Skill Tool

Skills surface to the model as two core tools: search_skills (find a skill by intent) and Skill (invoke a skill by name). Skill runs a child agent loop that uses the skill's content as its role prompt and returns that run's final text. You can let the model call these tools during a run, or invoke a skill directly from the SDK with session.tool('Skill', ...).

Register a skill directory (a folder of SKILL.md files) via skillDirs / skill_dirs, or pass inline skills through session options. A3S Code no longer ships default embedded skills, so builtinSkills / builtin_skills is only a compatibility flag. Registered skills become visible through Skill and search_skills.

Rust

Node.js

Python

Rust

use a3s_code_core::{Agent, SessionOptions};
use serde_json::json;

#[tokio::main]
async fn main() -> a3s_code_core::Result<()> {
    let agent = Agent::new("agent.acl").await?;
    let session = agent
        .session_builder(".")
        .options(SessionOptions::new().with_skill_dirs(["./skills"]))
        .build()
        .await?;

    println!("{:?}", session.tool_names());
    let run = session
        .send(
            "Search available skills, then apply the most relevant one.",
            None,
        )
        .await?;
    println!("{}", run.text);

    let result = session
        .tool(
            "Skill",
            json!({
                "skill_name": "release-review",
                "prompt": "Review this release patch for blockers and verification gaps."
            }),
        )
        .await?;
    if result.exit_code != 0 {
        return Err(a3s_code_core::CodeError::Tool {
            tool: "Skill".into(),
            message: result.output,
        });
    }
    println!("{}", result.output);

    session.close().await;
    agent.close().await;
    Ok(())
}

TypeScript

import { Agent } from '@a3s-lab/code';

const agent = await Agent.create('agent.acl');
const session = agent.session(process.cwd(), {
  skillDirs: ['./skills'], // a folder of SKILL.md files
});

// Skill and search_skills are core tools — confirm they're on the surface.
console.log(session.toolNames());

// Option A: let the model search for and apply a skill during a run.
const run = await session.run(
  'Search available skills, then apply the most relevant one.',
);
console.log(run.text);

// Option B: invoke a skill directly as a callable tool.
// Canonical args: { skill_name, prompt? }.
const result = await session.tool('Skill', {
  skill_name: 'release-review',
  prompt: 'Review this release patch for blockers and verification gaps.',
});
if (result.exitCode !== 0) {
  throw new Error(result.output);
}
console.log(result.output);

session.close();

Python

from a3s_code import Agent, SessionOptions

agent = Agent.create("agent.acl")
opts = SessionOptions()
opts.skill_dirs = ['./skills']  # a folder of SKILL.md files
session = agent.session('.', opts)

# Skill and search_skills are core tools — confirm they're on the surface.
print(session.tool_names())

# Option A: let the model search for and apply a skill during a run.
run = session.run('Search available skills, then apply the most relevant one.')
print(run.text)

# Option B: invoke a skill directly as a callable tool.
# Canonical args: { skill_name, prompt? }.
result = session.tool('Skill', {
    'skill_name': 'release-review',
    'prompt': 'Review this release patch for blockers and verification gaps.',
})
if result.exit_code != 0:
    raise RuntimeError(result.output)
print(result.output)

session.close()

package main

import (
	"context"
	"fmt"
	"log"

	code "github.com/A3S-Lab/Code/sdk/go/v9"
)

func main() {
	ctx := context.Background()
	agent, err := code.Create(ctx, "agent.acl")
	if err != nil {
		log.Fatal(err)
	}
	defer agent.Close(context.Background())
	session, err := agent.Session(ctx, ".", &code.SessionOptions{
		SkillDirs: []string{"./skills"},
	})
	if err != nil {
		log.Fatal(err)
	}
	defer session.Close(context.Background())

	names, err := session.ToolNames(ctx)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(names)
	run, err := session.Run(
		ctx,
		"Search available skills, then apply the most relevant one.",
	)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(run.Text)

	result, err := session.Tool(ctx, "Skill", map[string]any{
		"skill_name": "release-review",
		"prompt": "Review this release patch for blockers and verification gaps.",
	})
	if err != nil {
		log.Fatal(err)
	}
	if result.ExitCode != 0 {
		log.Fatal(result.Output)
	}
	fmt.Println(result.Output)
}

A SKILL.md declares its kind in frontmatter (instruction, the default, persona, or tool). The Skill tool grants the skill's allowed-tools only for the child run, composed with the session's permission policy, and revokes them afterwards. A skill that disables model invocation cannot be run through Skill. Skill administration is handled by SDK registration, skill directories, or project files — not by a model-visible management tool.

A runnable version ships at sdk/node/examples/skills/test_custom_skills_agents.ts.

#Skill Tool

Skill Tool