ajv-errors | Custom error messages in JSON Schemas for Ajv validator | JSON Processing library

Those are peer dependencies, not actual dependencies. Here is a good article explaining the difference. Peer dependencies are just to let users know what versions of various packages your installed package is compatible with. You don't need to fix these issues, as they are just letting you know which versions of various packages are compatible IF you want to use those packages sometime in the future.

In short, you can just start you project as-is, there is nothing that needs fixing.

One answer, if no one else has anything, is finding the package version sweet spot. Sure would be nice to not waste hours trying to piece this together:

As you spotted, the issue are the plugin versions.

Fastify v3 uses ajv v6 under the hood, so you must install&use ajv's plugins that support that version.

So you need to run:

0# is a relative json pointer for the current members name or array index.

ajv-errors bases this lookup on the { field: value } data it is validating rather than somewhere in the schema.

So inside a property errorMessage you have access to an object 1 level up:

Here a working snippet to play with; I think your issue is on the routes' schemas.

You already deduced it yourself: the allOf contains applies all subschemas and checks against all array items.

If you want only one error message, then you should decide the priority of countries and only check in subsequent allOf parts for the remaining countries and not all of them each time. E.g.

1. If the array contains a “USA” entry, neither “GERMANY” nor “FINLAND” should be present (as you already have it)
2. If the array contains a “GERMANY” entry, no “FINLAND” entry should be present (do not check for “USA” again)
3. If the array contains a “FINLAND” entry, the possible scenarios are already covered by the two checks above – no need for any additional check here.

That way, you never get multiple of these errors at the same time.