The fx adapter keeps model requests on the application host
Vercel’s fx adapter for AI SDK HarnessAgent now runs the agent runtime and model requests in the application’s host process, while built-in filesystem and shell tools execute in a sandbox. The libfx library removes the need to install the agent inside that sandbox or expose a bridging port. Model credentials stay on the host; custom tools and remote tool connections require their own permission review.
Vercel · Oct 27, 2026
Why it matters
This execution split makes it easier to reason about which process holds credentials and which operations touch the isolated filesystem.
More context
Update @ai-sdk/harness-fx and configure the documented sandbox adapter and host-side model access. Test saved-session resumption and tool permissions on a small workflow. The documentation identifies the harness packages as experimental.
Keep in mind
The sandbox boundary covers the documented built-in tools, not every possible custom tool. Experimental APIs may change; model and sandbox costs were not established by the announcement.
Original read