fx (by Vercel)

fx is a minimal coding agent harness and command-line tool built in Zig. It's designed to be tiny (about 6.4 MB), fast to start (cold starts in 10 microseconds), and easy to embed in larger systems or run as a standalone agent. You install it with a single curl command, and it works with local models, cloud APIs, or gateways — no heavy IDE or TUI required. The project is open source under Apache-2.0 and currently in an experimental stage (v0.0.4).

What makes fx stand out is its focus on minimalism and performance. The binary is small enough to fit in resource-constrained environments or agent sandboxes. It has a shell-like interface that preserves scroll history and produces minimal output, so it feels more like a Unix shell than a bulky terminal UI. It's context-efficient, with a small system prompt and tool set to save on token costs and improve time-to-first-token. You can extend it via skills, plugins, or MCPs, and it supports WebAssembly builds for even smaller footprints.

fx is best for developers and researchers who want a lightweight, embeddable coding agent that can be scripted or integrated into automated workflows. It's also useful for anyone who prefers a fast, no-frills terminal experience over a full IDE. Because it's model-agnostic, you can pair it with whatever LLM you like — local or cloud — making it a flexible tool for experimentation and production use alike.