diff --git a/CHANGELOG.md b/CHANGELOG.md
index e2ce1f736..1c5ab1a90 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -55,6 +55,7 @@ to include examples, links to docs, or any other relevant information.
 
 ### Fixed
 
+- Standalone activity start requests now include a unique request ID so RPC retries are deduplicated.
 - OpenTelemetry trace and span IDs propagated by concurrent workers no longer
   interfere with each other, preserving the correct parent-child hierarchy.
 - The `google-adk` extra now depends on `mcp`, so fresh installs of
diff --git a/temporalio/client/_impl.py b/temporalio/client/_impl.py
index 9595351f2..2799deb27 100644
--- a/temporalio/client/_impl.py
+++ b/temporalio/client/_impl.py
@@ -627,6 +627,7 @@ async def _build_start_activity_execution_request(
         req = temporalio.api.workflowservice.v1.StartActivityExecutionRequest(
             namespace=self._client.namespace,
             identity=self._client.identity,
+            request_id=str(uuid.uuid4()),
             activity_id=input.id,
             activity_type=temporalio.api.common.v1.ActivityType(
                 name=input.activity_type
@@ -677,8 +678,8 @@ async def _build_start_activity_execution_request(
         # Set priority
         req.priority.CopyFrom(input.priority._to_proto())
 
-        # Add request_id, links, and completion callbacks from the Nexus context
-        # if not in a Nexus context, this is a no-op
+        # Nexus starts use the inbound request ID so retries across the Nexus
+        # boundary resolve to the same activity execution.
         temporalio.nexus._operation_context._apply_nexus_context_to_start_activity_request(
             req
         )
diff --git a/tests/test_activity.py b/tests/test_activity.py
index 874e2b620..3a63f51ac 100644
--- a/tests/test_activity.py
+++ b/tests/test_activity.py
@@ -2,9 +2,11 @@
 import uuid
 from dataclasses import dataclass
 from datetime import timedelta
+from unittest import mock
 
 import pytest
 
+import temporalio.api.workflowservice.v1
 from temporalio import activity, workflow
 from temporalio.client import (
     ActivityExecutionCount,
@@ -78,6 +80,31 @@ def sync_increment(self, x: int) -> int:
         return x + 1
 
 
+async def test_start_activity_generates_request_id() -> None:
+    start_activity_execution = mock.AsyncMock(
+        return_value=temporalio.api.workflowservice.v1.StartActivityExecutionResponse(
+            run_id="activity-run-id"
+        )
+    )
+    service_client = mock.MagicMock()
+    service_client.config.identity = "test-identity"
+    service_client.workflow_service.start_activity_execution = start_activity_execution
+    client = Client(service_client)
+
+    for activity_id in ("activity-id-1", "activity-id-2"):
+        await client.start_activity(
+            increment,
+            1,
+            id=activity_id,
+            task_queue="task-queue",
+            start_to_close_timeout=timedelta(seconds=1),
+        )
+
+    requests = [call.args[0] for call in start_activity_execution.call_args_list]
+    assert all(uuid.UUID(request.request_id).version == 4 for request in requests)
+    assert requests[0].request_id != requests[1].request_id
+
+
 class TestDescribe:
     @pytest.fixture
     async def activity_handle(self, client: Client, env: WorkflowEnvironment):