claude-code-system-prompts/system-prompts/skill-run-electron-desktop-gui-app-example.md

Example: Electron / desktop GUI app

Electron apps have a window. A future agent in a headless container can't see a window. So your deliverable here is not a markdown file that says "npm start opens a window" — it's a driver script that launches the app under xvfb, exposes a REPL of commands (click, type, screenshot), and lets an agent poke the UI by sending lines of text.

The skill's SKILL.md then becomes a short manual for that driver.

What you're building

apps/desktop/
  .claude/skills/run-desktop/
    SKILL.md               ← short. "run the driver, here are the commands"
    driver.mjs             ← REPL: stdin commands → Playwright actions

The driver IS the product. Without it, the skill describes a GUI an agent can never touch.

Graduation path: if the driver grows launch helpers the project's real e2e suite wants to share, move it to e2e-playwright/driver.mjs (or scripts/drive.mjs) and update the skill's paths. The skill stays at .claude/skills/run-desktop/; the driver finds a better home.

Step 1 — get the app to launch AT ALL under xvfb

This is usually the hardest part and produces most of the Gotchas. The README will say "macOS/Windows only." Ignore that. Install xvfb + the Chromium shared libs, find the Electron binary, and launch it:

apt-get install -y xvfb libnss3 libgbm1 libasound2t64 libgtk-3-0 \
  libxss1 libxkbcommon0 libatk-bridge2.0-0 libcups2 libdrm2

# Build the app first. Often the "dev" script is electron-forge which
# does a Vite/webpack build THEN launches. You want just the build:
npm install
npx electron-forge start &   # builds .vite/build/ or dist/
sleep 20 && kill %1          # kill it once built — you'll launch yourself

# Now try the raw launch
xvfb-run -a node -e "
  const { _electron } = require('playwright-core');
  _electron.launch({
    executablePath: './node_modules/electron/dist/electron',
    args: ['--no-sandbox', '.'],
    timeout: 30000,
  }).then(app => {
    console.log('launched, windows:', app.windows().map(w => w.url()));
    return app.close();
  });
"

Iterate until it launches. Each missing .so → one more apt-get package → one more line in Prerequisites. Each launch timeout → check the nodeCliInspect fuse isn't disabled, check the build output exists.

--no-sandbox is almost always needed in containers. Electron's sandbox needs CAP_SYS_ADMIN or user namespaces. Neither by default.

Step 2 — build the REPL driver

Once you can launch it, turn that throwaway script into a REPL. Start minimal — you will add commands as you need them. The REPL is the right shape because an agent can run it inside tmux and iterate without relaunching the (slow) app on every interaction.

// .claude/skills/run-<unit>/driver.mjs
// REPL driver for <app>. Run under xvfb on headless Linux.
// Designed for agents: wrap in tmux, send-keys commands, capture-pane output.
import { _electron as electron } from 'playwright-core';
import * as readline from 'node:readline';
import * as fs from 'node:fs';
import * as path from 'node:path';

const APP_DIR = path.resolve(import.meta.dirname, '../../..');
const SHOT_DIR = process.env.SCREENSHOT_DIR || '/tmp/shots';
fs.mkdirSync(SHOT_DIR, { recursive: true });

let app = null;
let page = null;   // the window/page you actually interact with

const electronBin = process.platform === 'darwin'
  ? path.join(APP_DIR, 'node_modules/electron/dist/Electron.app/Contents/MacOS/Electron')
  : path.join(APP_DIR, 'node_modules/electron/dist/electron');

const COMMANDS = {
  async launch() {
    if (app) return console.log('already launched');
    app = await electron.launch({
      executablePath: electronBin,
      args: ['--no-sandbox', APP_DIR],
      env: { ...process.env, DISPLAY: process.env.DISPLAY || ':99' },
      timeout: 30_000,
    });
    // Electron has no clean "loaded" signal — this sleep is a blind guess.
    // Replace with a poll once you know what ready looks like for this app:
    // wait until windows() includes the expected URL, or waitForSelector on firstWindow().
    await new Promise(r => setTimeout(r, 8_000));
    // Find the real UI page. Often NOT firstWindow() — may be a
    // splash screen, or the real content is in a BrowserView overlay.
    page = app.windows().find(w => !w.url().startsWith('devtools://'))
        ?? await app.firstWindow();
    console.log('launched.', app.windows().length, 'windows:');
    for (const w of app.windows()) console.log(' ', w.url());
  },

  async ss(name) {
    if (!page) return console.log('ERROR: launch first');
    const f = path.join(SHOT_DIR, (name || `ss-${Date.now()}`) + '.png');
    await page.screenshot({ path: f });
    console.log('screenshot:', f);
  },

