Rust quick start

Scaffold

act-build init rust my-component
cd my-component
just init

act-build init carries the templates inside the binary — there is no copier, uvx, or Python toolchain involved. Pass --http / --fs to have it declare those capabilities in act.toml for you.

The template sets up Cargo.toml with a current act-sdk, a wit/ directory tracking act:tools (and act:sessions for stateful components) over act:core, a rust-toolchain.toml pinning nightly + wasm32-wasip2, and a justfile with build / test / publish recipes.

Write a tool

src/lib.rs:

use act_sdk::prelude::*;

#[act_component]
mod component {
    use super::*;

    #[act_tool(description = "Reverse a string", read_only)]
    fn reverse(
        /// Text to reverse
        text: String,
    ) -> ActResult<String> {
        Ok(text.chars().rev().collect())
    }
}

The #[act_component] macro generates the WIT tool-provider export. The #[act_tool] macro derives the JSON Schema from the function signature, collects std:* metadata (read_only, idempotent, destructive, timeout_ms), and registers the function with the component’s dispatcher.

Build

just build

This runs:

1. cargo build --release → target/wasm32-wasip2/release/<name>.wasm
2. act-build pack <name>.wasm → embeds the act:component custom section (CBOR-encoded metadata) and the contents of skill/ as an Agent Skill tar.

Verify

act info target/wasm32-wasip2/release/component_my_component.wasm --tools

Should print your component name, version, and the reverse tool with its auto-derived schema.

Run

just test                    # e2e suite: pytest driving the component over MCP
act call <wasm> reverse --args '{"text":"hello"}'

What act-sdk gives you

Macro / type	Purpose
#[act_component]	Wraps a module as the act:tools/tool-provider export
#[act_tool(…)]	Registers a fn as a tool, emits JSON Schema from the signature + doc comments
ActError / ActResult	Return errors with a std:* kind and localized message
Json<T>	Opt into JSON encoding (default is CBOR, per ACT-SPEC)
Content { mime, data }	Return raw bytes with an explicit MIME type
#[session_open] / #[session_close]	Mark the session lifecycle hooks of a stateful component
ActContext / SessionRegistry	Per-call context and per-session state

Agent Skills need no macro: act-build pack embeds a skill/ directory automatically. See Agent Skills.

Next

• Declare capabilities — wasi:filesystem, wasi:http
• Agent Skills — SKILL.md embedded in the component
• Testing — the just test pattern: pytest + a real MCP client