> ## Documentation Index
> Fetch the complete documentation index at: https://motiadev-add-real-system-tutorial-round-2.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Functions

> Author the functions a new worker contributes to an iii system.

## What "writing a function" means

A worker contributes capability by registering functions. Each function has an `id` of the form
`service::name`, a handler that receives the payload and returns a result, and optional JSON
Schemas that describe the request and response shape.

For how callers invoke functions (`worker.trigger` / `iii trigger` / event-bound triggers), see
[Using iii / Functions](../using-iii/functions) and
[Using iii / Triggers](../using-iii/triggers). This page is about the authoring surface.

## Register a function

Inside the worker, register the function with the SDK. The `id` is what callers pass as
`function_id`; the handler signature is the same regardless of how the invocation arrived (direct
call, HTTP trigger, cron, queue message).

<Tabs>
  <Tab title="Node / TypeScript">
    ```typescript theme={"theme":{"light":"catppuccin-latte","dark":"dark-plus"}}
    import { registerWorker } from "iii-sdk";

    const worker = registerWorker(process.env.III_URL);

    worker.registerFunction("math::add", async (payload: { a: number; b: number }) => {
      return { c: payload.a + payload.b };
    });
    ```
  </Tab>

  <Tab title="Python">
    ```python theme={"theme":{"light":"catppuccin-latte","dark":"dark-plus"}}
    import os
    from iii import register_worker, InitOptions

    worker = register_worker(
        os.environ.get("III_URL"),
        InitOptions(worker_name="math-worker"),
    )

    def add_handler(payload: dict) -> dict:
        return {"c": payload["a"] + payload["b"]}

    worker.register_function("math::add", add_handler)
    ```
  </Tab>

  <Tab title="Rust">
    ```rust theme={"theme":{"light":"catppuccin-latte","dark":"dark-plus"}}
    use iii_sdk::{InitOptions, RegisterFunction, register_worker};

    let url = std::env::var("III_URL").expect("III_URL must be set");
    let worker = register_worker(&url, InitOptions::default());

    worker.register_function(RegisterFunction::new("math::add", |input: AddInput| {
        Ok(serde_json::json!({ "c": input.a + input.b }))
    }))?;
    ```
  </Tab>
</Tabs>

## Attach request and response schemas

Attach JSON Schemas to the registration so the request and response shape are documented alongside
the function. The schemas are stored with the function and surface in the iii console and the
agent-readable skills.

<Note>
  Runtime validation is not yet supported. Attached schemas are metadata only; the engine does not
  reject payloads or handler return values that don't match them.
</Note>

<Tabs>
  <Tab title="Node / TypeScript">
    ```typescript theme={"theme":{"light":"catppuccin-latte","dark":"dark-plus"}}
    import { registerWorker } from "iii-sdk";

    const worker = registerWorker(process.env.III_URL);

    worker.registerFunction(
      "math::add",
      async (payload) => ({ c: payload.a + payload.b }),
      {
        request_schema: {
          type: "object",
          properties: { a: { type: "number" }, b: { type: "number" } },
          required: ["a", "b"],
        },
        response_schema: {
          type: "object",
          properties: { c: { type: "number" } },
          required: ["c"],
        },
      },
    );
    ```
  </Tab>

  <Tab title="Python">
    ```python theme={"theme":{"light":"catppuccin-latte","dark":"dark-plus"}}
    import os
    from iii import register_worker, InitOptions

    worker = register_worker(
        os.environ.get("III_URL"),
        InitOptions(worker_name="math-worker"),
    )

    worker.register_function(
        "math::add",
        add_handler,
        request_schema={
            "type": "object",
            "properties": {"a": {"type": "number"}, "b": {"type": "number"}},
            "required": ["a", "b"],
        },
        response_schema={
            "type": "object",
            "properties": {"c": {"type": "number"}},
            "required": ["c"],
        },
    )
    ```
  </Tab>

  <Tab title="Rust">
    ```rust theme={"theme":{"light":"catppuccin-latte","dark":"dark-plus"}}
    use iii_sdk::{InitOptions, RegisterFunction, register_worker};
    use serde_json::json;

    let url = std::env::var("III_URL").expect("III_URL must be set");
    let worker = register_worker(&url, InitOptions::default());

    worker.register_function(
        RegisterFunction::new("math::add", |input: AddInput| {
            Ok(serde_json::json!({ "c": input.a + input.b }))
        })
        .request_schema(json!({
            "type": "object",
            "properties": { "a": { "type": "number" }, "b": { "type": "number" } },
            "required": ["a", "b"],
        }))
        .response_schema(json!({
            "type": "object",
            "properties": { "c": { "type": "number" } },
            "required": ["c"],
        })),
    )?;
    ```
  </Tab>
</Tabs>

The schemas also feed the iii console and the agent-readable skills.

## Return values and errors

A function returns either a value (which the handler is responsible for shaping to match its
documented response schema) or an error. Errors raised inside the handler are propagated to the
caller as invocation errors with the worker's stack trace; the engine doesn't swallow them. Use
this distinction to express expected failures (return a structured error value) versus unexpected
ones (throw / raise / return `Err`).

## Unregister a function

`registerFunction` returns a handle with an `unregister()` method that removes the function from
the engine at runtime. When the worker disconnects, all of its functions are removed automatically
and pending invocations error out.

<Tabs>
  <Tab title="Node / TypeScript">
    ```typescript theme={"theme":{"light":"catppuccin-latte","dark":"dark-plus"}}
    const add = worker.registerFunction("math::add", async (payload) => {
      return { c: payload.a + payload.b };
    });

    add.unregister();
    ```
  </Tab>

  <Tab title="Python">
    ```python theme={"theme":{"light":"catppuccin-latte","dark":"dark-plus"}}
    add = worker.register_function("math::add", add_handler)

    add.unregister()
    ```
  </Tab>

  <Tab title="Rust">
    ```rust theme={"theme":{"light":"catppuccin-latte","dark":"dark-plus"}}
    let add = worker.register_function(RegisterFunction::new("math::add", |input: AddInput| {
        Ok(serde_json::json!({ "c": input.a + input.b }))
    }));

    add.unregister();
    ```
  </Tab>
</Tabs>

## What goes in Worker Docs

The function ids your worker exposes, what each one does, and any worker-specific semantics
(idempotency, rate limits, side effects) belong in this worker's Worker Docs. Keep iii-level
concepts (the registration surface, schema metadata, error propagation) here; document the
per-function specifics there.