diff --git a/features/actor_service_coverage.feature b/features/actor_service_coverage.feature
index d717eb47e..4dcfc5bd6 100644
--- a/features/actor_service_coverage.feature
+++ b/features/actor_service_coverage.feature
@@ -90,3 +90,26 @@ Feature: Actor service coverage
     And any existing actors are cleared
     When I ensure the default mock actor with env value "0"
     Then the result should be None
+
+  Scenario: Ensuring default mock actor is idempotent
+    Given an actor service with stubbed dependencies
+    When I ensure the default mock actor with env value "true"
+    Then the returned actor name should be "local/mock-default"
+    And the stored actor "local/mock-default" should be default
+    When I ensure the default mock actor with env value "true" again
+    Then the returned actor name should be "local/mock-default"
+    And only one "local/mock-default" actor should exist
+
+  Scenario: Ensuring default mock actor with existing default is safe
+    Given an actor service with stubbed dependencies
+    And an actor named "local/existing-default" exists as default
+    When I ensure the default mock actor with env value "true"
+    Then the returned actor name should be "local/existing-default"
+    And the stored actor "local/existing-default" should be default
+    And only one default actor should exist
+
+  Scenario: Ensuring default mock actor creates exactly one actor
+    Given an actor service with stubbed dependencies
+    When I ensure the default mock actor with env value "true"
+    Then exactly one actor should exist
+    And the stored actor "local/mock-default" should be default
diff --git a/features/steps/actor_service_steps.py b/features/steps/actor_service_steps.py
index 3b7d39ebc..090c003fa 100644
--- a/features/steps/actor_service_steps.py
+++ b/features/steps/actor_service_steps.py
@@ -296,3 +296,30 @@ def after_scenario(context: Context, scenario) -> None:
     if hasattr(context, "_cleanup_handlers"):
         for handler in context._cleanup_handlers:
             handler()
+
+
+@when('I ensure the default mock actor with env value "{value}" again')
+def step_ensure_default_mock_actor_again(context: Context, value: str) -> None:
+    os.environ["CLEVERAGENTS_TESTING_USE_MOCK_AI"] = value
+    context.result_actor = context.actor_service.ensure_default_mock_actor()
+
+
+@then('only one "{name}" actor should exist')
+def step_only_one_actor_exists(context: Context, name: str) -> None:
+    actors = context.actor_repo.list_all()
+    count = sum(1 for actor in actors if actor.name == name)
+    assert count == 1, f"Expected 1 actor named {name}, but found {count}"
+
+
+@then("only one default actor should exist")
+def step_only_one_default_actor(context: Context) -> None:
+    actors = context.actor_repo.list_all()
+    default_count = sum(1 for actor in actors if actor.is_default)
+    assert default_count == 1, f"Expected 1 default actor, but found {default_count}"
+
+
+@then("exactly one actor should exist")
+def step_exactly_one_actor(context: Context) -> None:
+    actors = context.actor_repo.list_all()
+    count = len(actors)
+    assert count == 1, f"Expected 1 actor, but found {count}"
diff --git a/src/cleveragents/application/services/actor_service.py b/src/cleveragents/application/services/actor_service.py
index 1b6c4335b..73476949f 100644
--- a/src/cleveragents/application/services/actor_service.py
+++ b/src/cleveragents/application/services/actor_service.py
@@ -182,6 +182,10 @@ class ActorService:
 
         This is idempotent and only runs when CLEVERAGENTS_TESTING_USE_MOCK_AI is true
         unless ``force`` is set to True (used by tests to provision a fallback actor).
+
+        The check for an existing default and the creation of the mock actor are
+        performed within a single atomic transaction to prevent TOCTOU race conditions
+        in concurrent environments (e.g., parallel test workers).
         """
         if not force and os.environ.get(
             "CLEVERAGENTS_TESTING_USE_MOCK_AI", ""
@@ -192,30 +196,32 @@ class ActorService:
         ):
             return None
 
+        # Perform both check and creation in a single atomic transaction
+        # to prevent TOCTOU race condition
         with self.unit_of_work.transaction() as ctx:
+            # Check if a default actor already exists
             existing_default = ctx.actors.get_default()
             if existing_default:
                 return existing_default
 
-        # No default yet: create or promote a mock actor
-        now = datetime.now()
-        mock_name = "local/mock-default"
-        mock_actor = Actor(
-            id=None,
-            name=mock_name,
-            provider="MockProvider",
-            model="mock-gpt-4",
-            config_blob={},
-            config_hash=Actor.compute_hash({}),
-            graph_descriptor=None,
-            unsafe=True,
-            is_built_in=True,
-            is_default=True,
-            created_at=now,
-            updated_at=now,
-        )
+            # No default yet: create or promote a mock actor
+            now = datetime.now()
+            mock_name = "local/mock-default"
+            mock_actor = Actor(
+                id=None,
+                name=mock_name,
+                provider="MockProvider",
+                model="mock-gpt-4",
+                config_blob={},
+                config_hash=Actor.compute_hash({}),
+                graph_descriptor=None,
+                unsafe=True,
+                is_built_in=True,
+                is_default=True,
+                created_at=now,
+                updated_at=now,
+            )
 
-        with self.unit_of_work.transaction() as ctx:
             existing = ctx.actors.get_by_name(mock_name)
             if existing:
                 if not existing.is_default: