---
title: VS Code Extension (Beta)
summary: Author fy-app.json with FSM validation and a live state-machine preview, right in VS Code. Download the beta build.
order: 1
---

# VS Code Extension

The FastYoke VS Code extension makes `fy-app.json` — the source-of-truth
spec for your app's entities and workflows — safe and legible to author,
without leaving your editor.

::callout{type="info" title="Beta"}
This is an early beta, distributed as a downloadable `.vsix`. It isn't on the
VS Code Marketplace yet — install it manually with the steps below. We'll
publish to the Marketplace and Open VSX once it's out of beta.
::

## What it does

- **Structural IntelliSense** — key and enum completion, hover, and
  validation for `fy-app.json`.
- **Semantic workflow checks** — the state-machine mistakes a plain JSON
  validator can't catch: a transition pointing at a state that isn't
  declared (with a "did you mean…" quick-fix), unreachable states, duplicate
  `(from, event)` pairs, a missing terminal state, and more. The rules are
  shared with the [`fy` CLI](/docs/cli), so the editor and the terminal never
  disagree.
- **Live FSM preview** — run **FastYoke: Preview FSM** (or click the icon in
  the editor-title bar of a `fy-app.json`) to open a state-machine diagram
  beside your spec. It redraws as you type, an entity picker switches between
  workflows, and clicking a state jumps your cursor to its definition.

Everything runs locally — the preview is a self-contained diagram with no
network calls and no telemetry.

## Install the beta

1. **[Download the beta (.vsix)](/vscode/fastyoke-vscode.vsix){download="fastyoke-vscode.vsix"}** — the current
   build is `0.1.0`.
2. Install it — either:
   - In VS Code, open the **Extensions** view → the **⋯** (Views and More
     Actions) menu → **Install from VSIX…** → choose the downloaded file; or
   - from a terminal:
     ```bash
     code --install-extension fastyoke-vscode.vsix
     ```
3. Open any `fy-app.json` and the extension activates automatically.

Editors built on VS Code — **Cursor, VSCodium, Windsurf** — install the same
`.vsix` the same way.

## See also

- [CLI](/docs/cli) — `fy`, which scaffolds and reads the same `fy-app.json`.
- [`fy-app.json` spec](/docs/cli/app-spec) — the file this extension validates.