Tool Use, Unified

Back to Articles

• Introduction

• Chat Templating

• Passing tools to a chat template

• Adding tool calls to the chat

• Adding tool responses to the chat

• Tool use in action

• The regrettable disunity of response formats

• Conclusion

There is now a unified tool use API across several popular families of models. This API means the same code is portable - few or no model-specific changes are needed to use tools in chats with Mistral, Cohere, NousResearch or Llama models. In addition, Transformers now includes helper functionality to make tool calling even easier, as well as complete documentation and examples for the entire tool use process. Support for even more models will be added in the near future.

Introduction

Tool use is a curious feature – everyone thinks it’s great, but most people haven’t tried it themselves. Conceptually, it’s very straightforward: you give some tools (callable functions) to your LLM, and it can decide to call them to help it respond to user queries. Maybe you give it a calculator so it doesn’t have to rely on its internal, unreliable arithmetic abilities. Maybe you let it search the web or view your calendar, or you give it (read-only!) access to a company database so it can pull up information or search technical documentation.

Tool use overcomes a lot of the core limitations of LLMs. Many LLMs are fluent and loquacious but often imprecise with calculations and facts and hazy on specific details of more niche topics. They don’t know anything that happened after their training cutoff date. They are generalists; they arrive into the conversation with no idea of you or your workplace beyond what you give them in the system message. Tools provide them with access to structured, specific, relevant, and up-to-date information that can help a lot in making them into genuinely helpful partners rather than just fascinating novelty.

The problems arise, however, when you actually try to implement tool use. Documentation is often sparse, inconsistent, and even contradictory - and this is true for both closed-source APIs as well as open-access models! Although tool use is simple in theory, it frequently becomes a nightmare in practice: How do you pass tools to the model? How do you ensure the tool prompts match the formats it was trained with? When the model calls a tool, how do you incorporate that into the chat? If you’ve tried to implement tool use before, you’ve probably found that these questions are surprisingly tricky and that the documentation wasn’t always complete and helpful.

Worse, different models can have wildly different implementations of tool use. Even at the most basic level of defining the available tools, some providers expect JSON schemas, while others expect Python function headers. Even among the ones that expect JSON schemas, small details often differ and create big API incompatibilities. This creates a lot of friction and generally just deepens user confusion. So, what can we do about all of this?

Chat Templating

Devoted fans of the Hugging Face Cinematic Universe will remember that the open-source community faced a similar challenge in the past with chat models. Chat models use control tokens like <|start_of_user_turn|> or <|end_of_message|> to let the model know what’s going on in the chat, but different models were trained with totally different control tokens, which meant that users needed to write specific formatting code for each model they wanted to use. This was a huge headache at the time.

Our solution to this was chat templates - essentially, models would come with a tiny Jinja template, which would render chats with the right format and control tokens for each model. Chat templates meant that users could write chats in a universal, model-agnostic format, trusting in the Jinja templates to handle any model-specific formatting required.

The obvious approach to supporting tool use, then, was to extend chat templates to support tools as well. And that’s exactly what we did, but tools created many new challenges for the templating system. Let’s go through those challenges and how we solved them. In the process, hopefully, you’ll gain a deeper understanding of how the system works and how you can make it work for you.

Passing tools to a chat template

Our first criterion when designing the tool use API was that it should be intuitive to define tools and pass them to the chat template. We found that most users wrote their tool functions first and then figured out how to generate tool definitions from them and pass those to the model. This led to an obvious approach: What if users could simply pass functions directly to the chat template and let it generate tool definitions for them?

The problem here, though, is that “passing functions” is a very language-specific thing to do, and lots of people access chat models through JavaScript or Rust instead of Python. So, we found a compromise that we think offers the best of both worlds: Chat templates expect tools to be defined as JSON schema, but if you pass Python functions to the template instead, they will be automatically converted to JSON schema for you. This results in a nice, clean API:

def get_current_temperature(location: str):
    """
    Gets the temperature at a given location.

