TypeError: Cannot instantiate typing.Union when running with latest openai Python package #1417
Description
Description
When using the latest openai Python package with the openai-agents SDK, calling Runner.run_sync() fails with:
TypeError: Cannot instantiate typing.Union
This happens because the ChatCompletionMessageToolCallParam type in openai is now a typing.Union instead of a concrete class, and the openai-agents code still attempts to instantiate it directly.
Environment
- Python: 3.12.11
- openai: 1.99.3 (also tested with 1.99.4)
- openai-agents: latest from PyPI
- OS: Windows 10 / 11
Steps to Reproduce
- Install latest packages:
pip install --upgrade openai openai-agents
Root Cause Analysis
In recent openai releases (>=1.99.2), ChatCompletionMessageToolCallParam is no longer a concrete Pydantic model — it’s now defined as a typing.Union of multiple tool call param types, e.g.:
ChatCompletionMessageFunctionToolCallParam | ChatCompletionMessageCustomToolCallParam
The openai-agents SDK still attempts to instantiate ChatCompletionMessageToolCallParam(...) directly in chatcmpl_converter.py.
In Python, typing.Union is not callable, so this raises:
TypeError: Cannot instantiate typing.Union
Workarounds
- Pin
openaito 1.99.1:pip install "openai==1.99.1" - Modify
chatcmpl_converter.pyto:
- Build a plain
dict, or - Instantiate a specific member of the union, e.g.:
ChatCompletionMessageFunctionToolCallParam(...)
instead of calling the union type alias.