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.