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

# Invocation hooks

> Observe or transform agent, model, and tool execution in a portable way.

Portable hooks apply the same policy around managed ADK and LangGraph execution.

| Phase        | Use it for                                        |
| ------------ | ------------------------------------------------- |
| Before agent | Request policy, context setup, or early rejection |
| After agent  | Result checks or accounting                       |
| Before model | Input policy or routing metadata                  |
| After model  | Result checks                                     |
| Before tool  | Tool policy and guardrails                        |
| After tool   | Result checks and audit enrichment                |

```python theme={null}
from harnest.lifecycle import lifecycle


@lifecycle.before_tool
async def check_tool(event):
    if event.tool_name == "delete_account":
        raise PermissionError("Tool is disabled")
```

## Hook properties

| Property       | Behavior                                                                     |
| -------------- | ---------------------------------------------------------------------------- |
| Ordering       | Explicit order, then source path                                             |
| Transformer    | Can replace the current value                                                |
| Observer       | Reads an event without replacing it                                          |
| Rejection      | Stops work before the protected phase                                        |
| Error observer | Cannot hide the original failure                                             |
| Agent identity | Model hooks receive the managed agent or SubAgent performing that model call |

The `agent_name` on a model lifecycle context is call-specific. When a managed
SubAgent calls a model, it contains that SubAgent's name rather than the root
agent's name. The authenticated user, session, and invocation IDs remain those
of the root request, including when sibling SubAgents run concurrently.

Keep business operations in Agent Tools. Hooks should apply policy, observation, or transformation around those operations.

## Portable or native?

| Requirement                           | Choose                       |
| ------------------------------------- | ---------------------------- |
| Same policy on ADK and LangGraph      | Portable hook                |
| ADK runner or plugin API              | ADK native integration       |
| LangGraph middleware or graph runtime | LangGraph native integration |

<Card title="Native integrations" icon="code" href="/harnest/runtime/lifecycle/native-integrations">
  Isolate behavior that must use the selected framework directly.
</Card>