fix(mcp): revert StreamableHTTPSessionManager to stateless mode

[michelligabriele]

PR #19809 changed stateless=True to stateless=False to enable progress notifications for MCP tool calls. This caused the mcp library to enforce mcp-session-id headers on all non-initialize requests, breaking MCP Inspector, curl, and any client without automatic session management.

Revert to stateless=True to restore compatibility with all MCP clients. The progress notification code already handles missing sessions gracefully (defensive checks + try/except), so no other changes are needed.

Fixes #20242

Relevant issues

Fixes #20242

Pre-Submission checklist

• I have Added testing in the tests/litellm/ directory, Adding at least 1 test is a hard requirement - see details
• My PR passes all unit tests on make test-unit
• 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

CI (LiteLLM team)

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• 45-49 passing tests: acceptable but needs attention
• <= 40 passing tests: unstable; be careful with your merges and assess the risk.

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Type

🐛 Bug Fix
✅ Test

Changes

litellm/proxy/_experimental/mcp_server/server.py

• Reverted StreamableHTTPSessionManager initialization from stateless=False back to stateless=True (line 152). This restores pre-v1.81.0 behavior where MCP clients are not required to send mcp-session-id headers on every request.

tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server.py

• Added test_streamable_http_session_manager_is_stateless() — regression test that asserts the session manager is initialized with stateless=True to prevent this from being accidentally changed again.

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Project	Deployment	Actions	Updated (UTC)
litellm	Ready	Preview, Comment	Feb 16, 2026 5:03pm

[greptile-apps]

Greptile Summary

This PR reverts a single configuration change in the MCP server: StreamableHTTPSessionManager is initialized with stateless=True instead of stateless=False (introduced in PR #19809). The stateless=False setting caused the MCP library to require 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 to stateless=True in StreamableHTTPSessionManager initialization (server.py:152)
• The existing progress notification code (lines 307-330) already handles missing sessions defensively via null checks and try/except, so no additional changes are needed
• Adds a regression test that asserts session_manager.stateless is True to prevent future regressions

Confidence Score: 5/5

• This PR is a safe, minimal revert of a single boolean flag with a regression test to prevent future breakage.
• The change is a one-line revert to the previous known-working configuration. The progress notification code already handles missing sessions gracefully with defensive checks and try/except blocks, so reverting to stateless mode introduces no new failure paths. The regression test is well-written and correctly validates the fix.
• No files require special attention.

Important Files Changed

Filename	Overview
litellm/proxy/_experimental/mcp_server/server.py	Single-line revert of stateless=False back to stateless=True on StreamableHTTPSessionManager initialization. Restores compatibility with MCP clients that don't send mcp-session-id headers. No other logic changes.
tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server.py	Adds regression test test_streamable_http_session_manager_is_stateless that asserts session_manager.stateless is True. No network calls; pure attribute check with proper ImportError handling.

Sequence Diagram

sequenceDiagram
    participant Client as MCP Client (Inspector/curl)
    participant Server as LiteLLM MCP Server
    participant SM as StreamableHTTPSessionManager
    
    Note over SM: stateless=True (reverted)
    
    Client->>Server: POST /mcp (no mcp-session-id)
    Server->>SM: handle_request(scope, receive, send)
    SM-->>Server: Process request (no session required)
    Server-->>Client: 200 OK with response
    
    Note over SM: stateless=False (broken behavior)
    
    Client->>Server: POST /mcp (no mcp-session-id)
    Server->>SM: handle_request(scope, receive, send)
    SM-->>Server: 400 Bad Request (missing mcp-session-id)
    Server-->>Client: Error: session header required

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