Tool arguments of type dict are invisible to OpenAI-powered agents #3654
Description
Initial Checks
- I confirm that I'm using the latest version of Pydantic AI
- I confirm that I searched for my issue in https://github.com/pydantic/pydantic-ai/issues before opening this issue
Description
Observed behavior
When a pydantic-ai agent powered by an OpenAI model has a tool with an argument of type dict that argument is invisible to the OpenAI model.
Expected behavior
All tool arguments should be visible to the model powering the agent for all model providers.
Example
With a basic agent like
from pydantic_ai import Agent
openai_agent = Agent("openai:gpt-4o")
@openai_agent.tool_plain
def tool_w_dict(name: str, data: dict[str, str]) -> str:
"""A tool with a dict argument.
Args:
name: A name string
data: A dictionary with arbitrary data
"""
return f"Got {name} with {data}"
@openai_agent.tool_plain
def tool_w_list(name: str, data: list[str]) -> str:
"""A tool with a list argument.
Args:
name: A name string
data: A list with arbitrary data
"""
return f"Got {name} with {data}"
running a request like
openai_result = openai_agent.run_sync("List all tools you have and their parameters with types")
print(openai_result.output)
produces results like
I have the following tools available:
1. **functions.tool_w_dict**
- **Parameters**:
- `name` (string): A name string.
2. **functions.tool_w_list**
- **Parameters**:
- `name` (string): A name string.
- `data` (string[]): A list with arbitrary data.
These tools can handle dictionary-based or list-based inputs, respectively.
Note that the tool_w_dict parameters do not include the dict-typed data argument from the function. This behavior is consistent across many attempts and multiple OpenAI models (I tried gpt-4o, gpt-4o-mini, and gpt-5).
The same agent powered by Anthropic's anthropic:claude-sonnet-4-20250514 model produces output which does include the data argument like
I have access to 2 tools:
## 1. tool_w_dict
**Description:** A tool with a dict argument.
**Parameters:**
- `name` (string, required): A name string
- `data` (object, required): A dictionary with arbitrary data
- Additional properties: string values only
## 2. tool_w_list
**Description:** A tool with a list argument.
**Parameters:**
- `name` (string, required): A name string
- `data` (array, required): A list with arbitrary data
- Items: string type only
cc @angsten (who originally discovered this behavior), @physicsrob
Example Code
#!/usr/bin/env python3
# /// script
# requires-python = ">=3.12"
# dependencies = ["pydantic-ai==1.27.0"]
# ///
"""
MRE: Dict-typed arguments are invisible to OpenAI models in pydantic-ai
Run with: uv run mre.py
Requires: OPENAI_API_KEY environment variable
Requires: ANTHROPIC_API_KEY environment variable
"""
import pydantic_ai
import openai
import sys
import anthropic
from pydantic_ai import Agent
openai_agent = Agent("openai:gpt-4o")
@openai_agent.tool_plain
def tool_w_dict(name: str, data: dict[str, str]) -> str:
"""A tool with a dict argument.
Args:
name: A name string
data: A dictionary with arbitrary data
"""
return f"Got {name} with {data}"
@openai_agent.tool_plain
def tool_w_list(name: str, data: list[str]) -> str:
"""A tool with a list argument.
Args:
name: A name string
data: A list with arbitrary data
"""
return f"Got {name} with {data}"
anthropic_agent = Agent("anthropic:claude-sonnet-4-20250514")
@anthropic_agent.tool_plain
def tool_w_dict(name: str, data: dict[str, str]) -> str:
"""A tool with a dict argument.
Args:
name: A name string
data: A dictionary with arbitrary data
"""
return f"Got {name} with {data}"
@anthropic_agent.tool_plain
def tool_w_list(name: str, data: list[str]) -> str:
"""A tool with a list argument.
Args:
name: A name string
data: A list with arbitrary data
"""
return f"Got {name} with {data}"
openai_result = openai_agent.run_sync(
"List all tools you have and their parameters with types",
)
anthropic_result = anthropic_agent.run_sync(
"List all tools you have and their parameters with types"
)
print("Library versions:")
print(f"{sys.version=}")
print(f"{pydantic_ai.__version__=}")
print(f"{openai.__version__=}")
print(f"{anthropic.__version__=}")
print("\n" * 2)
print("OpenAI Agent results:")
print(openai_result.output)
print("\n" * 2)
print("Anthropic Agent results:")
print(anthropic_result.output)Python, Pydantic AI & LLM client version
sys.version='3.12.12 (main, Oct 31 2025, 23:02:31) [Clang 21.1.4 ]'
pydantic_ai.__version__='1.27.0'
openai.__version__='2.9.0'
anthropic.__version__='0.75.0'