Customize the agent's persona, guidelines, and response style without overriding core behavior.

Prompt Slots

Prompt slots are session options that shape the agent's system prompt declaratively. Use them for host-level behavior — persona, coding standards, output style — that should not live inside each user prompt. The slots layer on top of the agent's built-in instructions, so core tool behavior (reading, writing, running commands) is preserved.

There are four slots:

Slot	Purpose
`role` / `role`	The persona the agent adopts.
`guidelines` / `guidelines`	Standards and rules the agent must follow.
`responseStyle` / `response_style`	How the agent should format its replies.
`extra` / `extra`	Freeform instructions appended verbatim.

Basic usage

Set any subset of the slots when you open a session. They apply to every turn of that session.

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

async function main() {
  // Node: create() takes a PATH to the ACL agent file.
  const agent = await Agent.create('agent.acl');

  const session = agent.session('/repo', {
    role: 'release-readiness reviewer',
    guidelines: 'Find blockers before improvements. Require command evidence for done claims.',
    responseStyle: 'concise, findings first',
  });

  const result = await session.send('Is this repo ready to ship?');
  console.log(result.text);

  await session.close();
}

main().catch((err) => {
  console.error(err);
  process.exit(1);
});

from a3s_code import Agent, SessionOptions

def main():
    # Python: create() takes the ACL SOURCE TEXT.
    agent = Agent.create(open('agent.acl').read())

    session = agent.session('/repo', SessionOptions(
        role='release-readiness reviewer',
        guidelines='Find blockers before improvements. Require command evidence for done claims.',
        response_style='concise, findings first',
    ))

    result = session.send('Is this repo ready to ship?')
    print(result.text)

    session.close()

main()

Each slot in turn

The four slots compose independently. A persona-only session, a reviewer with strict guidelines, and a session that appends a freeform instruction all use the same option set.

// 1. Custom role only.
let session = agent.session(workspace, {
  role: 'You are a senior Rust developer who specializes in async programming.',
});

// 2. Role + guidelines + response style.
session = agent.session(workspace, {
  role: 'You are a Python code reviewer.',
  guidelines: 'Always check for type hints. Flag any use of `eval()`.',
  responseStyle: 'Reply in bullet points. Be concise.',
});

// 3. Extra freeform instructions only.
session = agent.session(workspace, {
  extra: "Always end your response with '-- A3S'",
});

// Core tool behavior is preserved regardless of the slots.
session = agent.session(workspace, {
  role: 'You are a minimalist file manager.',
  guidelines: 'Only create files when explicitly asked.',
});
const result = await session.send(
  "Create a file called test.txt with the content 'prompt slots work'. Then read it back.",
);

# 1. Custom role only.
session = agent.session(workspace, SessionOptions(
    role='You are a senior Rust developer who specializes in async programming.',
))

# 2. Role + guidelines + response style.
session = agent.session(workspace, SessionOptions(
    role='You are a Python code reviewer.',
    guidelines='Always check for type hints. Flag any use of `eval()`.',
    response_style='Reply in bullet points. Be concise.',
))

# 3. Extra freeform instructions only.
session = agent.session(workspace, SessionOptions(
    extra="Always end your response with '-- A3S'",
))

# Core tool behavior is preserved regardless of the slots.
session = agent.session(workspace, SessionOptions(
    role='You are a minimalist file manager.',
    guidelines='Only create files when explicitly asked.',
))
result = session.send(
    "Create a file called test.txt with the content 'prompt slots work'. Then read it back.",
)

Slots customize personality and house rules; they do not disable tools or change the agent's core loop. Keep task-specific requests in the send message and reserve the slots for behavior that should hold across every turn of the session.

A runnable version ships at crates/code/sdk/node/examples/skills/test_prompt_slots.ts.

Prompt Slots

Prompt Slots

Basic usage

Each slot in turn

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