Crxjs Agent Skill

CRXJS

CRXJS is a Chrome extension development tool that provides true HMR for popup, options, content scripts, and side panels. It reads your manifest to auto-generate the extension output, handles content script injection, and manages the service worker build. Under the hood it is a Vite plugin (@crxjs/vite-plugin).

Current status

• Package: @crxjs/vite-plugin (v2.x stable, latest v2.4.0 as of March 2026)
• Scaffolding: npm create crxjs@latest (always use @latest)
• Maintained by: @Toumash and @FliPPeDround (since mid-2025)
• GitHub: github.com/crxjs/chrome-extension-tools (~4k stars)
• Vite compatibility: v3 through v8-beta

Quick start

# Scaffold new project (picks framework interactively)
npm create crxjs@latest

# Or add to existing Vite project
npm install @crxjs/vite-plugin -D

Vite config by framework

CRXJS is added as a Vite plugin. The setup varies slightly per framework.

React

// vite.config.ts
import { defineConfig } from "vite";
import react from "@vitejs/plugin-react";
import { crx } from "@crxjs/vite-plugin";
import manifest from "./manifest.json";

export default defineConfig({
  plugins: [react(), crx({ manifest })],
});

Use @vitejs/plugin-react (not plugin-react-swc) for best HMR compatibility. If you must use SWC, cast the manifest:

import { ManifestV3Export } from "@crxjs/vite-plugin";
const manifest = manifestJson as ManifestV3Export;

Vue

import vue from "@vitejs/plugin-vue";
import { crx } from "@crxjs/vite-plugin";
import manifest from "./manifest.json";

export default defineConfig({
  plugins: [vue(), crx({ manifest })],
});

Svelte

import { svelte } from "@sveltejs/vite-plugin-svelte";
import { crx } from "@crxjs/vite-plugin";
import manifest from "./manifest.json";

export default defineConfig({
  plugins: [svelte(), crx({ manifest })],
});

Vanilla TypeScript

import { crx } from "@crxjs/vite-plugin";
import manifest from "./manifest.json";

export default defineConfig({
  plugins: [crx({ manifest })],
});

defineManifest — type-safe dynamic manifest

Instead of a static JSON file, use CRXJS's defineManifest for dynamic values and full TypeScript autocompletion:

// manifest.ts
import { defineManifest } from "@crxjs/vite-plugin";
import pkg from "./package.json";

export default defineManifest((config) => ({
  manifest_version: 3,
  name: config.command === "serve" ? `[DEV] ${pkg.name}` : pkg.name,
  version: pkg.version,
  description: pkg.description,
  permissions: ["storage", "activeTab", "scripting"],
  action: {
    default_popup: "src/popup/index.html",
    default_icon: {
      "16": "public/icons/icon16.png",
      "48": "public/icons/icon48.png",
    },
  },
  background: {
    service_worker: "src/background/index.ts",
    type: "module",
  },
  content_scripts: [
    {
      matches: ["https://*/*"],
      js: ["src/content/index.ts"],
      css: ["src/content/styles.css"],
    },
  ],
  options_page: "src/options/index.html",
  side_panel: { default_path: "src/sidepanel/index.html" },
  icons: {
    "16": "public/icons/icon16.png",
    "48": "public/icons/icon48.png",
    "128": "public/icons/icon128.png",
  },
}));

Import in vite.config.ts:

import manifest from "./manifest";
// ... crx({ manifest })

Type declarations

Add to a src/vite-env.d.ts or src/crxjs.d.ts:

/// <reference types="@crxjs/vite-plugin/client" />

This enables types for ?script and ?script&module imports.

HMR behavior by context

| Context | HMR | How it works |

| --- | --- | --- |

| Popup | Full HMR | WebSocket-based, state preserved |

| Options page | Full HMR | Same as popup |

| Side panel | Full HMR | Same as popup |

| Content script (manifest) | True HMR | CRXJS injects loader + HMR client |

| Content script (dynamic) | True HMR | Via ?script import |

| Service worker | Auto-reload | Changes trigger full extension reload |

| Main world scripts | No HMR | Skipped by CRXJS loader |

Content script HMR works because CRXJS generates a loader script that imports an HMR preamble, the HMR client, and your actual script — enabling real module-level HMR without full page reload. This is CRXJS's main differentiator.

