**fx** is Vercel Labs' coding-agent harness and CLI, written in **Zig** and released under **Apache-2.0**. It went public on **11 August 2026** and has collected **1,951 stars and 198 forks** in eleven days. The project labels itself **experimental**, and the version numbers back that up: five releases between 17 and 21 August, still at **v0.0.5**. The design goal is stated plainly: minimalism and performance from the system prompt outward, landing in a **7.8 MiB binary**. Where most terminal agents have converged on a full-screen TUI, fx deliberately goes the other way — its output style and form factor aim to sit closer to a Unix shell than to an "IDE in the terminal". `fx ask "explain the changes in this repository"` is a single request; `fx` on its own opens an interactive session rooted at the current directory; `fx sessions` lists saved work and `fx session resume last` picks it back up. JSON and quiet requests stay noninteractive by default, with prompt text written to stderr so JSON stdout stays parseable. Authentication is unusually pluralistic. **`fx login`** signs in through the Vercel AI Gateway, but `fx login codex` and `fx login grok` attach an eligible **ChatGPT** or **Grok** subscription over OAuth instead. The project is explicit that neither of those OAuth tokens is sent to the Gateway: the Codex session lives at `~/.fx/chatgpt-auth.json` and the Grok session at `~/.fx/grok-auth.json`, each used only against its own authenticated catalog. `/setup` switches providers, `/model` lists the active provider's fetched models, and `/logout codex` drops one subscription without touching the others. Embeddability is the part that separates fx from the rest of the field. It builds as a native binary **or WebAssembly**, and a host application supplies network transport, session storage, configuration, permission handling and terminal I/O. Three surfaces are exposed: **`fx acp`** connects the native agent to editors and other Agent Client Protocol clients, **`createFxAgent()`** embeds the agent core in a JavaScript host via `fx-core.wasm`, and **`createFxTerminal()`** embeds the interactive terminal via `fx-term.wasm`. The WebAssembly SDK is flagged experimental. On top of that, the agent extends through **skills**, **MCP** servers and **subagents**, with fx-native project skills discovered from `.fx/skills`. The permission model is more considered than the version number suggests. fx starts in **`auto`** mode: routine, understood development actions run directly, while unresolved sensitive ones get one bounded automatic review, and a blocked action can return an exact approval request that the agent sends to the real permission screen. Ordinary question text never grants permission. Inside a saved session, `/permissions remember <allow|deny> <tool-name> <arguments-json>` stores an exact confirmed rule without running the action, and `/permissions revoke <rule-id>` removes it even after the workspace or file state has changed. Release **v0.0.5** on **21 August** added the Codex and Grok subscription routes and `.fx/skills` discovery, but also shipped two breaking changes: approved captured, background and monitor commands now run as **ordinary host subprocesses** with sandbox configuration, status fields and commands retired, and the `/provider` slash command was folded into `/setup`. That sandbox removal is the thing to weigh. Combined with an `auto` default and a self-declared experimental status, fx asks for more trust in its permission logic than a harness at v0.0.5 has had time to earn. Building from source needs **Zig 0.16.0+**, which is still a moving target of its own.