Under the gpt5 model, the completion interface cannot utilize the custom tool's large capability

[NiceCode666]

Confirm this is an issue with the Python library and not an underlying OpenAI API

• This is an issue with the Python library

Describe the bug

Under the gpt5 model, the completion interface cannot utilize the custom tool's large capability

I have confirmed multiple times that my format meets the API interface requirements, and the API interface is: https://platform.openai.com/docs/api-reference/chat/create#chat -create-tools-custom-tool-custom-format

The interface response result is: Error code: 400 - {'error': {'message': 'Invalid param', 'type': 'invalid_request_error', 'param': 'c44c810b-15b6-4c62-8e6d-1b8ee2ceece8', 'code': 'param_error'}}
But it is unclear which field is the problem and how to fix it. Please provide the correct usage method

To Reproduce

Run the code

Code snippets

definition = """
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
"""

ebnf_description = "Creates valid Lark grammar."

def custom_tool_ebnf_grammar_1():
    return {
                "type": "custom",
                "custom": {
                    "name": "ebnf_grammar",
                    "description": ebnf_description,
                    "format": {
                        "type": "grammar",
                        "grammar": {
                            "syntax": "lark",
                            "definition": definition,
                        }
                    },
                }
            }

def custom_tool_ebnf_grammar_2():
    return {
                "type": "custom",
                "custom": {
                    "name": "ebnf_grammar",
                    "description": ebnf_description,
                    "format": {
                        "type": "grammar",
                        "syntax": "lark",
                        "definition": definition,
                    },
                }
            }


def custom_tool_ebnf_grammar_3():
    return {
            "type": "custom",
            "name": "ebnf_grammar",
            "description": ebnf_description,
            "format": {
                "type": "grammar",
                "syntax": "lark",
                "definition": definition,
            },
        }

client = OpenAI()

ebnf_query = "use the ebnf_grammar tool to add four plus four."

def test_completion_chat_ebnf_grammar_1():
    print("===== start test_completion_chat_ebnf_grammar_1 ===== ")
    response = client.chat.completions.create(
        model="gpt-5",
        messages=[{"role": "user", "content": ebnf_query}],
        tools=[custom_tool_ebnf_grammar_1()],
        parallel_tool_calls=False,
        tool_choice="required"
    )
    print(response.choices[0].message.content)
    print("===== end test_completion_chat_ebnf_grammar_1 ===== ")

def test_completion_chat_ebnf_grammar_2():
    print("===== start test_completion_chat_ebnf_grammar_2 ===== ")
    response = client.chat.completions.create(
        model="gpt-5",
        messages=[{"role": "user", "content": ebnf_query}],
        tools=[custom_tool_ebnf_grammar_2()],
        parallel_tool_calls=False,
        tool_choice="required"
    )
    print(response.choices[0].message.content)
    print("===== end test_completion_chat_ebnf_grammar_2 ===== ")


def test_completion_chat_ebnf_grammar_3():
    print("===== start test_completion_chat_ebnf_grammar_3 ===== ")
    response = client.chat.completions.create(
        model="gpt-5",
        messages=[{"role": "user", "content": ebnf_query}],
        tools=[custom_tool_ebnf_grammar_3()],
        parallel_tool_calls=False,
        tool_choice="required"
    )
    print(response.choices[0].message.content)
    print("===== end test_completion_chat_ebnf_grammar_3 ===== ")

OS

macOS

Python version

Python 3.9

Library version

openai 1.108.1