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. Slots only change the edges of the system prompt: the runtime contract, the repository-tool contract, and the safety boundaries are always kept, so core tool behavior (reading, writing, running commands) is preserved.

There are five text slots (Node.js / Python names shown; Rust uses the SystemPromptSlots::with_* builders and Go uses the PromptSlots fields):

Slot	Effect on the system prompt
`role` / `role`	Replaces the default identity line.
`guidelines` / `guidelines`	Added to the `Guidelines` section.
`responseStyle` / `response_style`	Replaces the default `Response Format` section.
`outputLanguage` / `output_language`	Pins user-facing replies to a BCP-47 tag such as `zh-CN`.
`extra` / `extra`	Appended verbatim at the end.

Rust's SystemPromptSlots also has with_style, which forces one of the built-in prompt styles.

Basic usage

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

Rust

Node.js

Python

Rust

use a3s_code_core::{Agent, SessionOptions, SystemPromptSlots};

#[tokio::main]
async fn main() -> a3s_code_core::Result<()> {
    let agent = Agent::new("agent.acl").await?;
    let slots = SystemPromptSlots::default()
        .with_role("release-readiness reviewer")
        .with_guidelines(
            "Find blockers before improvements. Require command evidence for done claims.",
        )
        .with_response_style("concise, findings first");
    let session = agent
        .session_builder("/repo")
        .options(SessionOptions::new().with_prompt_slots(slots))
        .build()
        .await?;

    let result = session.send("Is this repo ready to ship?", None).await?;
    println!("{}", result.text);

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

TypeScript

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

async function main() {
  // create() accepts an .acl file path or inline ACL string.
  const agent = await Agent.create('agent.acl');

  const session = await agent.sessionAsync('/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.closeAsync();
  await agent.close();
}

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

Python

from a3s_code import Agent, SessionOptions

def main():
    # create() accepts an .acl file path or inline ACL string.
    agent = Agent.create('agent.acl')

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

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

    session.close()
    agent.close()

main()

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, "/repo", &code.SessionOptions{
		PromptSlots: &code.PromptSlots{
			Role: "release-readiness reviewer",
			Guidelines: "Find blockers before improvements. " +
				"Require command evidence for done claims.",
			ResponseStyle: "concise, findings first",
		},
	})
	if err != nil {
		log.Fatal(err)
	}
	defer session.Close(context.Background())

	result, err := session.Run(ctx, "Is this repo ready to ship?")
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(result.Text)
}

Each slot in turn

The 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.

Rust

Node.js

Python

Rust

use a3s_code_core::{Agent, SessionOptions, SystemPromptSlots};

#[tokio::main]
async fn main() -> a3s_code_core::Result<()> {
    let agent = Agent::new("agent.acl").await?;

    let role_only = agent
        .session_builder("/repo")
        .options(SessionOptions::new().with_prompt_slots(
            SystemPromptSlots::default().with_role(
                "You are a senior Rust developer who specializes in async programming.",
            ),
        ))
        .build()
        .await?;
    role_only.close().await;

    let reviewer = agent
        .session_builder("/repo")
        .options(SessionOptions::new().with_prompt_slots(
            SystemPromptSlots::default()
                .with_role("You are a Python code reviewer.")
                .with_guidelines("Always check for type hints. Flag any use of `eval()`.")
                .with_response_style("Reply in bullet points. Be concise."),
        ))
        .build()
        .await?;
    reviewer.close().await;

    let extra_only = agent
        .session_builder("/repo")
        .options(SessionOptions::new().with_prompt_slots(
            SystemPromptSlots::default().with_extra("Always end your response with '-- A3S'"),
        ))
        .build()
        .await?;
    extra_only.close().await;

    let file_manager = agent
        .session_builder("/repo")
        .options(SessionOptions::new().with_prompt_slots(
            SystemPromptSlots::default()
                .with_role("You are a minimalist file manager.")
                .with_guidelines("Only create files when explicitly asked."),
        ))
        .build()
        .await?;
    let result = file_manager
        .send(
            "Create test.txt with 'prompt slots work', then read it back.",
            None,
        )
        .await?;
    println!("{}", result.text);

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

TypeScript

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

const agent = await Agent.create('agent.acl');
const workspace = '/repo';

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

// 2. Role + guidelines + response style.
const reviewer = await agent.sessionAsync(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.',
});
await reviewer.closeAsync();

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

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

Python

from a3s_code import Agent, SessionOptions

agent = Agent.create('agent.acl')
workspace = '/repo'

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

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

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

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

package main

import (
	"context"
	"fmt"
	"log"

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

func open(
	ctx context.Context,
	agent *code.Agent,
	slots code.PromptSlots,
) *code.Session {
	session, err := agent.Session(ctx, "/repo", &code.SessionOptions{
		PromptSlots: &slots,
	})
	if err != nil {
		log.Fatal(err)
	}
	return session
}

func main() {
	ctx := context.Background()
	agent, err := code.Create(ctx, "agent.acl")
	if err != nil {
		log.Fatal(err)
	}
	defer agent.Close(context.Background())

	roleOnly := open(ctx, agent, code.PromptSlots{
		Role: "You are a senior Rust developer who specializes in async programming.",
	})
	roleOnly.Close(ctx)

	reviewer := open(ctx, agent, code.PromptSlots{
		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.",
	})
	reviewer.Close(ctx)

	extraOnly := open(ctx, agent, code.PromptSlots{
		Extra: "Always end your response with '-- A3S'",
	})
	extraOnly.Close(ctx)

	fileManager := open(ctx, agent, code.PromptSlots{
		Role:       "You are a minimalist file manager.",
		Guidelines: "Only create files when explicitly asked.",
	})
	defer fileManager.Close(context.Background())
	result, err := fileManager.Run(
		ctx,
		"Create test.txt with 'prompt slots work', then read it back.",
	)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(result.Text)
}

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 sdk/node/examples/skills/test_prompt_slots.ts.

#Prompt Slots

#Basic usage

#Each slot in turn

Prompt Slots

Basic usage

Each slot in turn