---
title: Interventions
description: "A composable control layer for authorization, guardrails, and steering with typed actions, ordered evaluation, and short-circuiting."
tags: [hooks, event-loop, tool-execution]
languages: [typescript]
---

Interventions are a composable control layer for agents. They provide a typed action model for common control concerns — authorization, guardrails, steering, and content transformation — with ordered evaluation and short-circuiting. Unlike raw [hooks](./hooks.mdx) and [plugins](../plugins/index.mdx) which mutate event objects directly, intervention handlers return typed decisions (`proceed`, `deny`, `guide`, `confirm`, `transform`) that the framework applies with well-defined semantics — enabling automatic short-circuiting, feedback accumulation, and conflict resolution.

## Basic Usage

Create an intervention handler by extending `InterventionHandler` and overriding the lifecycle methods you need. Register handlers via the `interventions` option in agent configuration:

```typescript
--8<-- "user-guide/concepts/agents/interventions.ts:basic_usage"
```

Handlers only need to override the lifecycle methods relevant to their concern — all methods default to `proceed()`.

## Action Types

Each lifecycle method returns one of five typed actions:

| Action | Factory | Description |
|--------|---------|-------------|
| Proceed | `InterventionActions.proceed()` | Allow the operation to continue unchanged |
| Deny | `InterventionActions.deny(reason)` | Block the operation. Short-circuits remaining handlers |
| Guide | `InterventionActions.guide(feedback)` | Cancel and provide feedback for the model to retry with |
| Confirm | `InterventionActions.confirm(prompt)` | Pause for human approval |
| Transform | `InterventionActions.transform(apply)` | Modify event content in-place before execution continues |

The following examples show each action type in a realistic handler:

```typescript
--8<-- "user-guide/concepts/agents/interventions.ts:action_types"
```

## Lifecycle Methods

Intervention handlers can override five lifecycle methods. Each method supports a specific subset of actions:

| Method | Valid Actions | When it Runs |
|--------|-------------|--------------|
| `beforeInvocation` | Proceed, Deny, Guide, Transform | Before the agent loop starts |
| `beforeToolCall` | Proceed, Deny, Guide, Confirm, Transform | Before each tool execution |
| `afterToolCall` | Proceed, Transform | After each tool execution |
| `beforeModelCall` | Proceed, Deny, Guide, Transform | Before each model API call |
| `afterModelCall` | Proceed, Guide, Transform | After each model response |

How actions behave depends on the lifecycle method:

| Action | Before events | After events |
|--------|--------------|--------------|
| **Deny** | Sets `event.cancel`, short-circuits remaining handlers | No effect (warns at runtime) |
| **Guide** | On `beforeToolCall`/`beforeInvocation`: cancels with accumulated feedback. On `beforeModelCall`: injects feedback as user message | Injects feedback and retries |
| **Confirm** | Pauses agent via interrupt/resume for human approval; denied responses set `event.cancel` | Not supported |
| **Transform** | Calls `action.apply(event)` — later handlers see modified content | Calls `action.apply(event)` |

## Evaluation Order and Short-Circuiting

Handlers evaluate in **registration order**. If any handler returns `deny()`, remaining handlers are skipped — the operation is blocked immediately. This enables efficient pipelines where fast checks (like authorization) run first and prevent expensive evaluations (like LLM-based steering) from running unnecessarily.

```typescript
--8<-- "user-guide/concepts/agents/interventions.ts:short_circuiting"
```

For `guide()` actions, all handlers continue to run and their feedback is accumulated — the model receives combined guidance from all guiding handlers.

## Error Handling

The `onError` property controls what happens when a handler throws an exception:

| Value | Behavior |
|-------|----------|
| `'throw'` | Rethrow the error (default). The invocation fails. |
| `'proceed'` | Log the error and continue as if `proceed()` was returned. |
| `'deny'` | Log the error and treat it as a `deny()` (fail-closed). |

```typescript
--8<-- "user-guide/concepts/agents/interventions.ts:error_handling"
```

Use `'deny'` for security-critical handlers where a failure should block execution. Use `'proceed'` for non-critical handlers like logging where availability is more important than enforcement.

## Relationship to Hooks and Plugins

Interventions are built on top of the [hooks](./hooks.mdx) system — under the hood, each lifecycle method registers a hook callback. The difference is in how they communicate with the framework.

[Hooks](./hooks.mdx) and [plugins](../plugins/index.mdx) mutate event properties directly (e.g., setting `event.cancel = "reason"`). The framework doesn't know _why_ something was cancelled — was it a hard authorization denial or soft guidance to retry differently? Multiple plugins modifying the same event can conflict silently with last-write-wins semantics.

Interventions return typed actions that the framework interprets. This enables:

- **Short-circuiting** — a `deny()` from an authorization handler skips all remaining handlers automatically. With hooks, each plugin must independently check `event.cancel` before doing work.
- **Feedback accumulation** — multiple handlers can return `guide()` and their feedback is combined into a single message to the model, rather than overwriting each other.
- **Human-in-the-loop** — `confirm()` integrates with the SDK's interrupt/resume system to pause for approval without the handler needing to manage interrupt lifecycle.
- **Ordered evaluation** — handlers always run in registration order with well-defined precedence (deny > confirm > guide > transform > proceed).
- **Error policies** — each handler declares its own failure mode via `onError`. A logging handler can use `'proceed'` (skip on failure), while an auth handler can use `'deny'` (fail closed). Hooks have no equivalent — a thrown error always propagates.

## Related topics

- [Hooks](./hooks.mdx) — Low-level event callbacks for observing and modifying agent behavior
- [Plugins](../plugins/index.mdx) — Bundle related hooks and tools into reusable modules
- [Interrupts](../interrupts.mdx) — The interrupt/resume system that `confirm()` builds on
