# The fx adapter now runs the harness on the host

Source: [Vercel](https://vercel.com/changelog/fx-harness-adapter-now-separates-host-and-sandbox)  
Feed7 permalink: https://feed7.dev/p/fx-harness-adapter-now-separates-host-and-sandbox-1lvyogi  
Published: 2026-10-29T06:00:00.000Z  
Trust: Official Source (official_source)

## Why Included

The AI SDK's fx adapter moves the harness and model requests onto the host while keeping built-in shell and filesystem tools sandboxed, so model credentials stay off the sandbox.

## Source Summary

The AI SDK's fx adapter now runs the harness and model requests in the **host process**, while built-in filesystem and shell operations remain in the sandbox. It removes the need for an fx install in the sandbox, a bridge, or an exposed sandbox port; model credentials stay on the host.

## Practical Implication

Builders using HarnessAgent can update `@ai-sdk/harness-fx` to gain tool filtering, mid-turn steering, schema-based structured output, image tool results, and sessions that can resume in a new host process.

## Agent-Ready Context

The AI SDK's fx adapter now runs the harness and model requests in the **host process**, while built-in filesystem and shell operations remain in the sandbox. It removes the need for an fx install in the sandbox, a bridge, or an exposed sandbox port; model credentials stay on the host.

Builders using HarnessAgent can update `@ai-sdk/harness-fx` to gain tool filtering, mid-turn steering, schema-based structured output, image tool results, and sessions that can resume in a new host process.

The material does not specify a version number or discuss compatibility and operational tradeoffs of the new host/sandbox boundary.

## Connected Context

Feed7 judgment across 944 accumulated Signals:

This clarifies the fx adapter boundary: harness execution, model requests, and credentials stay on the host, while built-in filesystem and shell tools remain sandboxed. It removes the sandbox install and bridge requirements and adds resumable sessions and other HarnessAgent capabilities. The source leaves compatibility and operational tradeoffs unspecified.

- [Cline is now available in the AI SDK harness layer](https://feed7.dev/p/cline-harness-adapter-1sv80a0) — Both describe a host-resident harness with sandboxed tools, so the fx update confirms that this boundary applies beyond the Cline adapter; it does not establish full runtime isolation.
- [Introducing Run SDK: secure eval for your agents](https://feed7.dev/p/introducing-run-12zpstq) — Run SDK also separates agent execution from trusted host access, but uses narrow host functions for generated code. Together they show distinct ways to constrain tool or code access, rather than one shared sandbox design.

## Context Map

- Layer: agent
- Domains: coding, security
- Topics: harness-engineering, sandboxing, agent-sdks

## Uncertainty

- The material does not specify a version number or discuss compatibility and operational tradeoffs of the new host/sandbox boundary.

## Agent Instruction

Use this item as source-backed context. Do not invent claims beyond the linked source. If this item conflicts with another source, call out the conflict.