fix: preserve per-user OAuth Authorization over MCP signer JWT on tools/calls

[RajanChavada]

Relevant issues

Fixes #31977

Linear ticket

Pre-Submission checklist

Please complete all items before asking a LiteLLM maintainer to review your PR

• I have added meaningful tests
• My PR passes all CI/CD checks (e.g., lint, format, unit tests)
• [ x My PR's scope is as isolated as possible; it only solves 1 specific problem
• I have requested a Greptile review by commenting @greptileai and received a Confidence Score of at least 4/5 before requesting a maintainer review

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Screenshots / Proof of Fix

pytest tests/.../test_mcp_hook_extra_headers.py → 29 passed. Added a
regression test on the v1 delegate_auth_to_upstream oauth2 path (the scenario
#31977 reports) asserting the resolved OAuth Authorization survives the signer
merge. Mutation-checked: reverting the fix re-fails the test.

Screenshot of error in the code:

Screenshot after the fix

Type

🐛 Bug Fix

Changes

On the MCP tools/call path, the signer's JWT unconditionally overwrote the
Authorization header via extra_headers.update(hook_extra_headers), even when a
per-user OAuth token had already been resolved into extra_headers. The
tools/list path already guards against this; this applies the same guard to
tools/call — the hook's non-Authorization headers still merge in.

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[greptile-apps]

Greptile Summary

This PR fixes a header-priority bug on the MCP tools/call path where the guardrail signer's JWT unconditionally overwrote a per-user OAuth token (or any previously resolved Authorization) via extra_headers.update(hook_extra_headers). The fix replaces the blanket update with a case-insensitive per-header loop that skips the hook's Authorization when one already exists in extra_headers, matching the guard that was already in place on the tools/list path.

• Core fix (mcp_server_manager.py, ~line 3352): replaces extra_headers.update(hook_extra_headers) with a loop that preserves any pre-existing Authorization header while still merging all other hook headers.
• Tests (test_mcp_hook_extra_headers.py): two tests corrected to assert the new precedence order; a new regression test covers the delegate_auth_to_upstream / v1-path scenario from [Bug]: MCP JWT signer overwrites OAuth Authorization header during tools/call #31977, mutation-checked by the author.

Confidence Score: 5/5

Safe to merge — the change is narrowly scoped to the hook-header merge step and cannot affect callers that have no existing Authorization in extra_headers.

The fix correctly mirrors the tools/list guard onto the tools/call path, uses case-insensitive header comparison, preserves all non-Authorization hook headers, and is covered by a mutation-verified regression test. The only gap is a missing debug log when the JWT is silently skipped — a minor observability concern with no correctness impact.

No files require special attention; both changed files are straightforward.

Important Files Changed

Filename	Overview
litellm/proxy/_experimental/mcp_server/mcp_server_manager.py	Replaces the blanket extra_headers.update(hook_extra_headers) with a case-insensitive per-header loop that skips the hook's Authorization when one is already present in extra_headers, matching the existing tools/list guard logic.
tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_hook_extra_headers.py	Two existing tests updated to assert the correct fixed priority (existing Authorization wins); a new regression test for the delegate_auth_to_upstream / v1-path scenario from issue #31977 is added. All tests use mocks only — no real network calls.

_{Reviews (1): Last reviewed commit: "fix: preserve per-user OAuth Authorizati..." | Re-trigger Greptile}

[greptile-apps]

When the hook's Authorization is silently skipped because an existing one was already in extra_headers, nothing is logged. A debug-level message here would help operators understand why the signer JWT was not forwarded, matching the level of observability that the old warning provided for the inverse scenario.

Suggested change

	if isinstance(header, str) and header.lower() == "authorization":
	if has_existing_authorization:
	continue
	if isinstance(header, str) and header.lower() == "authorization":
	if has_existing_authorization:
	verbose_logger.debug(
	"MCPServerManager: hook_extra_headers 'Authorization' skipped — "
	"an existing Authorization header (per-user OAuth or static) takes precedence."
	)
	continue

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...oxy/_experimental/mcp_server/mcp_server_manager.py	85.71%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[github-actions]

#31977 was fixed by #38555 on main, so this pull request is closed. Reopen it if something was missed.