diff --git a/CHANGELOG.md b/CHANGELOG.md
index be3444177..bf78bd257 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -28,6 +28,12 @@ to include examples, links to docs, or any other relevant information.
 
 ### Fixed
 
+- Starting an activity from a Nexus operation handler without a
+  `start_to_close_timeout`/`schedule_to_close_timeout`, or with a negative
+  `start_delay`, now fails immediately with a non-retryable `BAD_REQUEST`
+  instead of retrying as an `INTERNAL` error until `schedule_to_close_timeout`
+  elapses.
+
 ### Security
 
 ## [1.32.0] - 2026-08-24
diff --git a/temporalio/client/_impl.py b/temporalio/client/_impl.py
index 78471baf7..34816e161 100644
--- a/temporalio/client/_impl.py
+++ b/temporalio/client/_impl.py
@@ -136,6 +136,14 @@
     from ._client import Client
 
 
+class _StartActivityInputError(temporalio.exceptions.TemporalError, ValueError):
+    """A caller-fixable problem with a :py:meth:`Client.start_activity` call.
+
+    Raised for preconditions on ``StartActivityInput`` that are checked before
+    any RPC is issued.
+    """
+
+
 class _ClientImpl(OutboundInterceptor):  # pyright: ignore[reportUnusedClass]
     def __init__(self, client: Client) -> None:  # type: ignore
         # We are intentionally not calling the base class's __init__ here
@@ -526,11 +534,11 @@ async def terminate_workflow(self, input: TerminateWorkflowInput) -> None:
     async def start_activity(self, input: StartActivityInput) -> ActivityHandle[Any]:
         """Start an activity and return a handle to it."""
         if not (input.start_to_close_timeout or input.schedule_to_close_timeout):
-            raise ValueError(
+            raise _StartActivityInputError(
                 "Activity must have start_to_close_timeout or schedule_to_close_timeout"
             )
         if input.start_delay is not None and input.start_delay < timedelta(0):
-            raise ValueError("start_delay must be non-negative")
+            raise _StartActivityInputError("start_delay must be non-negative")
         req = await self._build_start_activity_execution_request(input)
 
         resp: temporalio.api.workflowservice.v1.StartActivityExecutionResponse
diff --git a/temporalio/nexus/_operation_context.py b/temporalio/nexus/_operation_context.py
index c44088f21..896f9c4aa 100644
--- a/temporalio/nexus/_operation_context.py
+++ b/temporalio/nexus/_operation_context.py
@@ -168,7 +168,7 @@ def _try_temporal_context() -> (
     return start_ctx or cancel_ctx
 
 
-def _try_start_operation_context() -> _TemporalStartOperationContext | None:  # pyright: ignore[reportUnusedFunction]
+def _try_start_operation_context() -> _TemporalStartOperationContext | None:
     """Return the active Nexus start-operation context, if any."""
     return _temporal_start_operation_context.get(None)
 
diff --git a/temporalio/worker/_nexus.py b/temporalio/worker/_nexus.py
index 90ba40382..16707244e 100644
--- a/temporalio/worker/_nexus.py
+++ b/temporalio/worker/_nexus.py
@@ -28,6 +28,7 @@
 import temporalio.bridge.proto.nexus
 import temporalio.bridge.worker
 import temporalio.client
+import temporalio.client._impl
 import temporalio.common
 import temporalio.converter
 import temporalio.nexus
@@ -611,6 +612,12 @@ def _exception_to_handler_error(err: BaseException) -> nexusrpc.HandlerError:
     # https://github.com/temporalio/sdk-typescript/blob/nexus/packages/worker/src/nexus.ts
     if isinstance(err, nexusrpc.HandlerError):
         return err
+    elif isinstance(err, temporalio.client._impl._StartActivityInputError):
+        handler_err = nexusrpc.HandlerError(
+            str(err),
+            type=nexusrpc.HandlerErrorType.BAD_REQUEST,
+            retryable_override=False,
+        )
     elif isinstance(err, ApplicationError):
         handler_err = nexusrpc.HandlerError(
             message="Handler failed with non-retryable application error",
diff --git a/tests/nexus/test_handler_error_conversion.py b/tests/nexus/test_handler_error_conversion.py
new file mode 100644
index 000000000..9bc7545c7
--- /dev/null
+++ b/tests/nexus/test_handler_error_conversion.py
@@ -0,0 +1,26 @@
+import nexusrpc
+import pytest
+
+import temporalio.client._impl
+import temporalio.worker._nexus
+
+
+@pytest.mark.parametrize(
+    "message",
+    [
+        "Activity must have start_to_close_timeout or schedule_to_close_timeout",
+        "start_delay must be non-negative",
+    ],
+)
+def test_start_activity_input_error_maps_to_nexus_bad_request(message: str):
+    """Pins the classification `_exception_to_handler_error` relies on to give
+    these two `start_activity` preconditions non-retryable BAD_REQUEST
+    semantics inside a Nexus operation handler, instead of falling through to
+    the generic, retryable INTERNAL branch.
+    """
+    handler_error = temporalio.worker._nexus._exception_to_handler_error(
+        temporalio.client._impl._StartActivityInputError(message)
+    )
+    assert handler_error.type == nexusrpc.HandlerErrorType.BAD_REQUEST
+    assert handler_error.retryable is False
+    assert handler_error.message == message
diff --git a/tests/test_activity.py b/tests/test_activity.py
index 3a63f51ac..aa6e28e8e 100644
--- a/tests/test_activity.py
+++ b/tests/test_activity.py
@@ -457,6 +457,19 @@ async def test_start_activity_rejects_negative_start_delay(client: Client):
         )
 
 
+async def test_start_activity_requires_a_timeout(client: Client):
+    with pytest.raises(
+        ValueError,
+        match="Activity must have start_to_close_timeout or schedule_to_close_timeout",
+    ):
+        await client.start_activity(
+            increment,
+            args=(1,),
+            id=str(uuid.uuid4()),
+            task_queue=str(uuid.uuid4()),
+        )
+
+
 async def test_get_result(client: Client, env: WorkflowEnvironment):
     if env.supports_time_skipping:
         pytest.skip(