fx: A 6MB coding agent that starts in 10 microseconds
fx :Tiny, open, native coding agent.
fx is a tiny, open-source coding agent harness and CLI written in Zig, designed for research and embeddability. Its binary is just 6.39MiB, cold-starts in 10µs, and runs as WebAssembly via the Zig toolchain. With a minimal memory footprint and a shell-like UI, fx is context-efficient and extensible through skills, plugins, and MCPs. It's model-agnostic, suitable for local or cloud inference, and ideal for resource-constrained environments.
fx cold starts in 10µs and does no unnecessary work or I/O prior to accepting user input, making it ideal for programmatic use.
- rsyring
For all the people asking "Why?", it seems like TFA has a pretty good list of features/attributes that it thinks sets it apart:
- fx is a coding agent harness and CLI written in Zig, optimized for research and embeddability as part of larger systems.
- It focuses on minimalism and performance across the board, from system prompt design, to its tools, feature set, and 6.39mib binary.
- For end users, its CLI output style and form factor aims to be closer to a Unix shell than a heavy "IDE in the terminal" TUI.
- It's open source (Apache-2.0), model-agnostic, and suitable for both local and cloud inference.
- Designed for instant installation and embedding in resource constrained environments and agent sandboxes.
- fx cold starts in 10µs and does no unnecessary work or I/O prior to accepting user input, making it ideal for programmatic use.
- Optimal fx.wasm builds produced by the Zig toolchain, which further reduce fx's size, making the network stack pluggable.
- fx contributes single-digit megabytes of memory baseline, allowing you to pack many instances in one machine.
- fx preserves scroll history by default, produces minimal output, and makes sparing use of complex TUI or paints
- Minimal system prompt and tools, to save on token costs and to yield optimal time-to-first-token performance (TTFT).
- Small core, extended via skills, plugins, MCPs, with a Unix-like philosophy to extensibility.
- Designed to work with local models, gateways, direct provider API access or subscri […]
- bodge5000
It does looks really interesting and definitely something I'll check out, but (genuine question), should "agent" and "agent harness" be used interchangeably as it is on here? It describes itself as an agent harness, but the tagline is "tiny, open, native coding agent".
I'm not sure harness is the right word either, but that seems to be what the industry has settled on so I'll concede on that, but surely the agent is the thing doing the work (which I guess is the model, or an instance of the model which is why agent is different?), whereas the harness is how the user interacts with the agent. We've had ways to describe that relationship before; client and server, frontend and backend, but again, I'll concede that the shiny new thing doesn't want to use boring old terminology, but I think some consistency and logic in the shiny new terminology is pretty important
That isn't specifically about fx of course, more of a general industry complaint
- kgeist
>Tiny ~6mb binary
I wonder why it's so large for a program written in Zig. It's basically just a loop that accepts user input, prepares the context, sends it to the LLM, parses the output, invokes the tools, and presents it all in the terminal. Add the built-in prompts and a few checks here and there (like blocking a write tool call before the file has been read first), and I'd expect a truly tiny native agent to be around 200-300 KB max.
- SmashDan
I'm not in the tech industry. Could someone explain why there are so many new coding agents, and why they're commonly upvoted on HackerNews? It seems like there's a new one in the top 10 every other day.
- vhantz
I wonder how long will the "curl my arbitrary script and pipe it to bash" will continue being a delivery method.