> ## 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.

# Client-hosted tools

> Declare typed work that runs in a browser, desktop, or mobile client.

Use `@client_tool` when the connected client—not the agent server—owns execution.

```python theme={null}
from harnest.tool import client_tool


@client_tool
def browser_open(url: str) -> dict[str, str]:
    """Open a URL and return visible page data."""
    ...
```

Harnest never executes the declaration body. It suspends the invocation and asks the client to run the tool.

## Client-tool properties

| Property            | Rule                                       |
| ------------------- | ------------------------------------------ |
| Declaration         | Typed stub under `tools/`                  |
| Execution           | Connected browser, desktop, or mobile host |
| Output              | Validated before the agent resumes         |
| Policy              | Client owns local execution and sandboxing |
| Runtime requirement | Active managed Harnest invocation          |

## Transport flow

| Transport | Request                       | Resume                           |
| --------- | ----------------------------- | -------------------------------- |
| JSON      | `requires_action`             | `POST /client-tools/{requestId}` |
| SSE       | `client_tool` required action | `POST /client-tools/{requestId}` |
| WebSocket | `client_tool.requested`       | `client_tool.result`             |

The submitted result is identity-bound, one-time, and validated against the declared return type.

## Combine with approval

`@client_tool` and `@require_human_approval` can appear in either decorator order. Approval runs first. After approval, the host receives the client-tool request.

<CardGroup cols={2}>
  <Card title="Human approvals" icon="user-check" href="/harnest/build/agent-tools/human-approvals">
    Bind approval to the exact user, session, invocation, action, and arguments.
  </Card>

  <Card title="Serving agents" icon="server" href="/harnest/runtime/serving/approvals-and-client-tools">
    Implement approval and client-tool responses over each transport.
  </Card>
</CardGroup>