fix(mcp): revert StreamableHTTPSessionManager to stateless mode (#21323) (#22030)

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

Author: michelligabriele

litellm/proxy/_experimental/mcp_server/server.py

@@ -149,7 +149,7 @@ class ListMCPToolsRestAPIResponseObject(MCPTool):
         app=server,
         event_store=None,
         json_response=False, # enables SSE streaming
-        stateless=False, # enables session state
+        stateless=True,
     )
 
     # Create SSE session manager

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

@@ -756,6 +756,31 @@ async def init_task():
 
 
 @pytest.mark.asyncio
+async def test_streamable_http_session_manager_is_stateless():
+    """
+    Test that the StreamableHTTPSessionManager is initialized with stateless=True.
+
+    Regression test for GitHub issue #20242 / PR #19809.
+    When stateless=False, the mcp library rejects non-initialize requests
+    that lack an mcp-session-id header, breaking clients like MCP Inspector,
+    curl, and any HTTP client without automatic session management.
+    """
+    try:
+        from litellm.proxy._experimental.mcp_server.server import session_manager
+    except ImportError:
+        pytest.skip("MCP server not available")
+
+    # The session manager must be stateless to avoid requiring mcp-session-id
+    # on every request. This was regressed by PR #19809 (stateless=True -> False).
+    assert session_manager.stateless is True, (
+        "StreamableHTTPSessionManager must be initialized with stateless=True. "
+        "stateless=False breaks MCP clients that don't manage session IDs. "
+        "See: https://github.com/BerriAI/litellm/issues/20242"
+    )
+
+
+@pytest.mark.asyncio
+@pytest.mark.no_parallel
 async def test_mcp_routing_with_conflicting_alias_and_group_name():
     """
     Tests (GH #14536) where an MCP server alias (e.g., "group/id")