[Python] Function call with OpenAI Agent SDK with Ollama fails

I'm having trouble getting function call working with OpenAI Agent SDK and Ollama. Suggestions/Solutions would be of great help. Thank you.

Using UV, python-12 and added "openai>=1.68.2" and "openai-agents>=0.0.15" dependencies to the project.

Below is my code:

from dotenv import load_dotenv
from agents import Agent, Runner, AsyncOpenAI, OpenAIChatCompletionsModel, function_tool
import asyncio

load_dotenv(override=True)

ollama_url = 'http://localhost:11434/v1'
ollama_api_key = 'dummy'
model = 'qwen3:4b'

@function_tool
def math_tool(a: int, b: int) -> int:
""" Send two integers and this returns their sum """
print(f"Adding {a} and {b}")
return a + b

async def main():
ollama_client = AsyncOpenAI(base_url=ollama_url, api_key=ollama_api_key)
ollama_qwen3_4b_model = OpenAIChatCompletionsModel(model=model, openai_client=ollama_client)

simple_agent = Agent(name="simple_agent", instructions="You are a simple agent that can perform basic tasks", tools=[math_tool], model=ollama_qwen3_4b_model)
output = await Runner.run(simple_agent, "What is 2 + 2?")
print(output)

asyncio.run(main())


It errors out, I assume below is the prime error.

File "/Users/RandomName/.local/share/uv/python/cpython-3.12.11-macos-aarch64-none/lib/python3.12/typing.py", line 501, in __call__
raise TypeError(f"Cannot instantiate {self!r}")
TypeError: Cannot instantiate typing.Union


This is the full trace when I run uv run ollama_simple.py

Traceback (most recent call last):
File "/Users/RandomName/Documents/ai/github/openai_agent_learning/ollama_agent_tool_simple/ollama_simple.py", line 33, in <module>
asyncio.run(main())
File "/Users/RandomName/.local/share/uv/python/cpython-3.12.11-macos-aarch64-none/lib/python3.12/asyncio/runners.py", line 195, in run
return runner.run(main)
^^^^^^^^^^^^^^^^
File "/Users/RandomName/.local/share/uv/python/cpython-3.12.11-macos-aarch64-none/lib/python3.12/asyncio/runners.py", line 118, in run
return self._loop.run_until_complete(task)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/RandomName/.local/share/uv/python/cpython-3.12.11-macos-aarch64-none/lib/python3.12/asyncio/base_events.py", line 691, in run_until_complete
return future.result()
^^^^^^^^^^^^^^^
File "/Users/RandomName/Documents/ai/github/openai_agent_learning/ollama_agent_tool_simple/ollama_simple.py", line 30, in main
output = await Runner.run(simple_agent, "What is 2 + 2?")
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/RandomName/Documents/ai/github/openai_agent_learning/ollama_agent_tool_simple/.venv/lib/python3.12/site-packages/agents/run.py", line 206, in run
return await runner.run(
^^^^^^^^^^^^^^^^^
File "/Users/RandomName/Documents/ai/github/openai_agent_learning/ollama_agent_tool_simple/.venv/lib/python3.12/site-packages/agents/run.py", line 434, in run
turn_result = await self._run_single_turn(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/RandomName/Documents/ai/github/openai_agent_learning/ollama_agent_tool_simple/.venv/lib/python3.12/site-packages/agents/run.py", line 956, in _run_single_turn
new_response = await cls._get_new_response(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/RandomName/Documents/ai/github/openai_agent_learning/ollama_agent_tool_simple/.venv/lib/python3.12/site-packages/agents/run.py", line 1168, in _get_new_response
new_response = await model.get_response(
^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/RandomName/Documents/ai/github/openai_agent_learning/ollama_agent_tool_simple/.venv/lib/python3.12/site-packages/agents/models/openai_chatcompletions.py", line 66, in get_response
response = await self._fetch_response(
^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/RandomName/Documents/ai/github/openai_agent_learning/ollama_agent_tool_simple/.venv/lib/python3.12/site-packages/agents/models/openai_chatcompletions.py", line 228, in _fetch_response
converted_messages = Converter.items_to_messages(input)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/RandomName/Documents/ai/github/openai_agent_learning/ollama_agent_tool_simple/.venv/lib/python3.12/site-packages/agents/models/chatcmpl_converter.py", line 443, in items_to_messages
new_tool_call = ChatCompletionMessageToolCallParam(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/RandomName/.local/share/uv/python/cpython-3.12.11-macos-aarch64-none/lib/python3.12/typing.py", line 1184, in __call__
result = self.__origin__(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/RandomName/.local/share/uv/python/cpython-3.12.11-macos-aarch64-none/lib/python3.12/typing.py", line 501, in __call__
raise TypeError(f"Cannot instantiate {self!r}")
TypeError: Cannot instantiate typing.Union

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