> ## Documentation Index
> Fetch the complete documentation index at: https://docs.agenticenv.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Error Control

> LLM fallback, extra iterations, and a per-tool circuit breaker

Walks through [`WithErrorControl`](/features/error-control) on one agent: fallback after an LLM error, one extra iteration, and a circuit breaker that skips a tool that keeps failing.

Source: [`examples/agent_with_error_control/`](https://github.com/agenticenv/agent-sdk-go/tree/main/examples/agent_with_error_control)

The sample uses stub LLM clients and a stub tool so you can run it without a provider key. Set `AGENT_RUNTIME=temporal` or `restate` in `examples/.env` to exercise those runtimes.

These callbacks are [error control](/features/error-control), not lifecycle [`WithHooks`](/features/hooks).

## What you will see

| Scenario | What happens |
| - | - |
| FallbackModel | Primary Generate returns `*LLMError` (`rate_limit`). The hook switches to the named client `"cheap"`. |
| ExtendIterations | `MaxIterations` is 1 and the stub still requests a tool. The hook grants one extra round; the next LLM call returns text. |
| Circuit breaker | The stub tool always fails with the same args. After two failures the third call is skipped. |

## Run

From `examples/`:

```bash theme={null}
go run ./agent_with_error_control
```

## Expected output

```
=== Scenario 1: OnLLMFailure FallbackModel ===
[error-control] OnLLMFailure: reason=rate_limit → FallbackModel
assistant: fallback model answered after rate limit

=== Scenario 2: OnMaxIterationsExceeded ExtendIterations ===
[error-control] OnMaxIterationsExceeded: last=fail_once → ExtendIterations extra=1
assistant: finished after extra iteration

=== Scenario 3: Circuit breaker skip ===
assistant: stopped looping
tools: total=3 failed=2
```

Hook lines go to **stderr**. Scenario 3's third tool call is skipped, so `failed` stays 2.

## Temporal and Restate

Pass the same `WithNamedLLMClients` and `WithErrorControl` on the agent that starts the run and on the worker. The SDK fingerprints hook **slots** (not bodies), the fallback name, named-client model/provider, and breaker thresholds.

On Restate, the hooks run in the process that serves the embedded endpoint.

## Learn more

<CardGroup cols={2}>
  <Card title="Error Control" icon="shield-halved" href="/features/error-control" horizontal>
    How to configure fallback, extra iterations, and the breaker
  </Card>

  <Card title="Hooks" icon="code-branch" href="/features/hooks" horizontal>
    Lifecycle middleware — a different API
  </Card>
</CardGroup>