  // Click via evaluate(), NOT locator.click(). If the content lives in a
  // BrowserView layered over the main window, Playwright's coordinate
  // math hits the wrong layer. DOM .click() always works.
  async click(sel) {
    if (!page) return console.log('ERROR: launch first');
    const r = await page.evaluate(s => {
      const el = document.querySelector(s);
      if (!el) return 'NOT_FOUND';
      el.click(); return 'OK';
    }, sel);
    console.log('click', sel, '→', r);
  },

  async 'click-text'(text) {
    if (!page) return console.log('ERROR: launch first');
    const r = await page.evaluate(t => {
      const els = [...document.querySelectorAll('button, a, [role="button"]')];
      const el = els.find(e => e.textContent?.trim() === t)
              ?? els.find(e => e.textContent?.includes(t));
      if (!el) return 'NOT_FOUND';
      el.click(); return 'OK: ' + el.tagName;
    }, text);
    console.log('click-text', JSON.stringify(text), '→', r);
  },

  async type(text)  { if (page) await page.keyboard.type(text, { delay: 30 }); },
  async press(key)  { if (page) await page.keyboard.press(key); },

  async wait(sel) {
    if (!page) return console.log('ERROR: launch first');
    try { await page.waitForSelector(sel, { timeout: 10_000 }); console.log('found:', sel); }
    catch { console.log('TIMEOUT:', sel); }
  },

  async eval(expr) {
    if (!page) return console.log('ERROR: launch first');
    try { console.log(JSON.stringify(await page.evaluate(expr))); }
    catch (e) { console.log('ERROR:', e.message); }
  },

  async text(sel) {
    if (!page) return console.log('ERROR: launch first');
    console.log(await page.evaluate(
      s => (s ? document.querySelector(s) : document.body)?.innerText ?? '(null)',
      sel || null));
  },

  // Introspection: essential for figuring out which window/webContents
  // actually has the UI. Electron apps often spawn several.
  async windows() {
    if (!app) return console.log('ERROR: launch first');
    for (const w of app.windows()) console.log(' ', w.url());
    const wcs = await app.evaluate(({ webContents }) =>
      webContents.getAllWebContents().map(w => ({ id: w.id, type: w.getType(), url: w.getURL() })));
    console.log('webContents:');
    for (const w of wcs) console.log(` [${w.id}] ${w.type}: ${w.url}`);
  },

  async quit() { if (app) await app.close().catch(()=>{}); app = null; page = null; },
  help() { console.log('commands:', Object.keys(COMMANDS).join(', ')); },
};

// Stop Electron from stealing stdin — use the raw fd.
const stdin = fs.createReadStream(null, { fd: fs.openSync('/dev/stdin', 'r') });
const rl = readline.createInterface({ input: stdin, output: process.stdout, prompt: 'driver> ' });

rl.on('line', async line => {
  const [cmd, ...rest] = line.trim().split(/\s+/);
  if (!cmd) return rl.prompt();
  const fn = COMMANDS[cmd];
  if (!fn) { console.log('unknown:', cmd, '— try: help'); return rl.prompt(); }
  try { await fn(rest.join(' ')); } catch (e) { console.log('ERROR:', e.message); }
  if (cmd === 'quit') { rl.close(); process.exit(0); }
  rl.prompt();
});
rl.on('close', async () => { await COMMANDS.quit(); process.exit(0); });

console.log('<app> driver — "help" for commands, "launch" to start');
rl.prompt();

This is a starting skeleton. As you try to reach interesting parts of the app you'll add app-specific commands: navigate to a particular view, focus a weird input type, bypass an auth gate, whatever. Those commands encode hard-won knowledge — keep them.

Step 3 — use it yourself, via tmux

Run the driver the same way the next agent will:

tmux new-session -d -s app -x 200 -y 50
tmux send-keys -t app 'cd /workspace/apps/desktop && xvfb-run -a node .claude/skills/run-desktop/driver.mjs' Enter
timeout 20 bash -c 'until tmux capture-pane -t app -p | grep -q "driver>"; do sleep 0.2; done'
tmux send-keys -t app 'launch' Enter
timeout 60 bash -c 'until tmux capture-pane -t app -p | grep -q "launched"; do sleep 0.2; done'
tmux send-keys -t app 'ss 01-landing' Enter
timeout 10 bash -c 'until tmux capture-pane -t app -p | grep -q "screenshot:"; do sleep 0.2; done'
tmux send-keys -t app 'windows' Enter    # which page has the real UI?
tmux capture-pane -t app -p

Then actually open /tmp/shots/01-landing.png. Is it the app? Is it blank? Is it a login screen? Each of these tells you what to do next.