    Args:
        location: The location to get the temperature for
    """
    return 22.0  # bug: Sometimes the temperature is not 22. low priority

tools = [get_current_temperature]    

chat = [
    {"role": "user", "content": "Hey, what's the weather like in Paris right now?"}
]

tool_prompt = tokenizer.apply_chat_template(
    chat, 
    tools=tools,
    add_generation_prompt=True,
    return_tensors="pt"
)

Internally, the get_current_temperature function will be expanded into a complete JSON schema. If you want to see the generated schema, you can use the get_json_schema function:

>>> from transformers.utils import get_json_schema

>>> get_json_schema(get_current_weather)
{
    "type": "function",
    "function": {
        "name": "get_current_temperature",
        "description": "Gets the temperature at a given location.",
        "parameters": {
            "type": "object",
            "properties": {
                "location": {
                    "type": "string",
                    "description": "The location to get the temperature for"
                }
            },
            "required": [
                "location"
            ]
        }
    }
}

If you prefer manual control or you’re coding in a language other than Python, you can pass JSON schemas like these directly to the template. However, when you’re working in Python, you can avoid handling JSON schema directly. All you need to do is define your tool functions with clear names, accurate type hints, and complete docstrings, including argument docstrings, since all of these will be used to generate the JSON schema that will be read by the template. Much of this is good Python practice anyway, and if you follow it, then you’ll find that no extra work is required - your functions are already usable as tools!

Remember: accurate JSON schemas, whether generated from docstrings and type hints or specified manually, are crucial for the model to understand how to use your tools. The model will never see the code inside your functions, but it will see the JSON schemas. The cleaner and more accurate they are, the better!

Adding tool calls to the chat

One detail that is often overlooked by users (and model documentation 😬) is that when a model calls a tool, this actually requires two messages to be added to the chat history. The first message is the assistant calling the tool, and the second is the tool response, the output of the called function.

Both tool calls and tool responses are necessary - remember that the model only knows what’s in the chat history, and it will not be able to make sense of a tool response if it can’t also see the call it made and the arguments it passed to get that response. “22” on its own is not very informative, but it’s very helpful if you know that the message preceding it was get_current_temperature("Paris, France").

This is one of the areas that can be extremely divergent between different providers, but the standard we settled on is that tool calls are a field of assistant messages, like so:

message = {
    "role": "assistant",
    "tool_calls": [
        {
            "type": "function",
             "function": {
                 "name": "get_current_temperature", 
                 "arguments": {
                     "location": "Paris, France"
                }
            }
        }
    ]
}
chat.append(message)

Adding tool responses to the chat

Tool responses are much simpler, especially when tools only return a single string or number.

message = {
    "role": "tool", 
    "name": "get_current_temperature", 
    "content": "22.0"
}
chat.append(message)

Tool use in action

Let’s take the code we have so far and build a complete example of tool-calling. If you want to use tools in your own projects, we recommend playing around with the code here - try running it yourself, adding or removing tools, swapping models, and tweaking details to get a feel for the system. That familiarity will make things much easier when the time comes to implement tool use in your software! To make that easier, this example is available as a notebook as well.

First, let’s set up our model. We’ll use Hermes-2-Pro-Llama-3-8B because it’s small, capable, ungated, and it supports tool calling. You may get better results on complex tasks if you use a larger model, though!

import torch
from transformers import AutoTokenizer, AutoModelForCausalLM

checkpoint = "NousResearch/Hermes-2-Pro-Llama-3-8B"
tokenizer = AutoTokenizer.from_pretrained(checkpoint)
model = AutoModelForCausalLM.from_pretrained(checkpoint, torch_dtype=torch.bfloat16, device_map="auto")

Next, we’ll set up our tool and the chat we want to use. Let’s use the get_current_temperature example from above:

def get_current_temperature(location: str):
    """
    Gets the temperature at a given location.

