nextjs - 💡(How to fix) Fix Next.js 16 App Router: API routes hang on Vercel (504) while pages work -- both Turbopack and webpack [2 comments, 2 participants]

export async function GET() {
  return new Response(JSON.stringify({ pong: true, ts: Date.now() }), {
    status: 200,
    headers: { "Content-Type": "application/json" },
  });
}

Tested with:
- next build (Turbopack) → hangs
- next build --webpack → hangs
- Deleting proxy.ts entirely → hangs
- Emptying next.config.ts → hangs
- serverExternalPackages for 5+ packages → hangs
- Lazy dynamic import() for all heavy deps → hangs
- Node.js 20.x and 24.x → hangs
- Deployment Protection disabled → hangs
- Fresh Vercel project → hangs

Zero runtime logs appear for API routes — the serverless function code never executes.

### Current vs. Expected behavior

**Current:** All App Router API routes return 504 (gateway timeout). Zero runtime logs —
serverless function code never executes. Even a zero-import `/api/ping` hangs. Pages (SSR) work perfectly.

**Expected:** API routes should execute and return responses, just like they do in the minimal repro and in local development.

**Key evidence this is a platform/framework issue, not application code:**
- The same `/api/ping` with zero imports works in the minimal repro but hangs in the full project
- Pages work fine in the same deployment
- Both Turbopack AND webpack produce the same hang
- Deleting proxy.ts (no middleware at all) still hangs
- Runtime logs show zero entries -- the function is never invoked

The difference between working and broken: the number of routes/source files importing heavy packages. The minimal repro has few routes. The full project has ~70 API routes + ~60 pages. The hang appears to be caused by the serverless function initialization phase (shared chunks or runtime bootstrap), not by route handler code.

### Provide environment information

```bash
Operating System:
  Platform: darwin
  Arch: arm64
Next.js version: 16.1.6
Node.js version: 20.x (also tested 24.x)
Package manager: pnpm 10.29.3
Monorepo: Turborepo with pnpm workspaces