Decorator to create an OpenAI custom tool.
Custom tools allow for tools with (potentially long) freeform string inputs.
See below for an example using LangGraph:
@custom_tool
def execute_code(code: str) -> str:
"""Execute python code."""
return "27"
model = ChatOpenAI(model="gpt-5", output_version="responses/v1")
agent = create_react_agent(model, [execute_code])
input_message = {"role": "user", "content": "Use the tool to calculate 3^3."}
for step in agent.stream(
{"messages": [input_message]},
stream_mode="values",
):
step["messages"][-1].pretty_print()
You can also specify a format for a corresponding context-free grammar using the
format kwarg:
from langchain_openai import ChatOpenAI, custom_tool
from langgraph.prebuilt import create_react_agent
grammar = """
start: expr
expr: term (SP ADD SP term)* -> add
| term
term: factor (SP MUL SP factor)* -> mul
| factor
factor: INT
SP: " "
ADD: "+"
MUL: "*"
%import common.INT
"""
format = {"type": "grammar", "syntax": "lark", "definition": grammar}
# highlight-next-line
@custom_tool(format=format)
def do_math(input_string: str) -> str:
"""Do a mathematical operation."""
return "27"
model = ChatOpenAI(model="gpt-5", output_version="responses/v1")
agent = create_react_agent(model, [do_math])
input_message = {"role": "user", "content": "Use the tool to calculate 3^3."}
for step in agent.stream(
{"messages": [input_message]},
stream_mode="values",
):
step["messages"][-1].pretty_print()
