fix: only set Nexus start on_conflict_options when there's something to attach

CHANGELOG.md

@@ -28,6 +28,9 @@ to include examples, links to docs, or any other relevant information.
 
 ### Fixed
 
+- Nexus-context workflow/activity starts no longer set `on_conflict_options` when there are no links
+  or callbacks to attach.
+
 ### Security
 
 ## [1.32.0] - 2026-08-24

temporalio/nexus/_operation_context.py

@@ -821,19 +821,17 @@ def _apply_nexus_context_to_start_workflow_request(  # pyright: ignore[reportUnu
     This is a no-op outside a Nexus operation context. Within one, it attaches
     inbound links and configures conflict handling to preserve Nexus metadata.
     The Nexus request ID and completion callbacks are added only when the
-    workflow is backing the Nexus operation.
+    workflow is backing the Nexus operation. on_conflict_options is populated
+    only when there are links or callbacks to attach.
     """
     nexus_ctx = _try_start_operation_context()
     if nexus_ctx is not None:
-        req.on_conflict_options.attach_request_id = True
-        req.on_conflict_options.attach_completion_callbacks = True
-        req.on_conflict_options.attach_links = True
-
         request_links = nexus_ctx._get_request_links()
 
         # Links are duplicated on request for compatibility with older server versions.
         req.links.extend(request_links)
 
+        callbacks: list[NexusCallback] = []
         if _in_nexus_backing_start_context():
             req.request_id = nexus_ctx.nexus_context.request_id
             callbacks = nexus_ctx._get_callbacks(
@@ -854,6 +852,11 @@ def _apply_nexus_context_to_start_workflow_request(  # pyright: ignore[reportUnu
                 for callback in callbacks
             )
 
+        if request_links or callbacks:
+            req.on_conflict_options.attach_request_id = True
+            req.on_conflict_options.attach_completion_callbacks = True
+            req.on_conflict_options.attach_links = True
+
 
 def _apply_start_workflow_response_to_nexus_context(  # pyright: ignore[reportUnusedFunction]
     workflow_handle: temporalio.client.WorkflowHandle[Any, Any],
@@ -872,16 +875,15 @@ def _apply_nexus_context_to_start_activity_request(  # pyright: ignore[reportUnu
     the Nexus request ID and configures conflict handling to preserve the Nexus
     metadata. Inbound links are attached to the completion callback when the
     activity backs the operation and to the request otherwise.
+    on_conflict_options is populated only when there are links or callbacks to
+    attach.
     """
     nexus_ctx = _try_start_operation_context()
     if nexus_ctx is not None:
-        req.on_conflict_options.attach_request_id = True
-        req.on_conflict_options.attach_completion_callbacks = True
-        req.on_conflict_options.attach_links = True
-
         req.request_id = nexus_ctx.nexus_context.request_id
         request_links = nexus_ctx._get_request_links()
 
+        callbacks: list[NexusCallback] = []
         if _in_nexus_backing_start_context():
             callbacks = nexus_ctx._get_callbacks(
                 OperationToken(
@@ -903,6 +905,11 @@ def _apply_nexus_context_to_start_activity_request(  # pyright: ignore[reportUnu
         else:
             req.links.extend(request_links)
 
+        if request_links or callbacks:
+            req.on_conflict_options.attach_request_id = True
+            req.on_conflict_options.attach_completion_callbacks = True
+            req.on_conflict_options.attach_links = True
+
 
 def _apply_start_activity_response_to_nexus_context(  # pyright: ignore[reportUnusedFunction]
     activity_id: str,

tests/nexus/test_link_propagation.py

@@ -130,6 +130,39 @@ def nexus_ctx() -> Generator[_TemporalStartOperationContext]:
         )
 
 
+@pytest.fixture
+def nexus_ctx_linkless() -> Generator[_TemporalStartOperationContext]:
+    """Like `nexus_ctx`, but with no inbound links and no callback URL."""
+    nexus_context = nexusrpc.handler.StartOperationContext(
+        service="svc",
+        operation="op",
+        headers={},
+        request_id="req-id",
+        callback_url=None,
+        inbound_links=[],
+        callback_headers={},
+        task_cancellation=_NexusTaskCancellation(),
+    )
+    ctx = temporalio.nexus._operation_context._TemporalStartOperationContext(
+        nexus_context=nexus_context,
+        client=mock.MagicMock(namespace=NAMESPACE),
+        info=lambda: temporalio.nexus.Info(
+            endpoint="endpoint", namespace=NAMESPACE, task_queue="tq"
+        ),
+        _runtime_metric_meter=mock.MagicMock(),
+        _worker_shutdown_event=mock.MagicMock(),
+    )
+    token = temporalio.nexus._operation_context._temporal_start_operation_context.set(
+        ctx
+    )
+    try:
+        yield ctx
+    finally:
+        temporalio.nexus._operation_context._temporal_start_operation_context.reset(
+            token
+        )
+
+
 def _make_client_impl(workflow_service: Any) -> _ClientImpl:
     client = mock.MagicMock()
     client.namespace = NAMESPACE
@@ -749,6 +782,30 @@ async def test_backing_activity_start_gets_nexus_request_fields() -> None:
     assert list(req.completion_callbacks[0].links) == [_inbound_nexus_link()]
 
 
+@pytest.mark.usefixtures("nexus_ctx_linkless")
+async def test_activity_start_omits_on_conflict_options_when_linkless() -> None:
+    impl = _make_client_impl(mock.MagicMock())
+
+    req = await impl._build_start_activity_execution_request(_start_activity_input())
+
+    assert req.request_id == "req-id"
+    assert not req.HasField("on_conflict_options")
+
+
+@pytest.mark.usefixtures("nexus_ctx_linkless")
+async def test_backing_activity_start_omits_on_conflict_options_when_linkless() -> None:
+    impl = _make_client_impl(mock.MagicMock())
+
+    with temporalio.nexus._operation_context._nexus_backing_start_context():
+        req = await impl._build_start_activity_execution_request(
+            _start_activity_input()
+        )
+
+    assert req.request_id == "req-id"
+    assert len(req.completion_callbacks) == 0
+    assert not req.HasField("on_conflict_options")
+
+
 # ── handler-level: backlinks land on the StartOperationResponse ──────────────────────────────
 
 # A response link that a handler stashes on ctx.outbound_links, mimicking what a signal RPC inside