The Agent Loop

ra's core loop is simple: send messages to the model, stream the response, execute any tool calls, repeat. Every step fires a middleware hook you can intercept. The loop handles iteration, token tracking, and tool execution — you control everything else through system prompts, skills, and middleware.

┌─────────────────────────────────────────────────┐
│                  beforeLoopBegin                │
└──────────────────────┬──────────────────────────┘
                       ▼
         ┌─── beforeModelCall ◄────────────┐
         │                                 │
         ▼                                 │
    Stream response                        │
    (onStreamChunk)                        │
         │                                 │
         ▼                                 │
   afterModelResponse                      │
         │                                 │
         ├── No tool calls? ──► afterLoopComplete
         │
         ▼
   beforeToolExecution
         │
         ▼
    Execute tools
         │
         ▼
   afterToolExecution
         │
         ▼
   afterLoopIteration ────────────────────►┘

How it works

1. Start — beforeLoopBegin fires once. Your middleware can set up logging, validate config, or inject initial context.

2. Model call — beforeModelCall fires with the full request (messages, tools, model). The model streams its response, firing onStreamChunk for every token. When the response is complete, afterModelResponse fires.

3. Tool execution — If the model requested tool calls, beforeToolExecution fires for each one, then the tool runs, then afterToolExecution fires with the result.

4. Iterate or complete — If tools were called, afterLoopIteration fires and the loop goes back to step 2 with the tool results appended to the conversation. If no tools were called, afterLoopComplete fires and the loop ends.

Loop controls

The loop tracks token usage per iteration, enforces maxIterations, and supports an AbortController. Any middleware can call ctx.stop() to halt the loop cleanly.

agent:
  maxIterations: 0      # default 0 = unlimited
  toolTimeout: 120000  # per-tool timeout in ms (default: 2 minutes)

// middleware/budget.ts — stop if we've used too many tokens
export default async (ctx) => {
  if (ctx.loop.usage.inputTokens + ctx.loop.usage.outputTokens > 100_000) {
    ctx.stop()
  }
}

See also

• Middleware — all available hooks and their context shapes
• Built-in Tools — tools available to the agent
• Context Control — how ra manages what the model sees
• Configuration — maxIterations and toolTimeout settings