It is already doable (not straight forward tbh)

const fastify = require('fastify')({
  logger: true,
  requestIdHeader: 'jdnsndfjegrnjsdnij',
  genReqId() {
    return undefined
  },
})
fastify.get('/', async (request, reply) => {
  return { hello: 'world' }
})

fastify.inject('/')

What I understood is that you don't want to log the reqId field

@Eomm Thank you for the quick reply.
What I meant was to disable requestIdHeader overwriting genReqId(), so untrusted requests can not overwrite my reqId.

This is the internal code:
const id = req.headers[requestIdHeader] || genReqId(req)

The only way is to (like you did) change the requestIdHeader to a bogus value. Which is honestly secure enough. Just not very clean in my opinion.

Oh, I got it! The issue here is to choose the priority of the genId option over the header 👍🏼

Yeah, I agree!

Would you like to submit a pr?

I think a better implementation is to move that || switch inside genReqId itself: https://github.com/fastify/fastify/blob/main/lib/reqIdGenFactory.js.

Would you like to send a PR?

@mcollina Sorry, I don't think I understand how/what you mean.

Simply req.headers[requestIdHeader] || genReqId(req) inside genReqId(factory) would obviously get overwritten by any custom genRegId(setup), therefore removing the requestIdHeader functionality. So, I don't think you mean it this way.

Just move the check inside genReqId(req).

Okey, I have a PR, it's my first one ever. It does not use the reqIdGenFactory because I could not get it to work using the factory just yet. Sorry. Have a look at it maybe, it's to hackish...

Maybe in 2 weeks I have time to dig deeper...