Function-Calling Tools: The Full Workflow

Working with the OpenAI Responses API

James Chapman

AI Curriculum Manager, DataCamp

def convert_currency(amount, from_currency, to_currency):
    url = f"https://api.frankfurter.dev/v1/latest?base={from_currency}&symbols={to_currency}"

    try:
        response = requests.get(url)
        response.raise_for_status()

        data = response.json()
        rate = data['rates'].get(to_currency)

        if rate is None:
            return f"Could not find exchange rate for {from_currency} to {to_currency}"

        converted_amount = amount * rate
        return f"{amount} {from_currency} = {converted_amount:.2f} {to_currency} (Rate: {rate})"

    except requests.exceptions.RequestException as e:
        return f"Error converting currency: {str(e)}"

Function-Calling Tools

Tool Definitions: An LLM's Manual

tools = [

    {
        "type": "function",
        "name": "convert_currency",
        "description": "Convert an amount from one currency to another using real-time exchange rates.",

        "parameters": {
            "type": "object",

            "properties": {

                "amount": {"type": "number", "description": "The amount of money to convert"},
                "from_currency": {"type": "string", "description": "The source currency code (e.g., 'USD', 'EUR')"},
                "to_currency": {"type": "string", "description": "The target currency code (e.g., 'USD', 'EUR')"}

            },

            "required": ["amount", "from_currency", "to_currency"],
            "additionalProperties": False

        }

    }

]

response = client.responses.create(
    model="gpt-5-mini",
    input=[{"role": "user", "content": "How much is $100 in euros?"}],
    tools=tools
)


for item in response.output:
    print(item)

ResponseReasoningItem(id='rs_0a46aaf8050644c4006924978291a481909d5f50b8b9f7e959',
                      summary=[], type='reasoning', content=None, encrypted_content=None, status=None)

ResponseFunctionToolCall(arguments='{"amount":100,"from_currency":"USD","to_currency":"EUR"}',
                         call_id='call_xTbWJuO6TiuFw84TK0hU9hRT', name='convert_currency',
                         type='function_call',
                         id='fc_0a46aaf8050644c40069249784fa248190874283efa2fc2c46', status='completed')

messages = [{"role": "user", "content": "How much is $100 in euros?"}]


response = client.responses.create(
    model="gpt-5",
    input=messages,
    tools=tools
)


messages += response.output

for item in response.output:
    if item.type == "function_call":
        if item.name == "convert_currency":

            currency_result = convert_currency(**json.loads(item.arguments))


            messages.append({
                "type": "function_call_output",
                "call_id": item.call_id,
                "output": json.dumps({"convert_currency": currency_result})
            })


response = client.responses.create(
    model="gpt-5-mini",
    instructions="Respond to the user input using the tool output.",
    tools=tools,
    input=messages,
)

print(response.output_text)

100 USD = 86.62 EUR (rate used: 1 USD = 0.86625 EUR).

Note: exchange rates fluctuate and actual amounts may differ due to fees or the
provider you use. Want to convert a different amount or use a different currency?

Let's practice!

Working with the OpenAI Responses API