How to increase timeout limit on Vercel Serverless functions

I have an API endpoint on a NextJS project that needs longer than 60s to run. I'm on a pro Vercel plan but for some reason cannot get the timeout limit to increase.

At the endpoint itself I've tried

export const maxDuration = 300
export const dynamic = 'force-dynamic'

which doesn't seem to do anything, I also tried adding a vercel.json file at the top level (above /src) like so:

{
    "functions": {
        "pages/api/**": {
            "memory": 3008,
            "maxDuration": 300
        },
    }
}

Which again isn't working. I've combed through the documentation (mostly here) and also a handful of threads (one example), none of which have helped.

I'm running NextJs version 13.5.6, am definitely on a pro plan, and Node v18, what am I doing wrong? Am really unsure of what else to try.

What you've done seems to be a mix of the Pages Router way the and the App Router way of doing it. Try to do it like in this example below:

With Pages Router (/pages/api/my-function/handler.ts):

export const config = {
  maxDuration: 5, // 5 seconds
};

export default function handler(
  request: NextApiRequest,
  response: NextApiResponse,
) {
  response.status(200).json({});
}

With App Router (/app/api/my-function/route.ts):

export const maxDuration = 5; // 5 seconds
export const dynamic = 'force-dynamic';

export function GET(request: Request) {
  return new Response('{}', { status: 200 });
}

If you want to override the maximum duration of a function for your whole project when using Vercel, you can do it in vercel.json (I think this is only valid for the App Router):

{
  "functions": {
    "app/api/**/*": {
      "maxDuration": 5
    }
  }
}

You can read more about all this on the official docs.