Tool Call

Toolcall (Function Calling) is an advanced capability on the StepFun platform. It lets models decide which external tools or functions to invoke to satisfy a user request. By using the Toolcall API, you can extend model capabilities to cover more use cases and offer richer features. Below is a minimal JSON request example that must include three key parameters:

{
  "model": "step-5-preview",
  "messages": [
    {
      "role": "user",
      "content": "Hey, can you calculate (80 + 20) / 5 for me?"
    }
  ],
  "tools": [
    {
      "type": "function",
      "function": {
        "name": "Calculator",
        "description": "This StepFun API provides basic arithmetic: addition, subtraction, multiplication, and division.",
        "parameters": {
          "properties": {
            "formula": {
              "type": "string",
              "description": "Expression to evaluate. Only integers are supported. Operators: +, -, *, /, and parentheses. Example: (1 + 2) * 3."
            }
          },
          "type": "object"
        }
      }
    }
  ]
}

model: model name

The following models support Toolcall requests:

step-5-preview
step-3.7-flash
step-3.5-flash
step-3.5-flash-2603

Official tools

Availability of built-in tools depends on the selected model. Support for function calling does not imply support for every built-in tool.

Web search: the built-in Chat Completions web_search tool is not available on step-5-preview. Integrate an external search service through a function tool and return its results to the model.
Knowledge base search: upload text to a knowledge base and search it to reduce hallucinations.

tools: list of functions

Use tools to describe the functions available locally. Up to 128 tools are allowed. Each entry has type function and contains a function object with the name, description, and parameters.

name: letters, numbers, -, _; ideally under 64 characters. Use a clear English name so the model understands it (regex guideline: ^[a-zA-Z*][a-zA-Z0-9-_]63$).
parameters: root type must be object. The content follows a subset of JSON Schema; keys follow the same naming rule. See JSON Schema for supported type values. description explains each parameter.
description: explains what the function does so the model can choose it; English or Chinese is supported.

messages: conversation context

See Chat Completion request object.

Examples

python
js
curl

from openai import OpenAI

client = OpenAI(api_key="YOUR_STEP_API_KEY", base_url="https://api.stepfun.ai/v1")

completion = client.chat.completions.create(
    model="step-5-preview",
    messages=[
        {
            "role": "user",
            "content": "Can you calculate (80 + 20) / 5 for me?"
        }
    ],
    tools=[
        {
            "type": "function",
            "function": {
                "name": "Calculator",
                "description": "This StepFun API provides basic arithmetic: addition, subtraction, multiplication, and division.",
                "parameters": {
                    "type": "object",
                    "properties": {
                        "formula": {
                            "type": "string",
                            "description": "Expression to evaluate. Only integers are supported. Operators: +, -, *, /, and parentheses. Example: '(1 + 2) * 3'."
                        }
                    }
                }
            }
        }
    ]
)

print(completion)

import OpenAI from "openai";

const openai = new OpenAI({
    apiKey: "YOUR_STEP_API_KEY",
    baseURL: "https://api.stepfun.ai/v1"
});

async function main() {
    const completion = await openai.chat.completions.create({
        model: "step-5-preview",
        messages: [
            {
                role: "user",
                content: "Can you calculate (80 + 20) / 5 for me?"
            }
        ],
        tools: [
            {
                type: "function",
                function: {
                    name: "Calculator",
                    description: "This StepFun API provides basic arithmetic: addition, subtraction, multiplication, and division.",
                    parameters: {
                        type: "object",
                        properties: {
                            formula: {
                                type: "string",
                                description: "Expression to evaluate. Only integers are supported. Operators: +, -, *, /, and parentheses. Example: '(1 + 2) * 3'."
                            }
                        }
                    }
                }
            }
        ]
    });

    console.log(JSON.stringify(completion));
}

main();

curl https://api.stepfun.ai/v1/chat/completions \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $STEP_API_KEY" \
  -d '{
    "model": "step-5-preview",
    "messages": [
      {
        "role": "user",
        "content": "Can you calculate (80 + 20) / 5 for me?"
      }
    ],
    "tools": [
      {
        "type": "function",
        "function": {
          "name": "Calculator",
          "description": "This StepFun API provides basic arithmetic: addition, subtraction, multiplication, and division.",
          "parameters": {
            "type": "object",
            "properties": {
              "formula": {
                "type": "string",
                "description": "Expression to evaluate. Only integers are supported. Operators: +, -, *, /, and parentheses. Example: (1 + 2) * 3."
              }
            }
          }
        }
      }
    ]
  }'

{
  "id": "b7b56af0-52a6-483f-a589-948182676a1b",
  "object": "chat.completion",
  "created": 1717744611,
  "model": "step-5-preview",
  "choices": [
    {
      "index": 0,
      "message": {
        "role": "assistant",
        "content": "",
        "tool_calls": [
          {
            "id": "call_ybVBO_JASgipgJH8xWbhKg",
            "type": "function",
            "function": {
              "name": "Calculator",
              "arguments": "{\"formula\": \"(80 + 20) / 5\"}"
            }
          }
        ]
      },
      "finish_reason": "stop"
    }
  ],
  "usage": {
    "prompt_tokens": 151,
    "completion_tokens": 25,
    "total_tokens": 176
  }
}

Chat

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Token Count

Error Codes

Tool Call

model: model name

Official tools

tools: list of functions

messages: conversation context

Examples

​model: model name

​Official tools

​tools: list of functions

​messages: conversation context

​Examples

model: model name

Official tools

tools: list of functions

messages: conversation context

Examples