Minimal CLI harness
fx is a minimal CLI and agent runtime written in Zig, with a small core and a shell-like interface rather than a heavy terminal TUI.
What fx is
fx is a tiny, open coding agent and CLI written in Zig. It is positioned as a minimal agent harness for developers who want a compact, embeddable tool rather than a terminal-heavy IDE experience.
The product can run natively in a shell, act as an ACP server for editors and applications, or load from JavaScript through a WebAssembly build. The documentation frames it as suitable for research, resource-constrained environments, and larger systems that need a configurable agent runtime.
Features
Low-overhead execution
The product is designed for fast startup and low overhead, with documentation describing instant time to prompt, no unnecessary work before input, and a small binary footprint.
Multiple runtime modes
fx can run natively in a terminal, serve as an ACP server for editors and other clients, or load as WebAssembly through the JavaScript SDK.
Configurable browser integration
The docs describe browser adapters for storage, authentication, command execution, and network requests, with separate adapters for terminal and agent use.
Provider flexibility
fx is model-agnostic and can work with local models, gateways, direct provider APIs, or subscriptions.
Extensible and embeddable
The runtime is built to be extended through skills, plugins, and MCPs, and the docs emphasize embeddability as part of larger systems.
Common ways to use fx
Terminal coding agent
Run fx in a project directory as a terminal-based coding agent when you want a small CLI that treats the current folder as the main workspace.
Editor and client integration
Connect fx through Agent Client Protocol when building or using an editor that wants an agent running as a native process over standard input and output.
JavaScript and browser embedding
Load the WebAssembly build from JavaScript when you need an embeddable agent runtime or an interactive terminal inside a browser or app.
Custom browser workflows
Use the browser adapter model to persist session state, restore config, complete device login, or proxy network requests in a custom app.
Research and resource-constrained deployments
Choose fx when you need a compact, extensible harness for research, sandboxed environments, or systems that need multiple instances with modest resource use.
Pros and Cons
Pros
- Open source under Apache-2.0.
- Small binary footprint and low baseline memory use are emphasized in the product copy.
- Supports native CLI use, ACP-based editor and app integration, and WebAssembly loading from JavaScript.
- Model and provider agnostic, with support for local models, gateways, direct APIs, or subscriptions.
- Docs describe browser adapters that let apps control storage, auth, command execution, and networking.
Cons
- The installation docs note that the release archive is not signature-checked or verified by a published checksum.
- The product is described as experimental in the rendered home page text, so users should expect frequent changes.
FAQ
What platforms does fx support?
fx supports macOS and Linux on x86_64 and arm64. The installation guide also notes that the installer needs curl or wget and tar.
Is fx open source?
Yes. The docs describe fx as open source under the Apache-2.0 license.
How can fx be used?
You can run the native CLI in a terminal, connect an editor or another application over Agent Client Protocol (ACP), or load the WebAssembly build from JavaScript.
How is fx installed safely?
The installation docs say the installer downloads a release archive over HTTPS from Vercel's release storage and does not check a signature or published checksum. They recommend reviewing the script and verifying artifacts yourself if your policy requires an audited install.
Does fx require a specific model provider?
The docs describe fx as model-agnostic and suitable for local models, gateways, direct provider API access, or subscriptions.
Quick Facts
- Category
- Developer Tool
- License
- Apache-2.0
- Primary interface
- CLI
- Language
- Zig
- Supported runtime
- Native process, ACP, WebAssembly
- Website
- fx.sh
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