tool: Define a tool

In ellmer: Chat with Large Language Models

Define a tool

Description

Annotate a function for use in tool calls, by providing a name, description, and type definition for the arguments.

Learn more in vignette("tool-calling").

Usage

tool(
  fun,
  description,
  ...,
  arguments = list(),
  name = NULL,
  convert = TRUE,
  annotations = list(),
  .name = deprecated(),
  .description = deprecated(),
  .convert = deprecated(),
  .annotations = deprecated()
)

Arguments

fun	The function to be invoked when the tool is called. The return value of the function is sent back to the chatbot. Expert users can customize the tool result by returning a ContentToolResult object.
description	A detailed description of what the function does. Generally, the more information that you can provide here, the better.
...	Use arguments instead.
arguments	A named list that defines the arguments accepted by the function. Each element should be created by a type_*() function. Use type_ignore() if you don't want the LLM to provide that argument (e.g., because the R function has a suitable default value).
name	The name of the function. This can be omitted if fun is an existing function (i.e. not defined inline).
convert	Should JSON inputs be automatically convert to their R data type equivalents? Defaults to TRUE.
annotations	Additional properties that describe the tool and its behavior. Usually created by tool_annotations(), where you can find a description of the annotation properties recommended by the Model Context Protocol.
.name, .description, .convert, .annotations	Please switch to the non-prefixed equivalents.

Value

An S7 ToolDef object.

ellmer 0.3.0

In ellmer 0.3.0, the definition of the tool() function changed quite a bit. To make it easier to update old versions, you can use an LLM with the following system prompt

Help the user convert an ellmer 0.2.0 and earlier tool definition into a
ellmer 0.3.0 tool definition. Here's what changed:

* All arguments, apart from the first, should be named, and the argument
  names no longer use `.` prefixes. The argument order should be function,
  name (as a string), description, then arguments, then anything

* Previously `arguments` was passed as `...`, so all type specifications
  should now be moved into a named list and passed to the `arguments`
  argument. It can be omitted if the function has no arguments.

```R
# old
tool(
  add,
  "Add two numbers together"
  x = type_number(),
  y = type_number()
)

# new
tool(
  add,
  name = "add",
  description = "Add two numbers together",
  arguments = list(
    x = type_number(),
    y = type_number()
  )
)
```

Don't respond; just let the user provide function calls to convert.

See Also

Other tool calling helpers: tool_annotations(), tool_reject()

Examples

# First define the metadata that the model uses to figure out when to
# call the tool
tool_rnorm <- tool(
  rnorm,
  description = "Draw numbers from a random normal distribution",
  arguments = list(
    n = type_integer("The number of observations. Must be a positive integer."),
    mean = type_number("The mean value of the distribution."),
    sd = type_number("The standard deviation of the distribution. Must be a non-negative number.")
  )
)
tool_rnorm(n = 5, mean = 0, sd = 1)

chat <- chat_openai()
# Then register it
chat$register_tool(tool_rnorm)

# Then ask a question that needs it.
chat$chat("Give me five numbers from a random normal distribution.")

# Look at the chat history to see how tool calling works:
chat
# Assistant sends a tool request which is evaluated locally and
# results are sent back in a tool result.

ellmer documentation built on July 14, 2026, 1:07 a.m.