fix: allow replacing worker client on default runtime

temporalio/worker/_worker.py

@@ -731,11 +731,13 @@ def client(self, value: temporalio.client.Client) -> None:
 
         Changing the client will make sure the worker starts using it for the
         next calls it makes. However, outstanding client calls will still
-        complete with the existing client. The new client cannot be "lazy" and
-        must be using the same runtime as the current client.
+        complete with the existing client. The new client cannot be "lazy" and,
+        when configured with an explicit runtime, must use the current client's
+        runtime.
         """
         bridge_client = _extract_bridge_client_for_worker(value)
-        if self._runtime is not bridge_client.config.runtime:
+        new_runtime = bridge_client.config.runtime
+        if new_runtime is not None and self._runtime is not new_runtime:
             raise ValueError(
                 "New client is not on the same runtime as the existing client"
             )

tests/worker/test_workflow.py

@@ -6279,6 +6279,22 @@ async def test_workflow_replace_worker_client_diff_runtimes_fail(
             worker.client = other_client
 
 
+async def test_workflow_replace_worker_client_implicit_runtime(
+    env: WorkflowEnvironment,
+):
+    # An implicit runtime must not be resolved just to compare it with the
+    # worker's explicit runtime. The worker owns the runtime for its bridge
+    # client replacement.
+    worker_runtime = Runtime(telemetry=TelemetryConfig())
+    worker_client = await env.connect_client(runtime=worker_runtime)
+    default_runtime_client = await Client.connect(
+        worker_client.service_client.config.target_host,
+        namespace=worker_client.namespace,
+    )
+    async with new_worker(worker_client, HelloWorkflow) as worker:
+        worker.client = default_runtime_client
+
+
 @activity.defn(dynamic=True)
 async def return_name_activity(_args: Sequence[RawValue]) -> str:
     return activity.info().activity_type