Pranit

Builder of @Vercel

[Pranit @fazxes]

I’ve spent the past four months building fx. We’re open sourcing it today.

I cared most about making it fast, small, and easy to embed:

10µs cold start
6.3 MiB native binary
single-digit megabytes of memory at baseline
structured output, ACP, and WebAssembly

Getting fx to this point has been a great experience, especially working closely with @EstebanSuarez and @rauchg and being mentored directly by both of them.

Try it and give us feedback. Join the X group chat for updates and a closer look at how we’re approaching fx and where it’s headed:

https://t.co/ihUJsRoj0Q

https://t.co/6TAVw5vgNl

[Vercel Developers @vercel_dev]

Introducing fx, a tiny, open, native coding agent from Vercel Labs.

Originally an internal tool, fx is a harness and CLI written in Zig, optimized for research and embedding in larger systems. Today, we're open sourcing it.

fx is built on three principles:

1. Fast. A single native binary, no runtime to install. It cold starts in 10µs and does no unnecessary work or I/O before accepting input. fx is the answer to "how fast can a coding agent be?"

2. Light. The 6.3MiB binary uses single-digit megabytes of memory at baseline, made for instant installation and embedding in resource-constrained environments and agent sandboxes.

3. Open. Apache-2.0, model and provider agnostic, suitable for local and cloud inference. Its small core extends through skills, plugins, and MCP.

Minimalism is an obsession throughout the entire harness: system prompt, tools, features, binary. The goal was to keep context usage and time to first token low, and make fx optimal for model benchmarking, sandboxing, evals, and gyms.

You can use fx directly or embed it as infrastructure. The CLI feels more like a Unix shell than an IDE in the terminal: it preserves scroll history, produces minimal output, and uses complex TUI rendering very, very sparingly. Programmatically, 𝚏𝚡 𝚊𝚜𝚔 --𝚓𝚜𝚘𝚗 gives structured output, 𝚏𝚡 𝚊𝚌𝚙 connects to editors and other clients, and WebAssembly can even run the whole thing inside the browser (see: https://t.co/wf2Trg47sC).

Privacy is a design constraint: no product telemetry, sessions and usage stay local, and no source code or prompts are shared with any endpoint other than inference. With local inference and auto-updates off, fx is fully hermetic.

fx is experimental. Use at your own risk and expect frequent changes. Chat with us on X (https://t.co/A2AB2YythC) or file issues (https://t.co/GEjTHSoa1J).

𝚌𝚞𝚛𝚕 -𝚏𝚜𝚂𝙻 𝚏𝚡.𝚜𝚑/𝚜𝚎𝚝𝚞𝚙.𝚜𝚑 | 𝚋𝚊𝚜𝚑

https://t.co/g2uEuXhGnt

[Pranit @fazxes]

@bergamaberg yea for sure TUI is still very limited and not as good as a GUI but I love TUI

[Pranit @fazxes]

software is hard (still)

[Leonard @LeonardCser]

how fast to startup?

@fx wins by a clear margin

[Pranit @fazxes]

@DuckDeleuze background server -- lol

[Pranit @fazxes]

@MelkeyDev after the steak last night I thought we touched enough grass

[Pranit @fazxes]

@bergamaberg yea well it depends on what u looking at, for GUI apps it's useless I agree but we're not talking about GUI here are we ?

[Pranit @fazxes]

@Arjith88 make the binary as small as possible

[Pranit @fazxes]

Beam from @reflection_ai is free on https://t.co/10YSWmz9e0 until tomorrow

Enjoy 🇺🇸

[Guillermo Rauch @rauchg]

Beam by Reflection is free on https://t.co/3Z4NESzNKd until Nov 12.
American open weights.
Zero data retention.
Enjoy 🇺🇸

[Pranit @fazxes]

@mjlbach @gm_mertd and a 🐐

[Pranit @fazxes]

@natemcgrady not a fan tbh

[Pranit @fazxes]

@nikolay_dp 🔥

[Pranit @fazxes]

Fable 5.2 is now available on https://t.co/10YSWmz9e0 - give it a spin!

[Pranit @fazxes]

@mitchellh @anthonyshew 🐐

[Pranit @fazxes]

@iamMrDuncan @joelniklaus @vercel Need to post benchmarks of our own 👀

[Pranit @fazxes]

@evilrabbit_ Woa

[Pranit @fazxes]

@mdlahfir @JoelFickson Shhhh 👀🤫

[Pranit @fazxes]

@heyblake @Superhuman https://t.co/mpY8cnQ1QH

[Pranit @fazxes]

@ThatsPranav Go for it

[Pranit @fazxes]

@rachpradhan Great work bro

[Pranit @fazxes]

Woa

[Chris Tate @ctatedev]

wterm now supports WebMCP

Agents can use the terminals on your page
→ List, read, type, press keys
→ Opt-in per terminal, read-only mode
→ React, Vue, Svelte + vanilla
→ No DOM scraping or fake key events

Below: 5 fx agents launch each other across 4 browser terminals