Dynamic content script imports

For content scripts injected programmatically (not in manifest), CRXJS provides special import suffixes:

// background.ts — ?script gives you a resolved path for executeScript
import contentScript from "./content?script";

chrome.action.onClicked.addListener(async (tab) => {
  await chrome.scripting.executeScript({
    target: { tabId: tab.id! },
    files: [contentScript],
  });
});

For main world injection (no HMR):

import mainWorldScript from "./inject?script&module";

await chrome.scripting.executeScript({
  target: { tabId },
  world: "MAIN",
  files: [mainWorldScript],
});

CRXJS plugin options

crx({
  manifest,
  browser: "chrome", // 'chrome' | 'firefox'
  contentScripts: {
    injectCss: true, // auto-inject CSS for content scripts
    hmrTimeout: 5000, // HMR connection timeout (ms)
  },
});

Development workflow

# Start dev server (outputs to dist/ with HMR)
npm run dev

# 1. Open chrome://extensions
# 2. Enable "Developer mode"
# 3. Click "Load unpacked"
# 4. Select the dist/ directory
# 5. Edit code — popup/content scripts update instantly via HMR
# 6. Service worker changes trigger automatic extension reload

After loading once, subsequent npm run dev sessions reconnect automatically. No need to re-load the extension unless manifest.json changes.

Production build

npm run build    # outputs to dist/

The dist/ directory is ready to zip and upload to Chrome Web Store:

cd dist && zip -r ../extension.zip .

Disable Vite's module preload to avoid CWS rejection of inline scripts:

build: {
  modulePreload: false;
}

Known issues and workarounds

Tailwind CSS HMR in content scripts

New Tailwind classes may not trigger CSS updates in content scripts. Workaround: restart dev server after adding new utility classes. Improved in v2.4.0 but not fully resolved. Ensure injectCss: true in config.

WebSocket connection errors (ws://localhost:undefined/)

Cause: port mismatch between dev server and HMR config. Fix: explicitly set both to the same value:

server: {
  port: 5173,
  strictPort: true,
  hmr: { port: 5173 },
}

"Manifest version 2 is deprecated" warning

If you see this, your manifest is being interpreted as MV2. Fix: ensure "manifest_version": 3 is set.

Content scripts not injecting on file:// URLs

Chrome requires the user to enable "Allow access to file URLs" in the extension settings at chrome://extensions. CRXJS cannot change this.

HMR stops working after Chrome update

CRXJS's HMR relies on injecting a content script that connects to the dev server's WebSocket. Chrome security updates occasionally break this. Fix: update to the latest CRXJS version, which tracks Chrome changes.

CRXJS vs alternatives

| Feature | CRXJS | WXT | Plasmo |

| --- | --- | --- | --- |

| Content script HMR | True HMR | File-based reload | Partial |

| Framework support | Any Vite framework | Any | React-focused |

| Abstraction level | Thin (Vite plugin) | Full framework | Full framework |

| Messaging helpers | None (use chrome.\* directly) | Built-in | Built-in |

| Storage wrappers | None | Built-in | Built-in |

| Cross-browser | Chrome + Firefox | Chrome + Firefox + Safari | Chrome + Firefox |

| File-based routing | No | Yes | Yes |

| Learning curve | Low (know Vite, know CRXJS) | Medium | Medium |

Choose CRXJS when: you want minimal abstraction over raw Chrome APIs and value content script HMR above all. CRXJS stays out of the way — no magic routing, no wrapper APIs, just your code with HMR.

Choose WXT when: you want conventions, built-in utilities, and cross-browser support.

Choose Plasmo when: you're React-focused and want the highest-level abstraction.

Project structure (recommended)

my-extension/
├── src/
│   ├── background/
│   │   └── index.ts
│   ├── content/
│   │   ├── index.ts
│   │   └── styles.css
│   ├── popup/
│   │   ├── index.html        <- CRXJS resolves HTML entry points
│   │   ├── App.tsx
│   │   └── main.tsx
│   ├── options/
│   │   ├── index.html
│   │   └── main.tsx
│   ├── sidepanel/
│   │   ├── index.html
│   │   └── main.tsx
│   └── shared/
│       ├── messages.ts
│       └── storage.ts
├── public/
│   └── icons/
├── manifest.ts               <- or manifest.json
├── vite.config.ts
├── tsconfig.json
└── package.json

CRXJS resolves HTML files referenced in the manifest automatically. Your popup.html can use standard <script type="module" src="./main.tsx"> and it works.

If you encounter a bug or unexpected behavior in CRXJS, open an issue at github.com/crxjs/chrome-extension-tools/issues.