diff --git a/agents-api/agents_api/workflows/task_execution/__init__.py b/agents-api/agents_api/workflows/task_execution/__init__.py
index 0e5f5055c..595ec3175 100644
--- a/agents-api/agents_api/workflows/task_execution/__init__.py
+++ b/agents-api/agents_api/workflows/task_execution/__init__.py
@@ -59,6 +59,7 @@
     from ...common.retry_policies import DEFAULT_RETRY_POLICY
     from ...env import (
         debug,
+        task_max_parallelism,
         temporal_heartbeat_timeout,
         temporal_schedule_to_close_timeout,
         testing,
@@ -304,7 +305,9 @@ async def _handle_MapReduceStep(
             return PartialTransition(output=None)
 
         parallelism = step.parallelism
-        if parallelism is None or parallelism == 1:
+        if parallelism is None:
+            parallelism = task_max_parallelism
+        if parallelism == 1:
             result = await execute_map_reduce_step(
                 context=self.context,
                 execution_input=self.context.execution_input,
diff --git a/agents-api/agents_api/workflows/task_execution/helpers.py b/agents-api/agents_api/workflows/task_execution/helpers.py
index 3bf023bb6..c0e4fb6f6 100644
--- a/agents-api/agents_api/workflows/task_execution/helpers.py
+++ b/agents-api/agents_api/workflows/task_execution/helpers.py
@@ -15,6 +15,7 @@
         TransitionTarget,
         Workflow,
         WorkflowStep,
+        YieldStep,
     )
     from ...common.protocol.tasks import (
         ExecutionInput,
@@ -274,6 +275,10 @@ async def execute_map_reduce_step_parallel(
     workflow.logger.info(f"MapReduce step: Processing {len(items)} items")
     results = initial
 
+    if isinstance(context.current_step.map, YieldStep):
+        msg = "Subworkflow step not supported in parallel map reduce"
+        raise ValueError(msg)
+
     parallelism = min(parallelism, task_max_parallelism)
     assert parallelism > 1, "Parallelism must be greater than 1"
 
diff --git a/agents-api/tests/test_workflow_helpers.py b/agents-api/tests/test_workflow_helpers.py
new file mode 100644
index 000000000..a1c8bf43f
--- /dev/null
+++ b/agents-api/tests/test_workflow_helpers.py
@@ -0,0 +1,119 @@
+import uuid
+from unittest.mock import patch
+
+from agents_api.autogen.openapi_model import (
+    Agent,
+    MapReduceStep,
+    PromptItem,
+    PromptStep,
+    TaskSpecDef,
+    TransitionTarget,
+    Workflow,
+    YieldStep,
+)
+from agents_api.common.protocol.tasks import (
+    ExecutionInput,
+    StepContext,
+)
+from agents_api.common.utils.datetime import utcnow
+from agents_api.workflows.task_execution.helpers import execute_map_reduce_step_parallel
+from ward import raises, test
+
+
+@test("execute_map_reduce_step_parallel: subworkflow step not supported")
+async def _():
+    async def _resp():
+        return "response"
+
+    subworkflow_step = YieldStep(
+        kind_="yield", workflow="subworkflow", arguments={"test": "$ _"}
+    )
+
+    step = MapReduceStep(
+        kind_="map_reduce",
+        map=subworkflow_step,
+        over="$ [1, 2, 3]",
+        parallelism=3,
+    )
+
+    execution_input = ExecutionInput(
+        developer_id=uuid.uuid4(),
+        agent=Agent(
+            id=uuid.uuid4(), name="test agent", created_at=utcnow(), updated_at=utcnow()
+        ),
+        agent_tools=[],
+        arguments={},
+        task=TaskSpecDef(
+            name="task1",
+            tools=[],
+            workflows=[Workflow(name="main", steps=[step])],
+        ),
+    )
+
+    context = StepContext(
+        execution_input=execution_input,
+        current_input={"current_input": "value 1"},
+        cursor=TransitionTarget(
+            workflow="main",
+            step=0,
+        ),
+    )
+    with patch("agents_api.workflows.task_execution.helpers.workflow") as workflow:
+        workflow.execute_activity.return_value = await _resp()
+        with raises(ValueError):
+            await execute_map_reduce_step_parallel(
+                context=context,
+                map_defn=step.map,
+                execution_input=execution_input,
+                items=["1", "2", "3"],
+                current_input={},
+            )
+
+
+@test("execute_map_reduce_step_parallel: parallelism must be greater than 1")
+async def _():
+    async def _resp():
+        return "response"
+
+    subworkflow_step = PromptStep(prompt=[PromptItem(content="hi there", role="user")])
+
+    step = MapReduceStep(
+        kind_="map_reduce",
+        map=subworkflow_step,
+        over="$ [1, 2, 3]",
+        parallelism=1,
+    )
+
+    execution_input = ExecutionInput(
+        developer_id=uuid.uuid4(),
+        agent=Agent(
+            id=uuid.uuid4(), name="test agent", created_at=utcnow(), updated_at=utcnow()
+        ),
+        agent_tools=[],
+        arguments={},
+        task=TaskSpecDef(
+            name="task1",
+            tools=[],
+            workflows=[Workflow(name="main", steps=[step])],
+        ),
+    )
+
+    context = StepContext(
+        execution_input=execution_input,
+        current_input={"current_input": "value 1"},
+        cursor=TransitionTarget(
+            workflow="main",
+            step=0,
+        ),
+    )
+    with patch("agents_api.workflows.task_execution.helpers.workflow") as workflow:
+        workflow.execute_activity.return_value = await _resp()
+        with raises(AssertionError):
+            await execute_map_reduce_step_parallel(
+                context=context,
+                map_defn=step.map,
+                execution_input=execution_input,
+                items=["1", "2", "3"],
+                current_input={},
+                parallelism=1,
+            )