[padraig-ohiceadha]

Fixes #3856 and #2776

Added some helper functions in src/mcp/shared/auth_utils.py to add, remove and replace query params to a URL and then used those in src/mcp/client/auth/oauth2.py in _perform_authorization_code_grant to conform to the rules in section 3.1 of the OAuth 2.1 spec.
There is one change to check if a fragment has been added to the authorization_url and if so to reject it. You could also make an argument to accept the URL and strip the fragment rather than to return an error
It then later when adding query params to the url calls the new function replace_query_params to append the params to the url without interfering with any existing query params and also validates that the URL doesn't already contain any of the params used by the authorization request.

Devin AI was used to document the code changes and to update the tests to cover the changes.

Motivation and Context

Not accepting (or removing fragments) from the authorization_url can help protect against fragment extension attacks, and validating that the authorization URL doesn't already contain parameters expected to be added by the request generation prevents attacks where the param provided with the authorization_url is used instead of the params added by the MCP client.

How Has This Been Tested?

It has been tested against a proprietary OAuth 2.1 authorization server which expects certain query params in the authorization_url obtained via OpenId Discovery are retained.

Breaking Changes

Users should not have to update anything

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I am assigned to the linked issue (or it is labeled help wanted, or I'm a maintainer)
• I have disclosed any AI assistance and can explain the change in my own words
• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[github-actions]

This PR has been closed automatically. This repo only keeps pull requests open when they come from a maintainer, or from a contributor a maintainer has assigned to the linked issue, and you aren't currently assigned to #3856.

If a maintainer assigns you to #3856, this PR reopens on its own and there's nothing more you need to do here. Assignment is a maintainer call based on capacity; comments that only ask to be assigned don't factor in. What does help is engaging on the issue itself by confirming the repro, explaining why it matters for your use case, or describing the approach you'd take.

You're welcome to keep pushing commits here (just avoid force-pushing, since GitHub can't reopen a rewritten branch), but that on its own won't get the PR reviewed or the issue assigned, and realistically most auto-closed PRs stay closed. There's no need to open a new PR either way.

CONTRIBUTING.md has the full reasoning, but in short:

• We're a small team with very little capacity to review community PRs right now.
• Many recent PRs are AI-generated with little human review, and reviewing one carefully still costs a maintainer as much time as it ever did. A well-described issue is usually more useful to us than the code.

Maintainers: reopen, remove missing-issue-link, or add bypass-issue-check to override.