From e1900cd25af155fa33b8a5faa22e0b1407c60581 Mon Sep 17 00:00:00 2001
From: Chris <64613756+wuchris-ch@users.noreply.github.com>
Date: Fri, 9 Oct 2026 20:50:37 -0700
Subject: [PATCH 1/2] Allow replacing a worker's client when both use the
 default runtime

Fixes #657.
---
 changelog/fixed/relay-racing-raccoon.md |  1 +
 temporalio/service.py                   |  3 +++
 temporalio/worker/_worker.py            |  6 +++--
 tests/worker/test_workflow.py           | 34 +++++++++++++++++++++++++
 4 files changed, 42 insertions(+), 2 deletions(-)
 create mode 100644 changelog/fixed/relay-racing-raccoon.md

diff --git a/changelog/fixed/relay-racing-raccoon.md b/changelog/fixed/relay-racing-raccoon.md
new file mode 100644
index 000000000..d2d84df6b
--- /dev/null
+++ b/changelog/fixed/relay-racing-raccoon.md
@@ -0,0 +1 @@
+Setting `Worker.client` no longer raises "New client is not on the same runtime as the existing client" when both clients connected without a `runtime`, since both use the default runtime. The check now compares the runtime each client actually connected with.
diff --git a/temporalio/service.py b/temporalio/service.py
index 926dfbd46..5c390f92a 100644
--- a/temporalio/service.py
+++ b/temporalio/service.py
@@ -418,6 +418,8 @@ def __init__(self, config: ConnectConfig) -> None:
         self._bridge_config = config._to_bridge_config()
         self._bridge_client: temporalio.bridge.client.Client | None = None
         self._bridge_client_connect_lock = asyncio.Lock()
+        # Runtime we connected with, so the default one if none was set
+        self._runtime: temporalio.runtime.Runtime | None = None
 
     async def _connected_client(self) -> temporalio.bridge.client.Client:
         # Fast path avoids touching the lock once connected. This keeps the
@@ -433,6 +435,7 @@ async def _connected_client(self) -> temporalio.bridge.client.Client:
                     runtime._core_runtime,
                     self._bridge_config,
                 )
+                self._runtime = runtime
             return self._bridge_client
 
     @property
diff --git a/temporalio/worker/_worker.py b/temporalio/worker/_worker.py
index 39faf615a..3c6effd80 100644
--- a/temporalio/worker/_worker.py
+++ b/temporalio/worker/_worker.py
@@ -491,7 +491,9 @@ def _init_from_config(self, client: temporalio.client.Client, config: WorkerConf
 
         self._activity_worker: _ActivityWorker | None = None
         self._runtime = (
-            bridge_client.config.runtime or temporalio.runtime.Runtime.default()
+            bridge_client._runtime
+            or bridge_client.config.runtime
+            or temporalio.runtime.Runtime.default()
         )
         activities = config.get("activities")
         if activities:
@@ -743,7 +745,7 @@ def client(self, value: temporalio.client.Client) -> None:
         must be using the same runtime as the current client.
         """
         bridge_client = _extract_bridge_client_for_worker(value)
-        if self._runtime is not bridge_client.config.runtime:
+        if self._runtime is not bridge_client._runtime:
             raise ValueError(
                 "New client is not on the same runtime as the existing client"
             )
diff --git a/tests/worker/test_workflow.py b/tests/worker/test_workflow.py
index 80ac3d27b..d89b1d902 100644
--- a/tests/worker/test_workflow.py
+++ b/tests/worker/test_workflow.py
@@ -6669,6 +6669,40 @@ async def test_workflow_replace_worker_client_diff_runtimes_fail(
             worker.client = other_client
 
 
+async def test_workflow_replace_worker_client_implicit_default_runtime(
+    env: WorkflowEnvironment,
+):
+    # Neither client sets a runtime, so both are on the default one
+    worker_client = await env.connect_client(runtime=None)
+    new_client = await env.connect_client(runtime=None)
+    async with new_worker(worker_client, HelloWorkflow) as worker:
+        worker.client = new_client
+        assert worker.client is new_client
+        result = await new_client.execute_workflow(
+            HelloWorkflow.run,
+            "Temporal",
+            id=f"workflow-{uuid.uuid4()}",
+            task_queue=worker.task_queue,
+        )
+        assert result == "Hello, Temporal!"
+
+
+async def test_workflow_replace_worker_client_implicit_default_runtime_mismatch_fail(
+    env: WorkflowEnvironment,
+):
+    # No runtime means the default one, which isn't the worker's
+    worker_client = await env.connect_client(
+        runtime=Runtime(telemetry=TelemetryConfig())
+    )
+    default_runtime_client = await env.connect_client(runtime=None)
+    async with new_worker(worker_client, HelloWorkflow) as worker:
+        with pytest.raises(
+            ValueError,
+            match="New client is not on the same runtime as the existing client",
+        ):
+            worker.client = default_runtime_client
+
+
 @activity.defn(dynamic=True)
 async def return_name_activity(_args: Sequence[RawValue]) -> str:
     return activity.info().activity_type

From e4baa5abd9098d1396d4a30d236d993e6c6bc1ec Mon Sep 17 00:00:00 2001
From: Chris <64613756+wuchris-ch@users.noreply.github.com>
Date: Sun, 11 Oct 2026 22:26:14 -0700
Subject: [PATCH 2/2] Drop the workflow run from the default runtime test

---
 tests/worker/test_workflow.py | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/tests/worker/test_workflow.py b/tests/worker/test_workflow.py
index d89b1d902..01a2fcea8 100644
--- a/tests/worker/test_workflow.py
+++ b/tests/worker/test_workflow.py
@@ -6678,13 +6678,6 @@ async def test_workflow_replace_worker_client_implicit_default_runtime(
     async with new_worker(worker_client, HelloWorkflow) as worker:
         worker.client = new_client
         assert worker.client is new_client
-        result = await new_client.execute_workflow(
-            HelloWorkflow.run,
-            "Temporal",
-            id=f"workflow-{uuid.uuid4()}",
-            task_queue=worker.task_queue,
-        )
-        assert result == "Hello, Temporal!"
 
 
 async def test_workflow_replace_worker_client_implicit_default_runtime_mismatch_fail(