diff --git a/temporalio/contrib/openai_agents/invoke_model_activity.py b/temporalio/contrib/openai_agents/invoke_model_activity.py
index 10b7370cb..198517fe2 100644
--- a/temporalio/contrib/openai_agents/invoke_model_activity.py
+++ b/temporalio/contrib/openai_agents/invoke_model_activity.py
@@ -24,6 +24,7 @@
     WebSearchTool,
 )
 from agents.models.multi_provider import MultiProvider
+from pydantic_core import to_jsonable_python
 from typing_extensions import Required, TypedDict
 
 from temporalio import activity, workflow
@@ -139,7 +140,7 @@ async def empty_on_invoke_handoff(
 
         # workaround for https://github.com/pydantic/pydantic/issues/9541
         # ValidatorIterator returned
-        input_json = json.dumps(input["input"], default=str)
+        input_json = json.dumps(to_jsonable_python(input["input"]))
         input_input = json.loads(input_json)
 
         def make_tool(tool: ToolInput) -> Tool:
diff --git a/tests/contrib/openai_agents/test_openai.py b/tests/contrib/openai_agents/test_openai.py
index 990849d62..d7fe0dc27 100644
--- a/tests/contrib/openai_agents/test_openai.py
+++ b/tests/contrib/openai_agents/test_openai.py
@@ -2,7 +2,7 @@
 import uuid
 from dataclasses import dataclass
 from datetime import timedelta
-from typing import Any, Optional, Union, no_type_check
+from typing import Any, Optional, Union, cast, no_type_check
 
 import pytest
 from agents import (
@@ -96,6 +96,7 @@ def __init__(
     ) -> None:
         global response_index
         response_index = 0
+        self.inputs: list[Union[str, list[TResponseInputItem]]] = []
         super().__init__(model, openai_client)
 
     async def get_response(
@@ -113,6 +114,7 @@ async def get_response(
         global response_index
         response = self.responses[response_index]
         response_index += 1
+        self.inputs.append(input)
         return response
 
 
@@ -843,11 +845,12 @@ async def test_agents_as_tools_workflow(client: Client, use_local_model: bool):
 
     model_params = ModelActivityParameters(start_to_close_timeout=timedelta(seconds=30))
     with set_open_ai_agent_temporal_overrides(model_params):
+        model = AgentAsToolsModel(  # type: ignore
+            "", openai_client=AsyncOpenAI(api_key="Fake key")
+        )
         model_activity = ModelActivity(
             TestProvider(
-                AgentAsToolsModel(  # type: ignore
-                    "", openai_client=AsyncOpenAI(api_key="Fake key")
-                )
+                model,
             )
             if use_local_model
             else None
@@ -900,6 +903,7 @@ async def test_agents_as_tools_workflow(client: Client, use_local_model: bool):
                     .activity_task_completed_event_attributes.result.payloads[0]
                     .data.decode()
                 )
+                assert isinstance(model.inputs[3][3]["content"], list)  # type: ignore
 
 
 class AirlineAgentContext(BaseModel):