return res.status(404).json({ message: 'File content not found' });
     }
+    // Set explicitly on every response that carries file bytes, so the header never depends on a default.
+    res.set('Cache-Control', 'private, no-store');
 
     const ct = String(file.contentType || '');
     const isText = isTextualContentType(ct);

backend/routes/pods.ts

@@ -52,6 +52,7 @@ interface AuthReq {
 interface Res {
   status: (n: number) => Res;
   json: (d: unknown) => void;
+  set: (field: string, value: string) => void;
   sendFile: (path: string) => void;
 }
 
@@ -324,6 +325,8 @@ router.get('/external-link/:linkId/qrcode', auth, async (req: AuthReq, res: Res)
     if (!link || link.type !== 'wechat' || !link.qrCodePath) return res.status(404).json({ message: 'QR code not found' });
     const pod = await Pod.findById(link.podId) as { members?: Array<{ toString: () => string }> } | null;
     if (!pod || !pod.members?.some((m) => m.toString() === req.user?.id)) return res.status(403).json({ message: 'Access denied' });
+    // Set explicitly: send() defaults to public, max-age=0 when no header is set.
+    res.set('Cache-Control', 'private, no-store');
     return res.sendFile(path.resolve(link.qrCodePath));
   } catch (error) {
     console.error('Error fetching QR code:', error);

backend/routes/uploads.ts

@@ -135,6 +135,8 @@ const setServedHeaders = (res: Res, fileName: string): void => {
   res.set('Content-Type', served.type);
   res.set('X-Content-Type-Options', 'nosniff');
   res.set('Content-Disposition', served.inline ? 'inline' : 'attachment');
+  // Set explicitly on every response that carries file bytes, so the header never depends on a default.
+  res.set('Cache-Control', 'private, no-store');
 };
 
 // Size cap is driven by the driver's declared max. multer returns 413 itself