Silent failure of inter-function fetch call

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1

My application has a multi-step process for analyzing uploaded files. The final step, which involves a fetch call from the /api/upload-complete serverless function to the /api/process serverless function, is failing silently.

Flow:

  1. Client uploads a file, which successfully creates a job (/api/jobs).
  2. File is successfully uploaded to Vercel Blob.
  3. Client calls /api/upload-complete. This function logs that it is successful and attempts to trigger /api/process via fetch.
  4. The Vercel logs show the /api/upload-complete function completes with a 200 OK status.
  5. However, no log entries for /api/process are ever created. The request appears to vanish.

Debugging Steps Taken: We have exhaustively debugged every potential cause with the help of Gemini, including:

  1. Environment Variables: Confirmed that BLOB_READ_WRITE_TOKEN, INTERNAL_TRIGGER_SECRET, KV_REST_API_URL, and KV_REST_API_TOKEN are all present and correctly configured in the production environment.
  2. KV Store: Created and linked a Vercel KV (Upstash) database and confirmed the KV_ prefix is correct. The database shows zero usage, confirming it’s never being reached.
  3. Function Runtime: Forced all relevant API routes (/api/jobs, /api/upload, /api/upload-complete, /api/process) to use the Node.js runtime via export const runtime = 'nodejs';. Vercel logs confirm they are running on Node.js 22.x.
  4. Testing the Target Function: We have proven the /api/process function is alive and deployed correctly by sending a GET request to it directly, which correctly returns a 405 Method Not Allowed error, and this error does appear in the Vercel logs.

QUestion: The /api/process function is reachable, but POST requests sent from other serverless functions within the same project are being dropped without any error or log record. This points to a potential platform-level issue with internal networking or request processing. Please investigate why the fetch call from /api/upload-complete to /api/process is failing to invoke the target function.