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]

@mjlbach @vercel_dev This is exactly the kind of thing I hoped people would build with fx. Remote sessions over Tailscale RPC feel like a natural fit. Excited to see where you take it.

[Pranit @fazxes]

@HARNESSROUTER Cool

[Pranit @fazxes]

@shafu0x @vercel niceee

[Pranit @fazxes]

@biraj21_ @vercel @shellular_dev 👀

[Pranit @fazxes]

@MarcelLuebben can you dm more details ?

[Pranit @fazxes]

fx v0.0.5:
▫️ 4.2% smaller (6.13mb)
▫️ Grok and Codex subscription support
▫️ Project-local skills
▫️ Improved security
▫️ Many bugfixes & QoL improvements

fx upgrade ⋅ curl -fsSL https://t.co/qR3nSdKCaP | bash
https://t.co/AFaqPCQkPK

[Pranit @fazxes]

@armandokirwin oh yea😉

[Pranit @fazxes]

Personal software era

[Guillermo Rauch @rauchg]

I used 𝚏𝚡 to build Mini, a personal web browser.

Many times I need to screen share, stream, or just test something without the 'bloat' and state of my browser. So I go Mini.

This was a cool test for https://t.co/OL0LzGtvAw and open models. It was the first time I experienced how cheap, but also how *good* these things can be.

'Personal software' is a hot topic. "I spent $25,000USD to cancel my $10 XYZ subscription" is a real thing 😂 (especially in Enterprises these days).

But I think open and local models will change this dynamic very profoundly. Software will really become spontaneous.

ps: shoutout to Orion browser who inspired me with 'focus mode'. I wanted my browser to be even more minimal, hackable, and run on Chromium.

[Pranit @fazxes]

@montonenico “Wow” moment

[monto @montonenico]

fx is so good

here it is running inside a Vercel function using just-bash as an in-memory fs 🤯

[Pranit @fazxes]

@alltheputs Dm’d you

[Pranit @fazxes]

@luongnv89 Rate limited, working to add more capacity

[Himanshu @codingstark]

vercel's coding agent fx, running inside a cloudflare worker 🤯

fx is wasm and needs jspi node hides it behind a flag, workerd just ships it on.

no child process. 36ms to a live tui streamed over a websocket.

https://t.co/J4gzlPaap9

[Pranit @fazxes]

@codingstark @CloudflareDev @vercel_dev Love this, more updates to wasm incoming…

[Pranit @fazxes]

@wazX_7 lmk how it goes

[Waz @wazX_7]

I wanted to try Ox Alpha in https://t.co/o6xROMMGow

I have been using https://t.co/o6xROMMGow from the day it was launched and I think it’s a great competitor to pi agent.

Really loved how fast it is and low less memory it consumes @fazxes

So I took some time and build /setup to run any OpenAI compatible endpoint with https://t.co/o6xROMMGow

Now I have connected fx with openrouter api key and started using Ox Alpha.

Not sure of performance yet excited to test it out today

[Pranit @fazxes]

@bakztfuture https://t.co/JL2fGZsMhn ships with both headless and TUI ( inline )

[Pranit @fazxes]

Possibilities are endless.

[biraj (shellular.dev) @biraj21_]

using @vercel's new coding agent https://t.co/QAdNUa3CRM from my phone using @shellular_dev!!

it's running on my hetzner vps.

i just installed and set it up in like 2 minutes while sitting in the bus :)

cc @rauchg @fazxes 👀

[Pranit @fazxes]

@maria_rcks 👀

[Pranit @fazxes]

@biraj21_ 🫡

[Pranit @fazxes]

@mjlbach @OpenAI @vercel 100% agreed I am cooking up something - will tag you on the pr 👀 lmk your thoughts