Extend the non-advancing cursor guard to ClientSessionGroup drains

The pagination guide says every drain fails loudly when a server returns
the same cursor it was handed, but the group aggregator's drain loop had
no such guard and would page forever. Guard it the same way the Client
drains are guarded.

Author: CJGjr

src/mcp/client/session_group.py

@@ -76,6 +76,9 @@ async def _drain_paginated(
     `fetch_page` is one of the ClientSession `list_*` methods that takes a
     `params=PaginatedRequestParams(...)` keyword. `attribute` is the name of
     the list attribute on the result (e.g. `"tools"`, `"prompts"`).
+
+    Raises:
+        RuntimeError: The server returned a pagination cursor that did not advance.
     """
     items: list[Any] = []
     cursor: str | None = None
@@ -86,6 +89,8 @@ async def _drain_paginated(
         next_cursor = getattr(result, "next_cursor", None)
         if next_cursor is None:
             return items
+        if next_cursor == cursor:
+            raise RuntimeError("Server returned a pagination cursor that did not advance; refusing to page forever.")
         cursor = next_cursor
 
 
@@ -435,8 +440,8 @@ async def _aggregate_components(self, server_info: types.Implementation, session
                 tools_temp[name] = tool
                 tool_to_session_temp[name] = session
                 component_names.tools.add(name)
-        except MCPError as err:  # pragma: no cover
-            logging.warning(f"Could not fetch tools: {err}")
+        except MCPError as err:
+            logging.warning(f"Could not fetch tools: {err}")  # pragma: no cover
 
         # Clean up exit stack for session if we couldn't retrieve anything
         # from the server.

tests/client/test_session_group.py

@@ -444,3 +444,24 @@ async def test_client_session_group_aggregates_paginated_tools(
     second_call_kwargs = mock_session.list_tools.await_args_list[1].kwargs
     assert second_call_kwargs["params"] is not None
     assert second_call_kwargs["params"].cursor == "page-2"
+
+
+@pytest.mark.anyio
+async def test_client_session_group_rejects_non_advancing_cursor(
+    mock_exit_stack: contextlib.AsyncExitStack,
+):
+    """A server that keeps returning the cursor it was handed must fail loudly, not page forever."""
+    mock_server_info = mock.Mock(spec=types.Implementation)
+    mock_server_info.name = "StuckCursorServer"
+    mock_session = mock.AsyncMock(spec=mcp.ClientSession)
+
+    stuck_tool = mock.Mock(spec=types.Tool)
+    stuck_tool.name = "tool_a"
+    mock_session.list_tools.return_value = mock.AsyncMock(tools=[stuck_tool], next_cursor="page-2")
+    mock_session.list_resources.return_value = mock.AsyncMock(resources=[], next_cursor=None)
+    mock_session.list_prompts.return_value = mock.AsyncMock(prompts=[], next_cursor=None)
+
+    group = ClientSessionGroup(exit_stack=mock_exit_stack)
+    with mock.patch.object(group, "_establish_session", return_value=(mock_server_info, mock_session)):
+        with pytest.raises(RuntimeError, match="did not advance"):
+            await group.connect_to_server(StdioServerParameters(command="test"))