Tools & ReAct

A tool is a callable operation the model can invoke. ReAct is the module that lets a program use tools: it reasons, picks a tool, sees the result, and repeats until it has an answer.

Defining a tool

The simplest way to make a tool is to annotate a method with @description and lift it with ToolFunction.fromMethod. The macro derives the tool's name, description, and argument schema from the method:

@description("Get the current weather for a city.")
def get_weather(city: String): String = s"The weather in $city is sunny and 75°F"

@description("Search the web for information.")
def search_web(query: String): String = s"Search results for '$query': [relevant information...]"

val getWeather: ToolFunction = ToolFunction.fromMethod(get_weather)
val searchWeb: ToolFunction  = ToolFunction.fromMethod(search_web)

The argument names and types (city: String) become the tool's schema, which is surfaced to the model so it knows how to call the tool.

Building a ReAct agent

ReAct takes a base signature and a set of tools. It runs a reason-act loop up to maxIterations times, then produces the signature's output:

val reactAgent = ReAct(
  baseSignature = Signature.fromString("question -> answer"),
  tools = Vector(getWeather, searchWeb),
  maxIterations = IterationLimit(5)
)

def call(question: String)(using RuntimeContext): Either[DspyError, String] =
  reactAgent((question = question)).map(_.output.answer)

Like ChainOfThought, ReAct prepends a reasoning field to the output. The full step-by-step trajectory is available on the prediction's .raw.

Native function calling

By default, tools reach the model through the text protocol. If your model supports native function calling, enable it at the adapter level and tools are sent as a structured tools array instead:

val nativeChatAdapter: Adapter = ChatAdapter(useNativeFunctionCalling = true)
val nativeJsonAdapter: Adapter = JSONAdapter(useNativeFunctionCalling = true)

This is an adapter setting, not a ReAct setting: the agent loop is unchanged, only how tool definitions are transmitted to the provider differs. It applies to any program whose predictor declares tools, and is gated on the model reporting function-calling support.

When to use it

You want	Reach for
The model to fetch data or take an action	ReAct with one or more tools
A tool from an existing method	ToolFunction.fromMethod(method)
Structured tool calls on a capable model	ChatAdapter(useNativeFunctionCalling = true)

Next: Composing programs.