    Args:
        location: The location to get the temperature for, in the format "city, country"
    """
    return 22.0  # bug: Sometimes the temperature is not 22. low priority to fix tho

tools = [get_current_temperature]    

chat = [
    {"role": "user", "content": "Hey, what's the weather like in Paris right now?"}
]

tool_prompt = tokenizer.apply_chat_template(
    chat, 
    tools=tools, 
    return_tensors="pt",
    return_dict=True,
    add_generation_prompt=True,
)
tool_prompt = tool_prompt.to(model.device)

Now we’re ready to generate the model’s response to the user query, given the tools it has access to:

out = model.generate(**tool_prompt, max_new_tokens=128)
generated_text = out[0, tool_prompt['input_ids'].shape[1]:]

print(tokenizer.decode(generated_text))

and we get:

<tool_call>
{"arguments": {"location": "Paris, France"}, "name": "get_current_temperature"}
</tool_call><|im_end|>

The model has requested a tool! Note how it correctly inferred that it should pass the argument “Paris, France” rather than just “Paris”, because that is the format recommended by the function docstring.

The model does not really have programmatic access to the tools, though - like all language models, it just generates text. It's up to you as the programmer to take the model's request and call the function. First, though, let’s add the model's tool request to the chat.

Note that this step can require a little bit of manual processing - although you should always add the request to the chat in the format below, the text of the tool call request, such as the <tool_call> tags, may differ between models. Usually, it's quite intuitive, but bear in mind you may need a little bit of model-specific json.loads() or re.search() when trying this in your own code!

message = {
    "role": "assistant", 
    "tool_calls": [
        {
            "type": "function", 
            "function": {
                "name": "get_current_temperature", 
                "arguments": {"location": "Paris, France"}
            }
        }
    ]
}
chat.append(message)

Now, we actually call the tool in our Python code, and we add its response to the chat:

message = {
    "role": "tool", 
    "name": "get_current_temperature", 
    "content": "22.0"
}
chat.append(message)

And finally, just as we did before, we format the updated chat and pass it to the model, so that it can use the tool response in conversation:

tool_prompt = tokenizer.apply_chat_template(
    chat, 
    tools=tools, 
    return_tensors="pt",
    return_dict=True,
    add_generation_prompt=True,
)
tool_prompt = tool_prompt.to(model.device)

out = model.generate(**tool_prompt, max_new_tokens=128)
generated_text = out[0, tool_prompt['input_ids'].shape[1]:]

print(tokenizer.decode(generated_text))

And we get the final response to the user, built using information from the intermediate tool calling step:

The current temperature in Paris is 22.0 degrees Celsius. Enjoy your day!<|im_end|>

The regrettable disunity of response formats

While reading this example, you may have noticed that even though chat templates can hide model-specific differences when converting from chats and tool definitions to formatted text, the same isn’t true in reverse. When the model emits a tool call, it will do so in its own format, so you’ll need to parse it out manually for now before adding it to the chat in the universal format. Thankfully, most of the formats are pretty intuitive, so this should only be a couple of lines of json.loads() or, at worst, a simple re.search() to create the tool call dict you need.

Still, this is the biggest part of the process that remains "un-unified." We have some ideas on how to fix it, but they’re not quite ready for prime time yet. “Let us cook,” as the kids say.

Conclusion

Despite the minor caveat above, we think this is a big improvement from the previous situation, where tool use was scattered, confusing, and poorly documented. We hope this makes it a lot easier for open-source developers to include tool use in their projects, augmenting powerful LLMs with a range of tools that add amazing new capabilities. From smaller models like Hermes-2-Pro-8B to the giant state-of-the-art behemoths like Mistral-Large, Command-R-Plus or Llama-3.1-405B, many of the LLMs at the cutting edge now support tool use. We think tools will be an integral part of the next wave of LLM products, and we hope these changes make it easier for you to use them in your own projects. Good luck!