fix: ServerRunner passes None to notification handlers when params absent

Matches Server._handle_notification: when the wire omits params, the
handler receives None, not an empty model. _make_context now accepts
typed_params=None.

Author: maxisbey

src/mcp/server/runner.py

@@ -227,13 +227,17 @@ async def _on_notify(
         if entry is None:
             logger.debug("no handler for notification %s", method)
             return
-        typed_params = entry.params_type.model_validate(params or {})
+        # Absent wire params reach the handler as `None`, not an empty model
+        # (matches the existing `Server._handle_notification`).
+        typed_params = entry.params_type.model_validate(params) if params is not None else None
         ctx = self._make_context(dctx, typed_params)
         # TODO: cast goes away when `ServerRequestContext = Context` lands.
         await cast(Any, entry.handler)(ctx, typed_params)
 
-    def _make_context(self, dctx: DispatchContext[TransportContext], typed_params: BaseModel) -> Context[LifespanT]:
-        meta = getattr(typed_params, "meta", None)
+    def _make_context(
+        self, dctx: DispatchContext[TransportContext], typed_params: BaseModel | None
+    ) -> Context[LifespanT]:
+        meta = getattr(typed_params, "meta", None) if typed_params is not None else None
         return Context(dctx, lifespan=self.lifespan_state, connection=self.connection, meta=meta)
 
     def _handle_initialize(self, params: Mapping[str, Any] | None) -> dict[str, Any]:

tests/server/test_runner.py

@@ -209,10 +209,14 @@ async def on_roots_changed(ctx: Any, params: Any) -> None:
     server.add_notification_handler("notifications/roots/list_changed", NotificationParams, on_roots_changed)
     async with connected_runner(server) as (client, _):
         await client.notify("notifications/roots/list_changed", None)
+        await client.notify("notifications/roots/list_changed", {})
         # DirectDispatcher delivers synchronously; one yield is enough.
         await anyio.lowlevel.checkpoint()
-    assert len(seen) == 1
+    assert len(seen) == 2
     assert isinstance(seen[0][0], Context)
+    # Absent wire params reach the handler as None; present-but-empty validates.
+    assert seen[0][1] is None
+    assert isinstance(seen[1][1], NotificationParams)
 
 
 @pytest.mark.anyio