# 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: ```bash 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. ```javascript // .claude/skills/run-/driver.mjs // REPL driver for . 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(' 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: ```bash 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 > > ```bash > apt-get install -y xvfb libnss3 libgbm1 libasound2t64 libgtk-3-0 \ > libxss1 libxkbcommon0 libatk-bridge2.0-0 libcups2 libdrm2 > ``` > > ## Build > > ```bash > npm install > npx electron-forge start # builds .vite/build/ — Ctrl-C once built > # > ``` > > ## Run (agent path) > > ```bash > cd apps/desktop > xvfb-run -a node .claude/skills/run-desktop/driver.mjs > ``` > > Wrap in tmux for interactive use: > > ```bash > 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/.png` | > | `click ` | click element (via DOM, not coords — see Gotchas) | > | `click-text ` | click button/link containing text | > | `type ` / `press ` | keyboard input | > | `wait ` | wait for element, 10s timeout | > | `eval ` | 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: `` — . > > ## Run (human path) > > ```bash > npm start # opens a window; useless headless. Ctrl-C to quit. > ``` > > ## Gotchas > > - **** — → > - > > ## 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 ``. `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.