Agent Loop

The Agent Loop implements the ReAct (Reasoning + Acting) pattern — the core execution cycle that turns an LLM into an autonomous agent.

How It Works

User Prompt
    │
    ▼
┌─────────────────────┐
│   LLM Call          │ ◄──────────────────┐
│   (with tools)      │                    │
└─────────┬───────────┘                    │
          │                                │
          ▼                                │
    ┌─────────────┐     Yes    ┌──────────┴──────────┐
    │ Tool calls? ├───────────►│ Execute tools       │
    └──────┬──────┘            │ (parallel if safe)  │
           │ No                └─────────────────────┘
           ▼
    ┌─────────────┐
    │ Return      │
    │ final answer│
    └─────────────┘

Basic Usage

import { AgentLoop } from '@opensin/sdk'

const agent = new AgentLoop({
  model: 'claude-sonnet-4-6',
  tools: toolRegistry,
  systemPrompt: 'You are a helpful coding assistant.',
  maxTurns: 20,
})

const result = await agent.run('Fix the failing test in auth.test.ts')
console.log(result.response)
console.log(`Completed in ${result.turns} turns`)

Configuration

Option	Type	Default	Description
model	string | ModelRouter	required	LLM model or router instance
tools	ToolRegistry	required	Available tools
systemPrompt	string	''	System instructions
maxTurns	number	25	Maximum ReAct iterations
earlyExit	boolean	true	Stop when no tool calls are made
streaming	boolean	false	Enable streaming responses
permissions	PermissionManager	null	Permission gate for tools
context	ContextCompressor	null	Context window manager
session	SessionManager	null	Session persistence

Events

The agent loop emits events at each stage:

agent.on('turn:start', ({ turn, messages }) => {
  console.log(`Turn ${turn} starting with ${messages.length} messages`)
})

agent.on('tool:call', ({ name, args }) => {
  console.log(`Calling tool: ${name}`)
})

agent.on('tool:result', ({ name, result, duration }) => {
  console.log(`Tool ${name} completed in ${duration}ms`)
})

agent.on('turn:end', ({ turn, response }) => {
  console.log(`Turn ${turn} complete`)
})

agent.on('complete', ({ turns, totalTokens }) => {
  console.log(`Agent finished in ${turns} turns, ${totalTokens} tokens`)
})

Streaming

Enable real-time output streaming:

const agent = new AgentLoop({
  model: 'claude-sonnet-4-6',
  tools: toolRegistry,
  streaming: true,
})

for await (const chunk of agent.stream('Explain this codebase')) {
  if (chunk.type === 'text') {
    process.stdout.write(chunk.text)
  } else if (chunk.type === 'tool_call') {
    console.log(`\nUsing tool: ${chunk.name}`)
  }
}

Turn Management

Control how the agent progresses through turns:

const agent = new AgentLoop({
  maxTurns: 10,
  earlyExit: true,
  onMaxTurns: 'summarize', // or 'error', 'continue'
})

• earlyExit: true: Stops when the LLM returns a response without tool calls
• onMaxTurns: 'summarize': Summarizes progress when hitting the turn limit
• onMaxTurns: 'error': Throws an error at the turn limit