Vercel Labs has open-sourced fx, the coding agent tool it uses internally. Written in Zig, fx is essentially a featherweight agent harness and CLI designed for model research, evaluation, or dropping into other products.
The entire program clocks in at roughly 6.4MiB, with a claimed cold-start time of 10 microseconds and base memory usage in the single-digit megabytes.
fx works with different models, supports Skills, MCP, and sub-agents, and can plug into editors via ACP. It can even run directly in a webpage — no local install required.
The tool doesn't collect extra usage data; sessions and history stay local by default. Hook it up to a local model and disable networking, and it runs fully offline.
fx is still experimental. Because it's so resource-light, it's well-suited for running lots of agents at once during model testing. But 6MB and 10µs only prove the program is light, not that it writes better code.
For now, its biggest selling point is that it's small, fast, and easy to embed in other products.
Vercel Developers posted the details on X:
https://twitter.com/vercel_dev/status/2089828083415355806