bug: MCP e2e tests fail — auth ContextVar not propagated into SessionManager task groups

[jquinter]

Bug Description

All 4 MCP e2e tests in tests/mcp_tests/test_proxy_mcp_e2e.py fail on main with auth errors:

AssertionError: assert 'Error: User not allowed to call this tool.' == '30'

Failing Tests

• TestProxyMcpSimpleConnections::test_proxy_mcp_stdio_roundtrip
• TestProxyMcpSimpleConnections::test_proxy_mcp_streamable_http_roundtrip
• TestProxyMcpSimpleConnections::test_proxy_mcp_lists_all_servers_without_header
• TestProxyMcpStatelessBehavior::test_independent_clients_no_shared_session

Root Cause

set_auth_context() at server.py:2063 stores auth info (user_api_key_auth, mcp_servers, etc.) in a ContextVar during the HTTP request handler. However, the MCP StreamableHTTPSessionManager spawns new anyio.TaskGroup tasks for handling protocol messages like call_tool and list_tools.

ContextVar values don't reliably propagate into these spawned tasks, so when get_auth_context() is called inside mcp_server_tool_call (line 302), it returns empty/stale data. This causes get_allowed_mcp_servers() to return an empty list → _get_allowed_mcp_servers_from_mcp_server_names returns empty → 403 "User not allowed to call this tool."

Relevant Code

• set_auth_context() — server.py:2197
• get_auth_context() — server.py:2227
• mcp_server_tool_call reads context — server.py:302
• handle_streamable_http_mcp sets context — server.py:2063

Suggested Fixes

1. Store auth on the MCP session object — persist auth across protocol messages within a session
2. Pass auth explicitly through the MCP handler chain instead of relying on ContextVars
3. Use copy_context().run() when spawning tasks in the session manager to inherit parent context

Environment

• All tests fail locally on main (macOS, Python 3.12+)
• Also fails in GitHub Actions CI

Related

• Discovered while investigating CI failures on PR fix(mcp): update test mocks for renamed filter_server_ids_by_ip_with_info #22327
• Regression test for MCP servers requiring session ID persistence fail (e.g., Notion) #20242 (stateless mode)