fix flaky fix-test_streamablehttp_client_resumption test (#1166)

src/mcp/server/streamable_http.py

@@ -837,9 +837,7 @@ class StreamableHTTPServerTransport:
                             response_id = str(message.root.id)
                             # If this response is for an existing request stream,
                             # send it there
-                            if response_id in self._request_streams:
+                            target_request_id = response_id
-                                target_request_id = response_id
-
                         else:
                             # Extract related_request_id from meta if it exists
                             if (

tests/shared/test_streamable_http.py

@@ -98,32 +98,33 @@ class SimpleEventStore(EventStore):
         send_callback: EventCallback,
     ) -> StreamId | None:
         """Replay events after the specified ID."""
-        # Find the index of the last event ID
+        # Find the stream ID of the last event
-        start_index = None
+        target_stream_id = None
-        for i, (_, event_id, _) in enumerate(self._events):
+        for stream_id, event_id, _ in self._events:
             if event_id == last_event_id:
-                start_index = i + 1
+                target_stream_id = stream_id
                 break
 
-        if start_index is None:
+        if target_stream_id is None:
-            # If event ID not found, start from beginning
+            # If event ID not found, return None
-            start_index = 0
+            return None
 
-        stream_id = None
+        # Convert last_event_id to int for comparison
-        # Replay events
+        last_event_id_int = int(last_event_id)
-        for _, event_id, message in self._events[start_index:]:
-            await send_callback(EventMessage(message, event_id))
-            # Capture the stream ID from the first replayed event
-            if stream_id is None and len(self._events) > start_index:
-                stream_id = self._events[start_index][0]
 
-        return stream_id
+        # Replay only events from the same stream with ID > last_event_id
+        for stream_id, event_id, message in self._events:
+            if stream_id == target_stream_id and int(event_id) > last_event_id_int:
+                await send_callback(EventMessage(message, event_id))
+
+        return target_stream_id
 
 
 # Test server implementation that follows MCP protocol
 class ServerTest(Server):
     def __init__(self):
         super().__init__(SERVER_NAME)
+        self._lock = None  # Will be initialized in async context
 
         @self.read_resource()
         async def handle_read_resource(uri: AnyUrl) -> str | bytes:
@@ -159,6 +160,16 @@ class ServerTest(Server):
                     description="A tool that triggers server-side sampling",
                     inputSchema={"type": "object", "properties": {}},
                 ),
+                Tool(
+                    name="wait_for_lock_with_notification",
+                    description="A tool that sends a notification and waits for lock",
+                    inputSchema={"type": "object", "properties": {}},
+                ),
+                Tool(
+                    name="release_lock",
+                    description="A tool that releases the lock",
+                    inputSchema={"type": "object", "properties": {}},
+                ),
             ]
 
         @self.call_tool()
@@ -214,6 +225,39 @@ class ServerTest(Server):
                     )
                 ]
 
+            elif name == "wait_for_lock_with_notification":
+                # Initialize lock if not already done
+                if self._lock is None:
+                    self._lock = anyio.Event()
+
+                # First send a notification
+                await ctx.session.send_log_message(
+                    level="info",
+                    data="First notification before lock",
+                    logger="lock_tool",
+                    related_request_id=ctx.request_id,
+                )
+
+                # Now wait for the lock to be released
+                await self._lock.wait()
+
+                # Send second notification after lock is released
+                await ctx.session.send_log_message(
+                    level="info",
+                    data="Second notification after lock",
+                    logger="lock_tool",
+                    related_request_id=ctx.request_id,
+                )
+
+                return [TextContent(type="text", text="Completed")]
+
+            elif name == "release_lock":
+                assert self._lock is not None, "Lock must be initialized before releasing"
+
+                # Release the lock
+                self._lock.set()
+                return [TextContent(type="text", text="Lock released")]
+
             return [TextContent(type="text", text=f"Called {name}")]
 
 
@@ -825,7 +869,7 @@ async def test_streamablehttp_client_tool_invocation(initialized_client_session)
     """Test client tool invocation."""
     # First list tools
     tools = await initialized_client_session.list_tools()
-    assert len(tools.tools) == 4
+    assert len(tools.tools) == 6
     assert tools.tools[0].name == "test_tool"
 
     # Call the tool
@@ -862,7 +906,7 @@ async def test_streamablehttp_client_session_persistence(basic_server, basic_ser
 
             # Make multiple requests to verify session persistence
             tools = await session.list_tools()
-            assert len(tools.tools) == 4
+            assert len(tools.tools) == 6
 
             # Read a resource
             resource = await session.read_resource(uri=AnyUrl("foobar://test-persist"))
@@ -891,7 +935,7 @@ async def test_streamablehttp_client_json_response(json_response_server, json_se
 
             # Check tool listing
             tools = await session.list_tools()
-            assert len(tools.tools) == 4
+            assert len(tools.tools) == 6
 
             # Call a tool and verify JSON response handling
             result = await session.call_tool("test_tool", {})
@@ -962,7 +1006,7 @@ async def test_streamablehttp_client_session_termination(basic_server, basic_ser
 
             # Make a request to confirm session is working
             tools = await session.list_tools()
-            assert len(tools.tools) == 4
+            assert len(tools.tools) == 6
 
     headers = {}
     if captured_session_id:
@@ -1026,7 +1070,7 @@ async def test_streamablehttp_client_session_termination_204(basic_server, basic
 
             # Make a request to confirm session is working
             tools = await session.list_tools()
-            assert len(tools.tools) == 4
+            assert len(tools.tools) == 6
 
     headers = {}
     if captured_session_id:
@@ -1048,32 +1092,32 @@ async def test_streamablehttp_client_session_termination_204(basic_server, basic
 
