fix: prevent silent failure on GET stream so clients don't hang

src/mcp/client/streamable_http.py

@@ -210,6 +210,18 @@ async def handle_get_stream(self, client: httpx.AsyncClient, read_stream_writer:
                     # Stream ended normally (server closed) - reset attempt counter
                     attempt = 0
 
+            except httpx.HTTPStatusError as exc:  # pragma: lax no cover
+                # Handle HTTP errors that are retryable
+                if exc.response.status_code == 405:
+                    # Method Not Allowed - server doesn't support GET for SSE
+                    logger.warning(
+                        "Server does not support GET for SSE events (405 Method Not Allowed). "
+                        "Server-initiated messages will not be available."
+                    )
+                    return
+                # For other HTTP errors, log and retry
+                logger.debug(f"GET stream HTTP error: {exc.response.status_code} - {exc}")
+                attempt += 1
             except Exception:  # pragma: lax no cover
                 logger.debug("GET stream error", exc_info=True)
                 attempt += 1

tests/issues/test_streamable_http_405_get_stream.py

@@ -0,0 +1,100 @@
+"""Test for streamable_http client handling of 405 Method Not Allowed on GET requests.
+
+This test verifies the fix for the race condition where the client hangs when connecting
+to servers (like GitHub MCP) that don't support GET for SSE events.
+"""
+
+import logging
+
+import anyio
+import httpx
+import pytest
+from starlette.applications import Starlette
+from starlette.requests import Request
+from starlette.responses import JSONResponse, Response
+from starlette.routing import Route
+
+from mcp.client.session import ClientSession
+from mcp.client.streamable_http import streamable_http_client
+from mcp.types import InitializeResult
+
+
+async def mock_github_endpoint(request: Request) -> Response:
+    """Mock endpoint that returns 405 for GET (like GitHub MCP)."""
+    if request.method == "GET":
+        return Response(
+            content="Method Not Allowed",
+            status_code=405,
+            headers={"Allow": "POST, DELETE"},
+        )
+    elif request.method == "POST":
+        body = await request.json()
+        if body.get("method") == "initialize":
+            return JSONResponse(
+                {
+                    "jsonrpc": "2.0",
+                    "id": body.get("id"),
+                    "result": {
+                        "protocolVersion": "2025-03-26",
+                        "serverInfo": {"name": "mock_github_server", "version": "1.0"},
+                        "capabilities": {"tools": {}},
+                    },
+                },
+                headers={"mcp-session-id": "test-session"},
+            )
+        elif body.get("method") == "notifications/initialized":
+            return Response(status_code=202)
+        elif body.get("method") == "tools/list":
+            return JSONResponse(
+                {
+                    "jsonrpc": "2.0",
+                    "id": body.get("id"),
+                    "result": {
+                        "tools": [
+                            {
+                                "name": "test_tool",
+                                "description": "A test tool",
+                                "inputSchema": {"type": "object", "properties": {}},
+                            }
+                        ]
+                    },
+                }
+            )
+    return Response(status_code=405)
+
+
+@pytest.mark.anyio
+async def test_405_get_stream_does_not_hang(caplog: pytest.LogCaptureFixture):
+    """Test that client handles 405 on GET gracefully and doesn't hang."""
+    app = Starlette(routes=[Route("/mcp", mock_github_endpoint, methods=["GET", "POST"])])
+
+    with caplog.at_level(logging.INFO):
+        async with httpx.AsyncClient(
+            transport=httpx.ASGITransport(app=app), base_url="http://testserver", timeout=5.0
+        ) as http_client:
+            async with streamable_http_client("http://testserver/mcp", http_client=http_client) as (
+                read_stream,
+                write_stream,
+                _,
+            ):
+                async with ClientSession(read_stream, write_stream) as session:
+                    # Initialize sends the initialized notification internally
+                    init_result = await session.initialize()
+                    assert isinstance(init_result, InitializeResult)
+
+                    # Give the GET stream task time to fail with 405
+                    await anyio.sleep(0.2)
+
+                    # This should not hang and will now complete successfully
+                    tools_result = await session.list_tools()
+                    assert len(tools_result.tools) == 1
+                    assert tools_result.tools[0].name == "test_tool"
+
+    # Verify the 405 was logged and no retries occurred
+    log_messages = [record.getMessage() for record in caplog.records]
+    assert any("Server does not support GET for SSE events (405 Method Not Allowed)" in msg for msg in log_messages), (
+        f"Expected 405 log message not found in: {log_messages}"
+    )
+
+    reconnect_messages = [msg for msg in log_messages if "reconnecting" in msg.lower()]
+    assert len(reconnect_messages) == 0, f"Should not retry on 405, but found: {reconnect_messages}"