Vercel Labs releases fx, an open source coding agent

Vercel Labs announced fx in August 2026 and released it as open source. fx is a coding agent that a script or a different tool can start from the command line. The numbers are from Vercel: the fx binary is 6.3 MiB and the cold start is 10 microseconds. No other group tested these numbers, and the release is experimental.

How it works

fx writes its text in the usual terminal history and does not fill the screen. The same engine works as a command tool and as a library for JavaScript, with a WebAssembly version. A script can get JSON output and read it.

The numbers

The numbers are from the company: the memory use is some megabytes and the start time is 10 microseconds. No other group tested these numbers. Release 0.0.13 on 4 October added an Ultrafast mode for some OpenAI models.

What is not known

• First users wrote that setup sends data through a gateway of the company.
• Some users wanted to know how fx is different from OpenCode.
• A different JSON tool has the same name. The team checks how they can work together.
• Versions of fx before this one cannot read new libfx checkpoints.

Insight: The company wants to control the layer that connects coding agents

The company puts fx with 6 other coding agents behind one interface. A user wrote that fx is the entry point that puts more users on Vercel AI. This reads as a move to control the layer that connects coding agents. The company gets more traffic through its gateway. Teams that select tools must check which gateway an agent uses.