feat: add idle timeout for StreamableHTTP sessions (#1994)

tests/server/test_streamable_http_manager.py

@@ -313,3 +313,80 @@ async def test_unknown_session_id_returns_404():
         assert error_data["id"] == "server-error"
         assert error_data["error"]["code"] == INVALID_REQUEST
         assert error_data["error"]["message"] == "Session not found"
+
+
+@pytest.mark.anyio
+async def test_idle_session_is_reaped():
+    """After idle timeout fires, the session returns 404."""
+    app = Server("test-idle-reap")
+    manager = StreamableHTTPSessionManager(app=app, session_idle_timeout=0.05)
+
+    async with manager.run():
+        sent_messages: list[Message] = []
+
+        async def mock_send(message: Message):
+            sent_messages.append(message)
+
+        scope = {
+            "type": "http",
+            "method": "POST",
+            "path": "/mcp",
+            "headers": [(b"content-type", b"application/json")],
+        }
+
+        async def mock_receive():  # pragma: no cover
+            return {"type": "http.request", "body": b"", "more_body": False}
+
+        await manager.handle_request(scope, mock_receive, mock_send)
+
+        session_id = None
+        for msg in sent_messages:  # pragma: no branch
+            if msg["type"] == "http.response.start":  # pragma: no branch
+                for header_name, header_value in msg.get("headers", []):  # pragma: no branch
+                    if header_name.decode().lower() == MCP_SESSION_ID_HEADER.lower():
+                        session_id = header_value.decode()
+                        break
+                if session_id:  # pragma: no branch
+                    break
+
+        assert session_id is not None, "Session ID not found in response headers"
+
+        # Wait for the 50ms idle timeout to fire and cleanup to complete
+        await anyio.sleep(0.1)
+
+        # Verify via public API: old session ID now returns 404
+        response_messages: list[Message] = []
+
+        async def capture_send(message: Message):
+            response_messages.append(message)
+
+        scope_with_session = {
+            "type": "http",
+            "method": "POST",
+            "path": "/mcp",
+            "headers": [
+                (b"content-type", b"application/json"),
+                (b"mcp-session-id", session_id.encode()),
+            ],
+        }
+
+        await manager.handle_request(scope_with_session, mock_receive, capture_send)
+
+        response_start = next(
+            (msg for msg in response_messages if msg["type"] == "http.response.start"),
+            None,
+        )
+        assert response_start is not None
+        assert response_start["status"] == 404
+
+
+def test_session_idle_timeout_rejects_non_positive():
+    with pytest.raises(ValueError, match="positive number"):
+        StreamableHTTPSessionManager(app=Server("test"), session_idle_timeout=-1)
+    with pytest.raises(ValueError, match="positive number"):
+        StreamableHTTPSessionManager(app=Server("test"), session_idle_timeout=0)
+
+
+def test_session_idle_timeout_rejects_stateless():
+    with pytest.raises(RuntimeError, match="not supported in stateless"):
+        StreamableHTTPSessionManager(app=Server("test"), session_idle_timeout=30, stateless=True)