Tool Use (Function Calling)

TensorZero has first-class support for tool use, a feature that allows LLMs to interact with external tools (e.g. APIs, databases, web browsers). Tool use is available for most model providers supported by TensorZero. See Integrations for a list of supported model providers. You can define a tool in your configuration file and attach it to a TensorZero function that should be allowed to call it. Alternatively, you can define a tool dynamically at inference time.

The term “tool use” is also commonly referred to as “function calling” in the industry. In TensorZero, the term “function” refers to TensorZero functions, so we’ll stick to the “tool” terminology for external tools that the models can interact with and “function” for TensorZero functions.

You can also find a complete runnable example on GitHub.

Basic Usage

Defining a tool in your configuration file

You can define a tool in your configuration file and attach it to the TensorZero functions that should be allowed to call it. Only functions that are of type chat can call tools. A tool definition has the following properties:

name: The name of the tool.
description: A description of the tool. The description helps models understand the tool’s purpose and usage.
parameters: The path to a file containing a JSON Schema for the tool’s parameters.

Optionally, you can provide a strict property to enforce type checking for the tool’s parameters. This setting is only supported by some model providers, and will be ignored otherwise.

tensorzero.toml

[tools.get_temperature]
description = "Get the current temperature for a given location."
parameters = "tools/get_temperature.json"
strict = true # optional, defaults to false

[functions.weather_chatbot]
type = "chat"
tools = ["get_temperature"]
# ...

Example: JSON Schema for the `get_temperature` tool

If we wanted the get_temperature tool to take a mandatory location parameter and an optional units parameter, we could use the following JSON Schema:

tools/get_temperature.json

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "description": "Get the current temperature for a given location.",
  "properties": {
    "location": {
      "type": "string",
      "description": "The location to get the temperature for (e.g. \"New York\")"
    },
    "units": {
      "type": "string",
      "description": "The units to get the temperature in (must be \"fahrenheit\" or \"celsius\"). Defaults to \"fahrenheit\".",
      "enum": ["fahrenheit", "celsius"]
    }
  },
  "required": ["location"],
  "additionalProperties": false
}

See “Advanced Usage” below for information on how to define a tool dynamically at inference time.

Making inference requests with tools

Once you’ve defined a tool and attached it to a TensorZero function, you don’t need to change anything in your inference request to enable tool use By default, the function will determine whether to use a tool and the arguments to pass to the tool. If the function decides to use tools, it will return one or more tool_call content blocks in the response. For multi-turn conversations supporting tool use, you can provide tool results in subsequent inference requests with a tool_result content block.

Example: Multi-turn conversation with tool use

You can also find a complete runnable example on GitHub.

from tensorzero import TensorZeroGateway, ToolCall  # or AsyncTensorZeroGateway

with TensorZeroGateway.build_http(
    gateway_url="http://localhost:3000",
) as t0:
    messages = [{"role": "user", "content": "What is the weather in Tokyo (°F)?"}]

    response = t0.inference(
        function_name="weather_chatbot",
        input={"messages": messages},
    )

    print(response)

    # The model can return multiple content blocks, including tool calls
    # In a real application, you'd be stricter about validating the response
    tool_calls = [
        content_block
        for content_block in response.content
        if isinstance(content_block, ToolCall)
    ]
    assert len(tool_calls) == 1, "Expected the model to return exactly one tool call"

    # Add the tool call to the message history
    messages.append(
        {
            "role": "assistant",
            "content": response.content,
        }
    )

    # Pretend we've called the tool and got a response
    messages.append(
        {
            "role": "user",
            "content": [
                {
                    "type": "tool_result",
                    "id": tool_calls[0].id,
                    "name": tool_calls[0].name,
                    "result": "70",  # imagine it's 70°F in Tokyo
                }
            ],
        }
    )

    response = t0.inference(
        function_name="weather_chatbot",
        input={"messages": messages},
    )

    print(response)

See “Advanced Usage” below for information on how to customize the tool calling behavior (e.g. making tool calls mandatory).

Advanced Usage

Restricting allowed tools at inference time

You can restrict the set of tools that can be called at inference time by using the allowed_tools parameter. For example, suppose your TensorZero function has access to several tools, but you only want to allow the get_temperature tool to be called during a particular inference. You can achieve this by setting allowed_tools=["get_temperature"] in your inference request.

Defining tools dynamically at inference time

You can define tools dynamically at inference time by using the additional_tools property. (In the OpenAI-compatible API, you can use the tools property instead.) You should only use dynamic tools if your use case requires it. Otherwise, it’s recommended to define tools in the configuration file. You can define a tool dynamically with the additional_tools property. This field accepts a list of objects with the same structure as the tools defined in the configuration file, except that the parameters field should contain the JSON Schema itself (rather than a path to a file with the schema).

Customizing the tool calling strategy

You can control how and when tools are called by using the tool_choice parameter. The supported tool choice strategies are:

none: The function should not use any tools.
auto: The model decides whether or not to use a tool. If it decides to use a tool, it also decides which tools to use.
required: The model should use a tool. If multiple tools are available, the model decides which tool to use.
{ specific = "tool_name" }: The model should use a specific tool. The tool must be defined in the tools section of the configuration file or provided in additional_tools.

The tool_choice parameter can be set either in your configuration file or directly in your inference request.

Calling multiple tools in parallel

You can enable parallel tool calling by setting the parallel_tool_calling parameter to true. If enabled, the models will be able to request multiple tool calls in a single inference request (conversation turn). You can specify parallel_tool_calling in the configuration file or in the inference request.

Integrating with Model Context Protocol (MCP) servers

You can use TensorZero with tools offered by Model Context Protocol (MCP) servers with the functionality described above. See our MCP (Model Context Protocol) Example on GitHub to learn how to integrate TensorZero with an MCP server.

Introduction

Gateway

Optimization

Evaluations

UI

Tool Use (Function Calling)

Basic Usage

Defining a tool in your configuration file

Making inference requests with tools

Advanced Usage

Restricting allowed tools at inference time

Defining tools dynamically at inference time

Customizing the tool calling strategy

Calling multiple tools in parallel

Integrating with Model Context Protocol (MCP) servers

Learn More

API Reference: Inference

API Reference: Inference (OpenAI-Compatible)

Configuration Reference

Introduction

Gateway

Optimization

Evaluations

UI

​Basic Usage

​Defining a tool in your configuration file

​Making inference requests with tools

​Advanced Usage

​Restricting allowed tools at inference time

​Defining tools dynamically at inference time

​Customizing the tool calling strategy

​Calling multiple tools in parallel

​Integrating with Model Context Protocol (MCP) servers

​Learn More

API Reference: Inference

API Reference: Inference (OpenAI-Compatible)

Configuration Reference

Basic Usage

Defining a tool in your configuration file

Making inference requests with tools

Advanced Usage

Restricting allowed tools at inference time

Defining tools dynamically at inference time

Customizing the tool calling strategy

Calling multiple tools in parallel

Integrating with Model Context Protocol (MCP) servers

Learn More