Reject interpreter pool activity executors

CHANGELOG.md

@@ -42,6 +42,8 @@ to include examples, links to docs, or any other relevant information.
 
 - `temporalio.contrib.strands` activity and MCP tools now give the model the
   Activity's failure message and expose its exception to after-tool hooks.
+- Reject unsupported `InterpreterPoolExecutor` activity executors when creating
+  a worker instead of failing during activity execution.
 
 - Encoding a datetime search attribute without a timezone now raises
   `ValueError("Timezone must be present on all search attribute dates")` on

temporalio/worker/_worker.py

@@ -172,7 +172,8 @@ def __init__(
             activity_executor: Concurrent executor to use for non-async
                 activities. This is required if any activities are non-async.
                 :py:class:`concurrent.futures.ThreadPoolExecutor` is
-                recommended. If this is a
+                recommended. ``concurrent.futures.InterpreterPoolExecutor``
+                is not supported. If this is a
                 :py:class:`concurrent.futures.ProcessPoolExecutor`, all
                 non-async activities must be picklable. ``max_workers`` on the
                 executor should at least be ``max_concurrent_activities`` or a
@@ -417,6 +418,13 @@ def _init_from_config(self, client: temporalio.client.Client, config: WorkerConf
         Client is safe to take separately since it can't be modified by worker plugins.
         """
         self._config = config
+        if sys.version_info >= (3, 14) and isinstance(
+            config.get("activity_executor"), concurrent.futures.InterpreterPoolExecutor
+        ):
+            raise ValueError(  # pyright: ignore[reportUnreachable]
+                "InterpreterPoolExecutor is not supported as an activity_executor. "
+                "Use ThreadPoolExecutor or ProcessPoolExecutor instead."
+            )
         if not (
             config.get("activities")
             or config.get("nexus_service_handlers")

tests/worker/test_worker.py

@@ -5,11 +5,13 @@
 import multiprocessing
 import multiprocessing.context
 import os
+import sys
 import uuid
 from collections.abc import Awaitable, Callable, Sequence
 from contextlib import contextmanager
 from datetime import timedelta
 from typing import Any
+from unittest.mock import Mock
 from urllib.request import urlopen
 
 import nexusrpc
@@ -77,6 +79,36 @@ def test_load_default_worker_binary_id():
     assert val1 == val2
 
 
+@pytest.mark.skipif(
+    sys.version_info < (3, 14),
+    reason="InterpreterPoolExecutor requires Python 3.14 or newer",
+)
+@pytest.mark.parametrize("subclass", [False, True])
+def test_activity_executor_rejects_interpreter_pool(subclass: bool):
+    if sys.version_info >= (3, 14):
+
+        class CustomInterpreterPoolExecutor(concurrent.futures.InterpreterPoolExecutor):  # pyright: ignore[reportUnreachable]
+            pass
+
+        executor_class = concurrent.futures.InterpreterPoolExecutor
+        if subclass:
+            executor_class = CustomInterpreterPoolExecutor
+
+        client = Mock(spec=Client)
+        client.config.return_value = {"plugins": []}
+        with executor_class(max_workers=1) as executor:
+            with pytest.raises(
+                ValueError,
+                match="InterpreterPoolExecutor is not supported as an activity_executor",
+            ):
+                Worker(
+                    client,
+                    task_queue="test-interpreter-pool",
+                    activities=[never_run_activity],
+                    activity_executor=executor,
+                )
+
+
 @activity.defn
 async def never_run_activity() -> None:
     raise NotImplementedError