Trying to add a user which does not exist as contributor using a user access token results in a 403 forbidden

[xmo-odoo]

🏷️ Discussion Type

Bug

💬 Feature/Topic Area

API

Body

Endpoint: PUT /repos/{owner}/{repo}/collaborators/{username}

• if the username exists, yields success
• if the username does not exist and a UAT is being used, results in a 403
• if the username does not exist and a classic PAT is being used, results in a 404

Trying to access the non-existent user using the UAT does result in the expected 404, same as the classic PAT.

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[AhmadHassan-BTed]

Yess : ( that 403 is totally misleading.

It's essentially a "permission denied" error that the API throws because the UAT auth layer fails to resolve the nonexistent username against your app's installation scopes. It’s definitely a bug in the API's error-handling logic, since a classic PAT correctly returns a 404.

As, you've already verified the UAT can see that the user doesn't exist via separate lookups, it's clearly not a security thing; just sloppy API design. Your only move is to keep doing that pre-check lookup in your code before hitting the PUT endpoint, so you can handle the 404 yourself and ignore their garbage 403.

[emireln]

This looks like inconsistent error mapping between classic PATs and fine-grained user access tokens (UATs) on the collaborators endpoint.

What you are seeing

• Existing username → invitation succeeds (expected)
• Non-existent username + classic PAT → 404 Not Found
• Non-existent username + UAT → 403 Forbidden

Why 403 instead of 404 with a UAT
Fine-grained tokens are scoped to explicit repositories/resources. On some REST routes GitHub intentionally returns 403 when the token cannot act on the target resource, even when the underlying problem is "user not found". That avoids leaking whether an arbitrary login exists to a token that may not have org/user lookup rights.

Since GET /users/{username} returns 404 for the same non-existent login with your UAT, the user lookup path is fine — the collaborator PUT path is applying different authorization/error semantics.

Practical workaround

1. Validate the username first with GET /users/{username}.
2. Only call PUT /repos/{owner}/{repo}/collaborators/{username} after the user exists.
3. In automation, treat both 403 and 404 as "cannot add collaborator" when using UATs.

If you want to report it
Include: UAT vs PAT, exact endpoint, response status + X-GitHub-Request-Id, and note that user lookup returns 404 while collaborator add returns 403.

Hope that helps!