 @pytest.mark.anyio
 async def test_streamablehttp_client_resumption(event_server):
-    """Test client session to resume a long running tool."""
+    """Test client session resumption using sync primitives for reliable coordination."""
     _, server_url = event_server
 
     # Variables to track the state
     captured_session_id = None
     captured_resumption_token = None
     captured_notifications = []
-    tool_started = False
     captured_protocol_version = None
+    first_notification_received = False
 
     async def message_handler(
         message: RequestResponder[types.ServerRequest, types.ClientResult] | types.ServerNotification | Exception,
     ) -> None:
         if isinstance(message, types.ServerNotification):
             captured_notifications.append(message)
-            # Look for our special notification that indicates the tool is running
+            # Look for our first notification
             if isinstance(message.root, types.LoggingMessageNotification):
-                if message.root.params.data == "Tool started":
+                if message.root.params.data == "First notification before lock":
-                    nonlocal tool_started
+                    nonlocal first_notification_received
-                    tool_started = True
+                    first_notification_received = True
 
     async def on_resumption_token_update(token: str) -> None:
         nonlocal captured_resumption_token
         captured_resumption_token = token
 
-    # First, start the client session and begin the long-running tool
+    # First, start the client session and begin the tool that waits on lock
     async with streamablehttp_client(f"{server_url}/mcp", terminate_on_close=False) as (
         read_stream,
         write_stream,
@@ -1088,7 +1132,7 @@ async def test_streamablehttp_client_resumption(event_server):
             # Capture the negotiated protocol version
             captured_protocol_version = result.protocolVersion
 
-            # Start a long-running tool in a task
+            # Start the tool that will wait on lock in a task
             async with anyio.create_task_group() as tg:
 
                 async def run_tool():
@@ -1099,7 +1143,9 @@ async def test_streamablehttp_client_resumption(event_server):
                         types.ClientRequest(
                             types.CallToolRequest(
                                 method="tools/call",
-                                params=types.CallToolRequestParams(name="long_running_with_checkpoints", arguments={}),
+                                params=types.CallToolRequestParams(
+                                    name="wait_for_lock_with_notification", arguments={}
+                                ),
                             )
                         ),
                         types.CallToolResult,
@@ -1108,15 +1154,19 @@ async def test_streamablehttp_client_resumption(event_server):
 
                 tg.start_soon(run_tool)
 
-                # Wait for the tool to start and at least one notification
+                # Wait for the first notification and resumption token
-                # and then kill the task group
+                while not first_notification_received or not captured_resumption_token:
-                while not tool_started or not captured_resumption_token:
                     await anyio.sleep(0.1)
+
+                # Kill the client session while tool is waiting on lock
                 tg.cancel_scope.cancel()
 
-    # Store pre notifications and clear the captured notifications
+    # Verify we received exactly one notification
-    # for the post-resumption check
+    assert len(captured_notifications) == 1
-    captured_notifications_pre = captured_notifications.copy()
+    assert isinstance(captured_notifications[0].root, types.LoggingMessageNotification)
+    assert captured_notifications[0].root.params.data == "First notification before lock"
+
+    # Clear notifications for the second phase
     captured_notifications = []
 
     # Now resume the session with the same mcp-session-id and protocol version
@@ -1125,54 +1175,48 @@ async def test_streamablehttp_client_resumption(event_server):
         headers[MCP_SESSION_ID_HEADER] = captured_session_id
     if captured_protocol_version:
         headers[MCP_PROTOCOL_VERSION_HEADER] = captured_protocol_version
-
     async with streamablehttp_client(f"{server_url}/mcp", headers=headers) as (
         read_stream,
         write_stream,
         _,
     ):
         async with ClientSession(read_stream, write_stream, message_handler=message_handler) as session:
-            # Don't initialize - just use the existing session
-
-            # Resume the tool with the resumption token
-            assert captured_resumption_token is not None
-
-            metadata = ClientMessageMetadata(
-                resumption_token=captured_resumption_token,
-            )
             result = await session.send_request(
                 types.ClientRequest(
                     types.CallToolRequest(
                         method="tools/call",
-                        params=types.CallToolRequestParams(name="long_running_with_checkpoints", arguments={}),
+                        params=types.CallToolRequestParams(name="release_lock", arguments={}),
+                    )
+                ),
+                types.CallToolResult,
+            )
+            metadata = ClientMessageMetadata(
+                resumption_token=captured_resumption_token,
+            )
+
+            result = await session.send_request(
+                types.ClientRequest(
+                    types.CallToolRequest(
+                        method="tools/call",
+                        params=types.CallToolRequestParams(name="wait_for_lock_with_notification", arguments={}),
                     )
                 ),
                 types.CallToolResult,
                 metadata=metadata,
             )
-
-            # We should get a complete result
             assert len(result.content) == 1
             assert result.content[0].type == "text"
-            assert "Completed" in result.content[0].text
+            assert result.content[0].text == "Completed"
 
             # We should have received the remaining notifications
-            assert len(captured_notifications) > 0
+            assert len(captured_notifications) == 1
 
-            # Should not have the first notification
+            assert captured_notifications[0].root.params.data == "Second notification after lock"
-            # Check that "Tool started" notification isn't repeated when resuming
-            assert not any(
-                isinstance(n.root, types.LoggingMessageNotification) and n.root.params.data == "Tool started"
-                for n in captured_notifications
-            )
-            # there is no intersection between pre and post notifications
-            assert not any(n in captured_notifications_pre for n in captured_notifications)
 
 
 @pytest.mark.anyio
 async def test_streamablehttp_server_sampling(basic_server, basic_server_url):
     """Test server-initiated sampling request through streamable HTTP transport."""
-    print("Testing server sampling...")
     # Variable to track if sampling callback was invoked
     sampling_callback_invoked = False
     captured_message_params = None