fx debuts as a 6MB coding agent CLI written in Zig

fx is a coding agent harness and command-line tool written in Zig, published as open source under the Apache-2.0 license at fx.sh. The current release is v0.0.4, and the project's own status line marks it experimental, warning that it will be making frequent changes and that people should use it at their own risk.

The project is built around minimalism and speed rather than a large feature set. The whole binary measures 6.39 MiB, rounded to about 6MB in the page's own marketing copy, and fx starts up in about 10 microseconds, doing no unnecessary work or I/O before it is ready to accept input, according to the page. Its memory baseline sits in the single-digit megabytes, which the makers say lets many instances run on one machine. The system prompt and tool set are kept minimal by design, meant to cut token costs and help a model start responding faster.

Rather than a full-screen terminal interface, fx aims for something closer to a Unix shell: it keeps scroll history by default, produces minimal output, and avoids heavy interface elements. It is model and provider agnostic, designed to work with local models, gateways, direct provider API access or subscriptions, so it is not tied to one vendor's model or hosting. A companion fx.wasm build, produced through the Zig toolchain, shrinks the tool further and makes its networking layer swappable. Extensibility is meant to come from outside the small core, through skills, plugins and MCP connections, following what the page calls a Unix-like philosophy.

The product page's own terminal screenshot shows fx running in auto mode with a model listed as glm-5.2, though the page does not say whether that is fx's default model or simply what was configured for the screenshot. No maintainer, team or company is named anywhere in the source, and no pricing or license cost is mentioned beyond the Apache-2.0 grant itself. All of the performance numbers, the binary size, the cold start time, and the memory baseline, are the project's own stated figures, with no benchmark or method given for any of them.

Key facts

fx is an open source (Apache-2.0) coding agent CLI and harness written in Zig, currently at v0.0.4 and labeled experimental by its own makers.
The binary measures 6.39 MiB, rounded to about 6MB in the page's own marketing copy.
fx claims a cold start of about 10 microseconds and a memory baseline in the single-digit megabytes.
It is model and provider agnostic, working with local models, gateways, direct provider API access or subscriptions, and ships with fx.wasm builds made through the Zig toolchain.
The page's own demo screenshot shows fx running in auto mode with a model called glm-5.2, though it does not say whether that is the default.

Why it matters

fx bets specifically on being small and fast rather than feature rich: a 6.39 MiB binary, a cold start of about 10 microseconds, and a memory baseline in the single-digit megabytes. By its own account, that is meant to make it embeddable inside larger systems, research pipelines and agent sandboxes, and to let many instances run on a single machine, rather than serving mainly as a standalone assistant for one developer. It is also explicit about the interface tradeoff behind that choice: rather than build a heavy "IDE in the terminal" experience, it aims for something closer to a plain Unix shell, with minimal output and a preserved scroll history.

Who it affects

By the page's own framing, fx is aimed at people building agent infrastructure, not at someone who wants a full, IDE-like terminal experience: researchers who need to run many agent instances at once, and teams embedding a coding agent as one component inside a larger automated system or a sandboxed environment. Because it is model and provider agnostic, teams already committed to a particular model host, whether local models, a gateway, a direct provider API, or a subscription, are not required to switch to use it. The same audience is served by its approach to extensibility: skills, plugins and MCP connections sit outside a small core, for people who want to build on top of fx rather than wait for new features to land in the core itself.

How to use it

fx is open source under the Apache-2.0 license and available from fx.sh, currently at version v0.0.4. Its own status line calls it experimental and warns of frequent changes, so anyone adopting it now should expect the interface and behavior to keep shifting. It works with local models, gateways, direct provider API access or subscriptions, and ships with fx.wasm builds produced through the Zig toolchain, which the page says shrink it further and make its networking layer swappable. The page's own terminal screenshot shows fx running in auto mode with a model listed as glm-5.2, though it does not say whether that is fx's default or just what was set up for the screenshot. No pricing is mentioned anywhere in the source; beyond the Apache-2.0 grant, the page says nothing about cost.

How solid is it

Every number here, the 6.39 MiB binary size, the 10-microsecond cold start, the single-digit-megabyte memory baseline, comes from fx's own product page rather than an independent benchmark, and the page gives no methodology for how any of them was measured. The software itself is at v0.0.4, and its makers label it experimental in their own status line. The source names no maintainer, team or company behind the project, so there is no way from this page alone to judge who is building it or how established the effort is.

Risks and caveats

An experimental v0.0.4 label paired with a stated policy of frequent changes means anyone embedding fx as a dependency inside another system should expect its interface and behavior to shift without much notice. The size, speed and memory figures are self-reported, with no outside benchmark to check them against. The page names no pricing or license cost beyond the Apache-2.0 grant, so anyone who needs commercial support or a service agreement has nothing here to go on.

“fx is a coding agent harness and CLI written in Zig, optimized for research and embeddability as part of larger systems.”

— fx, product page (fx.sh)