fix: set oauth2_flow when building MCPServer in _execute_with_mcp_client

[joereyna]

Summary

• MCPServer.has_client_credentials requires oauth2_flow == "client_credentials" to be set explicitly
• _execute_with_mcp_client was building MCPServer without setting oauth2_flow, so has_client_credentials always returned False
• This caused two bugs: M2M tokens were never auto-fetched, and the incoming Authorization header was forwarded instead of being dropped
• Fix: set oauth2_flow="client_credentials" when client_id and client_secret are present

Test plan

• test_m2m_credentials_forwarded_to_server_model — server.has_client_credentials now returns True
• test_m2m_drops_incoming_oauth2_headers — incoming Authorization header is now dropped for M2M servers
• make test-unit passes

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Project	Deployment	Actions	Updated (UTC)
litellm	Ready	Preview, Comment	Mar 16, 2026 10:38pm

[greptile-apps]

Greptile Summary

This PR fixes a bug in _execute_with_mcp_client where MCPServer was built without an oauth2_flow value, causing has_client_credentials to always return False. This blocked M2M token auto-fetching and incorrectly forwarded the incoming Authorization header to downstream MCP servers.

Key changes:

• Adds oauth2_flow: Optional[Literal["client_credentials", "authorization_code"]] to NewMCPServerRequest in _types.py, correctly matching the MCPServer field type and allowing callers to opt in explicitly.
• Updates _execute_with_mcp_client to prefer the caller-supplied request.oauth2_flow, falling back to auto-detection (client_credentials when client_id, client_secret, and token_url are all present).
• Adds a safety guard: if oauth2_flow resolves to "client_credentials" but token_url is absent, it resets to None to avoid dropping the incoming auth header with nothing to replace it.
• Un-skips two regression tests (test_m2m_credentials_forwarded_to_server_model, test_m2m_drops_incoming_oauth2_headers) that now correctly exercise the fixed code paths.

Confidence Score: 4/5

• Safe to merge — the fix correctly restores M2M OAuth behaviour without breaking other flows, and is covered by the previously-skipped tests.
• The logic is sound: explicit oauth2_flow takes priority, the heuristic still requires token_url, and the guard prevents auth header loss. The one minor gap is that the guard silently ignores an explicitly set client_credentials flow without any log entry, making it hard to debug if a caller omits token_url. No breaking changes to existing code paths.
• No files require special attention beyond the minor logging gap in rest_endpoints.py at the token_url guard.

Important Files Changed

Filename	Overview
litellm/proxy/_experimental/mcp_server/rest_endpoints.py	Core logic fix: _execute_with_mcp_client now correctly propagates oauth2_flow to MCPServer, with explicit field preference and a safety guard for missing token_url. Minor concern: explicit client_credentials override is silently downgraded with no log when token_url is absent.
litellm/proxy/_types.py	Adds oauth2_flow: Optional[Literal["client_credentials", "authorization_code"]] = None to NewMCPServerRequest, correctly matching the type used in MCPServer. Addresses the previously missing field that prevented explicit M2M opt-in.
tests/test_litellm/proxy/_experimental/mcp_server/test_rest_endpoints.py	Removes @pytest.mark.skip from two previously broken tests. Both use monkeypatch to avoid real network calls and correctly assert the fixed behaviour: M2M credentials forwarded to MCPServer and incoming Authorization header dropped for M2M servers.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[_execute_with_mcp_client called] --> B[Extract client_id, client_secret, scopes]
    B --> C{request.oauth2_flow set?}
    C -- Yes --> D[Use request.oauth2_flow]
    C -- No --> E{client_id AND client_secret AND token_url?}
    E -- Yes --> F[_oauth2_flow = 'client_credentials']
    E -- No --> G[_oauth2_flow = None]
    D --> H{Guard: oauth2_flow == 'client_credentials' AND token_url missing?}
    F --> H
    G --> I[Build MCPServer with _oauth2_flow]
    H -- Yes --> J[Reset _oauth2_flow = None - silent downgrade]
    H -- No --> I
    J --> I
    I --> K{server_model.has_client_credentials?}
    K -- True --> L[Drop incoming oauth2_headers - M2M flow]
    K -- False --> M[Forward incoming oauth2_headers]
    L --> N[Create MCP client and run operation]
    M --> N

Loading

Comments Outside Diff (1)

1. litellm/proxy/_experimental/mcp_server/rest_endpoints.py, line 915-916 (link)

   Silent override of explicit oauth2_flow without a warning

   When a caller explicitly sets oauth2_flow="client_credentials" (via the newly-added NewMCPServerRequest.oauth2_flow field) but omits token_url, the guard silently resets _oauth2_flow to None. The incoming Authorization header is then forwarded as-is, which is correct behaviour — but the caller gets no feedback that their explicitly requested M2M flow was ignored.

   Because the whole function body is wrapped in except BaseException, even a ValueError would be swallowed and surface only as the generic "Failed to connect to MCP server" message. A verbose_logger.warning call here would at least produce a diagnosable log entry:

   if _oauth2_flow == "client_credentials" and not request.token_url:
       verbose_logger.warning(
           "_execute_with_mcp_client: oauth2_flow='client_credentials' requested but "
           "token_url is not set — falling back to forwarding the incoming auth header."
       )
       _oauth2_flow = None

_{Last reviewed commit: d58b0a9}

[codspeed-hq]

Merging this PR will not alter performance

✅ 16 untouched benchmarks

---

_{Comparing joereyna:fix/mcp-rest-m2m-oauth2-flow (d58b0a9) with main (278c9ba)}

[joereyna]

@greptileai please review this pr

[joereyna]

@greptileai review this draft