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Vercel functions errorVercel FUNCTION_INVOCATION_FAILED (500 Internal Server Error)

An API route or server function works locally, and on Vercel it returns 500 with FUNCTION_INVOCATION_FAILED. Vercel’s page lists the causes in a few lines. Here they are in Vercel’s words, with what to check first, or hand it over at a fixed price.

Short answer

The function crashed while handling the request. Open the deployment’s Logs tab and find the error from the failing request. Typical causes are an error your code didn’t catch, or a setting such as an environment variable that’s missing in production. Fix that, wrap the handler in try/catch so it returns a real error response, and redeploy.

What makes the function fail, and the fix

  1. 01

    An error nothing caught

    “Node.js or Bun threw an unhandled rejection/uncaught exception.” Vercel, FUNCTION_INVOCATION_FAILED

    An exception or a rejected promise that nothing handles stops the function, and Vercel answers with this 500. The stack trace is under the Logs tab. Catch errors in the handler and return a response with a clear status, and await every promise so a rejection lands in your try/catch.

  2. 02

    A setting the function needs is missing

    “Double-check the function configuration to ensure that it's set up correctly, including any environment variables or other settings” Vercel, FUNCTION_INVOCATION_FAILED

    Vercel’s checklist includes verifying the function’s configuration and environment variables. Code that reads an API key or database URL fails when that variable isn’t set for Production. Variables are set per environment, and a change only applies to new deployments, so check the variable and then redeploy.

  3. 03

    The runtime itself crashed

    “The runtime process (Node.js, Bun, Python, etc.) has crashed.” Vercel, FUNCTION_INVOCATION_FAILED

    Sometimes the process running your code stops, not just the request. Vercel says to check the code for errors and infinite loops.

Checked against the vendor’s docs on : Vercel docs: FUNCTION_INVOCATION_FAILED; Vercel docs: Environment variables.

Rather hand it over?

Fixed scope, fixed price, paid upfront. The work happens in writing; no call needed.

Related: Vercel function timeout (504) · Vercel env variables undefined · Vercel Module not found · Vercel 413 payload too large

Questions

More on this error

Is this the same as a 504 timeout?

No. FUNCTION_INVOCATION_FAILED is a 500: the function failed while running. A function that runs too long is a separate timeout error, with its own fix page linked below.

Why does it work on my machine?

Locally your code reads your own environment variables and runs on your setup. On Vercel it gets only the variables set for that environment, so start with the logs and the Production variables.

Can you fix it for me?

If one bug is the whole problem, the App Fix Ticket covers it: a written diagnosis first, then the fix. If the app has more going on, the Launch Check is a written check of the whole app. Each issue found is rated critical, high, medium or low, with where it is and how it was confirmed. No issues worth fixing, no charge. If the Launch Check finds nothing rated medium or higher, Simon refunds the full $450. You keep the report either way.

A different error?

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Send the error and what you were doing. You get a written scope, a fixed price and a timeline; we reply by email, usually within one business day.

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