# The fx adapter now runs the harness on the host

> Source: <https://vercel.com/changelog/fx-harness-adapter-now-separates-host-and-sandbox>
> Published: 2026-10-27 06:00:00+00:00

The fx adapter for the AI SDK’s `HarnessAgent` now runs the harness and model requests in the same process as your application. Built-in filesystem and shell operations remain isolated in the sandbox.

The adapter uses `libfx`, the TypeScript library for fx. It no longer needs to install fx inside the sandbox, run a sandbox bridge, or expose a sandbox port. Model credentials stay on the host rather than being forwarded into the sandbox.

The rewrite also adds:

**Built-in tool filtering:** choose which built-in tools the agent can use

**Mid-turn steering:** send new instructions while a turn is running

**Structured output:** generate responses that match a schema

**Image tool results:** return images to the model through `toModelOutput()`

Sessions can be saved and resumed in a new host process.

Update `@ai-sdk/harness-fx` to the latest version to use the new adapter. Learn more in the [fx harness documentation](https://ai-sdk.dev/providers/ai-sdk-harnesses/fx).