Hope this is the right place since there doesn't seem to be a vercel discord server

So, I tried adding a cron job to my nextjs project. I followed the doc to the letter, meaning I added an api handler with the following content:

src/pages/api/cron.ts

import { doCron } from '@/server/routers/cron'
import type { NextApiRequest, NextApiResponse } from 'next'
 
export default async function handler(req: NextApiRequest, res: NextApiResponse) {
    if (req.headers.authorization !== `Bearer ${process.env.CRON_SECRET}`) {
        return res.status(401).end('Unauthorized');
    }
    
    try {
        await doCron()
        res.status(200).json({ status: 'ok' })
    } catch (err) {
        console.log(err)
        res.status(400).json({ status: 'ko' })
    }
}

Then added a vercel.json file with the following content:

vercel.json

{
    "crons": [
      {
        "path": "/api/cron",
        "schedule": "0 */4 * * *"
      }
    ]
  }

However, upon pushing the changes, I saw an error message on github with Vercel's icon, telling me "all checks have failed". There's a link that says "details", but clicking it does nothing.
I tried looking at my vercel deployment history and logs, to see if they had more detail, but the new commit doesn't even appear in the deployment history on vercel.

I tried removing the vercel.json file, and suddenly, the commit was deployed as usual. Then I re-created it, and deployments broke once again.
I passed my vercel.json file through both a JSON validator, and a CRON validator, just in case, but both told me everything was okay.

Any idea what I might have missed here? Clearly my vercel.json file is wrong, but I haven't been able to find why and what the fix would be.