Keep going — click into the main feature, fill a form, see the result show up, screenshot it. The driver grows whatever commands you need (focus-input, goto-settings, login-as-test-user…). When one real flow works end-to-end, you're done building and ready to write.

Step 4 — write SKILL.md

Keep it short. The driver is the meat; SKILL.md is the manual. Structure that works:

---

name: run-desktop description: Build, run, and drive the Electron desktop app. Use when asked to start the desktop app, take a screenshot of it, build it, or interact with its UI.

is an Electron desktop app. For agent/automated use, drive it via the Playwright REPL at .claude/skills/run-desktop/driver.mjs under xvfb. Launch is slow (~10s) and the interesting UI lives in a BrowserView, not the main window — the driver handles both.

All paths are relative to apps/desktop/.

Prerequisites

apt-get install -y xvfb libnss3 libgbm1 libasound2t64 libgtk-3-0 \
  libxss1 libxkbcommon0 libatk-bridge2.0-0 libcups2 libdrm2

Build

npm install
npx electron-forge start   # builds .vite/build/ — Ctrl-C once built
# <any patch you had to apply: sed a feature gate, etc.>

Run (agent path)

cd apps/desktop
xvfb-run -a node .claude/skills/run-desktop/driver.mjs

Wrap in tmux for interactive use:

tmux new-session -d -s app -x 200 -y 50
tmux send-keys -t app 'cd apps/desktop && xvfb-run -a node .claude/skills/run-desktop/driver.mjs' Enter
timeout 20 bash -c 'until tmux capture-pane -t app -p | grep -q "driver>"; do sleep 0.2; done'
tmux send-keys -t app 'launch' Enter
timeout 60 bash -c 'until tmux capture-pane -t app -p | grep -q "launched"; do sleep 0.2; done'
tmux send-keys -t app 'ss landing' Enter
tmux capture-pane -t app -p

Screenshots land in /tmp/shots/ (override: SCREENSHOT_DIR).

Commands

command	what it does
launch	launch the app, wait for windows
ss [name]	screenshot → /tmp/shots/<name>.png
click <css-sel>	click element (via DOM, not coords — see Gotchas)
click-text <text>	click button/link containing text
type <text> / press <key>	keyboard input
wait <css-sel>	wait for element, 10s timeout
eval <js>	evaluate in the page, print JSON
text [css-sel]	print innerText
windows	list all windows + webContents (find the real UI)
quit	close app, exit

Plus any app-specific commands you built: <your-command> — .

Run (human path)

npm start   # opens a window; useless headless. Ctrl-C to quit.

Gotchas

• — → <fix/workaround>
• <etc. — only things you actually hit, not generic advice>

Troubleshooting

• Launch timeout (30s): build output missing? → re-run the build step. nodeCliInspect fuse disabled? → Playwright can't attach; don't disable that fuse in dev builds.
• "Missing X server": forgot xvfb-run. Headless Linux needs it.
• Stale Xvfb locks: rm -f /tmp/.X*-lock; pkill Xvfb

Obstacles you will hit (and they go in Gotchas)

These are real patterns from real Electron apps. You'll hit some subset:

• firstWindow() gives you a splash/loading screen, not the app. Wait longer, or find the right page by URL, or wait for a specific selector that only appears when the app is actually ready.

• The real UI is in a BrowserView, not a BrowserWindow. Playwright sees it as a separate "window" with a different URL. The windows command exists exactly for figuring this out. getBrowserViews() may also return empty on newer Electron — use webContents.getAllWebContents() instead.

• locator.click() clicks the wrong thing. Playwright computes click coordinates relative to the main window. If your content is in a BrowserView overlay, those coordinates hit the window behind it. The driver skeleton uses page.evaluate(el => el.click()) for this reason — DOM click bypasses coordinates entirely.

• Feature gates block the thing you need to test. The app checks a plan tier, or an env flag, or a feature flag baked into SSR HTML. Find where the check happens (grep the built output for the gate name) and patch it for your local run — a sed on the build output, an env var override, or (for SSR-embedded flags) intercept the response via CDP Fetch.enable and rewrite it in-flight. Document exactly what you patched and why.

• contentEditable inputs (ProseMirror, Tiptap, Slate) aren't <textarea>. fill() won't work. Focus the element, then use keyboard.type(). Add a focus <sel> command if the app has these.

• Electron steals stdin. The fs.openSync('/dev/stdin', 'r') + createReadStream trick in the skeleton protects your REPL's input.

• Native modules fail to load (keychain, notifications, etc.). Usually non-fatal — the core app runs, those features no-op. Note it and move on.