fx

fx is a coding agent harness and command-line tool written in Zig by Vercel. It focuses on minimalism and performance: the entire agent ships as a roughly 6 MB binary that cold-starts in microseconds, and the system prompt and toolset are kept deliberately small to save tokens and reach the first output fast.

Instead of an IDE-style terminal interface, fx behaves more like a Unix shell. It keeps scroll history, produces minimal output, and stays out of your way. It is model-agnostic: local models, gateways, direct provider APIs, and subscriptions all work.

The small core is built for embedding. Sandboxes, cloud inference, and other agent systems can drop fx in as a dependency, and it extends through skills, plugins, and MCP servers, all under an Apache 2.0 license.