let's say we have this sample code.

import { createError } from '@fastify/error';

const app = fastify();

const MY_CUSTOM_ERROR = createError('MY_CUSTOM_ERROR', 'description', 400);

app.route({
  method: 'POST',
  url: '/',
  schema: {
    body: { type: "object", properties: { foo: { type: "string" } } }
  },
  handler() {
    throw new MY_CUSTOM_ERROR();
  }
})

in case of validation error, the response body would be like this:

{
  "statusCode": 400,
  "error": "Bad Request",
  "message": "body must have required property 'foo'"
}

but in case of throwing my custom error, the response body would be like this:

{
  "statusCode": 400,
  "code": "MY_CUSTOM_ERROR"
  "error": "Bad Request",
  "message": "description"
}

as you can see, code key is not available in framework level errors.
this behavior is already confused our front-end developers and i'm kinda agree with them.
what's is your suggestion? I think we could add code key on every framework level errors