fix(mcp): backport StreamableHTTPSessionManager stateless fix to v1.81.12-stable

[michelligabriele]

Cherry-pick of #21323 onto v1.81.12-stable.

Reverts stateless=False → stateless=True in StreamableHTTPSessionManager and includes a regression test. The original fix was merged to main on Feb 16 but missed the v1.81.12-stable cut.

Fixes #20242

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Project	Deployment	Actions	Updated (UTC)
litellm	Ready	Preview, Comment	Feb 24, 2026 6:57pm

[greptile-apps]

Greptile Summary

Cherry-pick of #21323 onto v1.81.12-stable. Reverts the stateless=False change introduced by PR #19809 back to stateless=True in StreamableHTTPSessionManager. The stateless=False setting caused the MCP library to enforce mcp-session-id headers on all non-initialize requests, breaking MCP Inspector, curl, and any HTTP client without automatic session management (issue #20242).

• Reverts stateless=False → stateless=True in server.py line 152
• Adds a regression test (test_streamable_http_session_manager_is_stateless) that asserts session_manager.stateless is True
• Adds @pytest.mark.no_parallel to the adjacent test_mcp_routing_with_conflicting_alias_and_group_name test
• The progress notification code already handles missing sessions defensively, so no other changes are needed

Confidence Score: 5/5

• This PR is a minimal, well-scoped one-line fix with a regression test — safe to merge.
• The change is a single boolean flag revert (stateless=False → stateless=True) that restores the original behavior. It is a clean cherry-pick of an already-merged fix from main (fix(mcp): revert StreamableHTTPSessionManager to stateless mode #21323). The regression test is mock-only and verifies the fix. No new dependencies, no architectural changes, no risk of side effects.
• No files require special attention.

Important Files Changed

Filename	Overview
litellm/proxy/_experimental/mcp_server/server.py	One-line change reverting stateless=False → stateless=True in StreamableHTTPSessionManager constructor. Fixes regression from PR #19809 that broke MCP clients without session management.
tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server.py	Adds a regression test asserting session_manager.stateless is True, plus adds @pytest.mark.no_parallel to an adjacent test. No real network calls — all mock-based.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[MCP Client Request] --> B{stateless flag?}
    B -->|stateless=True| C[Accept request without mcp-session-id]
    B -->|stateless=False| D{Has mcp-session-id header?}
    D -->|Yes| E[Process request]
    D -->|No| F[Reject request ❌]
    C --> E
    E --> G[Return response to client]

    style B fill:#f9f,stroke:#333
    style C fill:#9f9,stroke:#333
    style F fill:#f99,stroke:#333

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