Reusable, validated tools that an LLM can invoke during a conversation.

Tools give the model access to external capabilities — looking up data, calling APIs, running computations. When the model decides to use a tool, Omni's loop automatically executes it and feeds the result back, continuing until the model produces a final text response.

A tool is a %Tool{} struct with a name, description, optional input schema, and optional handler function. You can create one directly with Omni.tool/1 for quick, inline use (see the Tools section in Omni), or define a tool module with use Omni.Tool to bundle the schema, handler, and metadata in a reusable unit.

Defining a tool module

A tool module implements schema/0 and call/1:

defmodule MyApp.Tools.GetWeather do
  use Omni.Tool, name: "get_weather", description: "Gets the weather for a city"

  def schema do
    import Omni.Schema
    object(%{city: string(description: "City name")}, required: [:city])
  end

  def call(input) do
    WeatherAPI.fetch(input.city)
  end
end

schema/0 returns a plain map following JSON Schema conventions. The Omni.Schema helpers are a convenient way to build these, but any map with the right structure works — you can construct schemas by hand or with other libraries. Import Omni.Schema inside the callback if you use it — it is not auto-imported by use Omni.Tool.

call/1 receives the validated input with atom keys matching the schema, regardless of the string keys the LLM sends. Return any term — it will be serialized and sent back to the model as a tool result.

Stateful tools

When a tool needs runtime state (a database connection, configuration, an API client), implement init/1 and call/2:

defmodule MyApp.Tools.DbLookup do
  use Omni.Tool, name: "db_lookup", description: "Looks up a record by ID"

  def schema do
    import Omni.Schema
    object(%{id: integer()}, required: [:id])
  end

  def init(repo), do: repo

  def call(input, repo) do
    repo.get(Record, input.id)
  end
end

init/1 receives the argument passed to new/1 and returns the state. call/2 receives validated input and that state. The state is captured in a closure at construction time, so each new/1 call can bind different state.

Using tools

Pass tool structs in the context:

weather = MyApp.Tools.GetWeather.new()
db_lookup = MyApp.Tools.DbLookup.new(MyApp.Repo)

context = Omni.context(
  messages: [Omni.message("What's the weather in Paris?")],
  tools: [weather, db_lookup]
)

{:ok, response} = Omni.generate_text({:anthropic, "claude-sonnet-4-5-20250514"}, context)

The loop executes tool uses automatically and continues the conversation until the model responds with text (or hits :max_steps).

Schema-only tools

A %Tool{} struct with handler: nil defines a tool the model can invoke but Omni won't auto-execute. The loop breaks and returns the response containing ToolUse blocks for you to handle manually. This is useful when tool execution requires human approval or happens outside your application.

schema = Omni.Schema.object(%{query: Omni.Schema.string()}, required: [:query])
tool = Omni.tool(name: "search", description: "Web search", input_schema: schema)

Dynamic descriptions

When a tool's description needs to include runtime context — for example, environment details or configuration injected at construction time — override description/1. It receives the state returned by init/1:

defmodule MyApp.Tools.Search do
  use Omni.Tool, name: "search", description: "Searches the knowledge base"

  def schema do
    import Omni.Schema
    object(%{query: string()}, required: [:query])
  end

  @impl Omni.Tool
  def description(opts) do
    base = description()
    case Keyword.get(opts, :extra) do
      nil -> base
      extra -> base <> "

" <> extra

    end
  end

  def init(opts), do: opts

  def call(input, _opts) do
    KnowledgeBase.search(input.query)
  end
end

# Base description
tool = MyApp.Tools.Search.new()
tool.description
#=> "Searches the knowledge base"

# With extra context
tool = MyApp.Tools.Search.new(extra: "Only return results from 2024.")
tool.description
#=> "Searches the knowledge base

Only return results from 2024."

The default description/1 delegates to description/0, so existing tools are unaffected.

Callbacks as an alternative to options

The name: and description: options on use Omni.Tool are convenient shorthand, but both are optional. You can omit them and implement name/0 and description/0 as callbacks directly:

defmodule MyApp.Tools.Ping do
  use Omni.Tool

  @impl Omni.Tool
  def name, do: "ping"

  @impl Omni.Tool
  def description, do: "Returns pong"

  def schema, do: Omni.Schema.object(%{})
  def call(_input), do: "pong"
end

If you omit an option without implementing the callback, the compiler will warn about the missing callback.

How execution works

Calling new/0 or new/1 on a tool module:

1. Calls init/1 with the given argument (defaults to nil)
2. Calls description/1 with the init state to resolve the description
3. Calls schema/0 to capture the input schema
4. Returns a %Tool{} struct with a handler closure bound to the init state

When the model invokes the tool, execute/2 validates the LLM's string-keyed input against the schema (via Omni.Schema.validate/2), casts keys to match the schema's key types, then calls the handler. Direct handler calls (tool.handler.(input)) bypass this validation.