Drive Chrome via the DevTools Protocol from JavaScript. Run JS snippets through the `browser-harness-js` CLI — it auto-spawns a long-lived bun HTTP server holding a fully-typed CDP `Session`, and every call (`browser-harness-js 'await session.Page.navigate(...)'`) executes against the same persisten

CDP — browser harness js skill Custom codegen'd CDP SDK (every method from browser protocol.json + js protocol.json gets a typed wrapper) plus a tiny HTTP server that holds one persistent CDP Session . The browser harness js CLI auto starts the server on first use and forwards JS snippets to it. The SDK lives in the skill's sdk/ directory. In the rest of this doc, <skill dir refers to wherever npx skills add installed the skill (Claude Code: ~/.claude/skills/cdp ; Cursor: ~/.cursor/skills/cdp ; other agents vary). The CLI should be on PATH as browser harness js . Setup (once, first use) npx skills add drops the skill into your agent's skills directory but does NOT put the CLI on PATH. Before the first call, verify it's reachable and symlink it into any directory on your PATH if not: The CLI auto installs bun on first run if it's missing (the server is Bun native). Set BROWSER HARNESS SKIP BUN INSTALL=1 to opt out. How to use Just run browser harness js '<JS ' . The first call spawns the server in the background; subsequent calls hit the same process and so reuse the same session , the same WebSocket to Chrome, and any globals you set. Output is the raw result content — no {ok,result} envelope. Result type stdout string bare text, no JSON quotes (e.g. Example Domain ) number / boolean 42 , true object / array (non empty) compact JSON (e.g. {"frameId":"..."} , [1,2,3] ) undefined / null / "" / {} / [] empty (no output) Errors go to stderr , exit code 1 . The CDP error message and JS stack are printed verbatim, e.g.: Detect failure with if browser harness js '...'; then ...; else handle error; fi or by checking $? . Multi line snippets via stdin (heredoc). Important: a multi statement snippet does NOT auto return the last expression — write return X explicitly. Single expression snippets passed as the first argument DO auto return. CLI commands Command Behavior browser harness js '<js ' Auto start server if needed, eval the JS, print result. browser harness js <<EOF…EOF Same, code from stdin. browser harness js status Print health JSON (uptime, connected, sessionId) or exit 1 if down. browser harness js start Explicit start (no op if already running). browser harness js stop Graceful shutdown. Drops session state. browser harness js restart Stop + start fresh. browser harness js logs tail f the server log ( /tmp/browser harness js.log ). Env vars: CDP REPL PORT (default 9876 ), CDP REPL LOG (default /tmp/browser harness js.log ). API surface inside snippets These globals are pre loaded — no imports needed: session — the persistent Session . Has every CDP domain mounted: session.Page , session.DOM , session.Runtime , session.Network , … 56 domains, 652 methods total. listPageTargets() — list real page targets via CDP's Target.getTargets (works on Chrome 144+ too), with chrome:// and devtools:// URLs filtered out. No args — uses the connected session. detectBrowsers() — scan OS specific profile dirs for running Chromium based browsers with remote debugging on. Returns [{name, profileDir, port, wsPath, wsUrl, mtimeMs}] , sorted by most recently launched. resolveWsUrl(opts) — resolve a WS URL from {wsUrl} {port, host?} {profileDir} . For the no args auto detect flow, call session.connect() directly instead. CDP — the generated namespaces ( CDP.Page , CDP.Runtime , …) for type name reference. Calling a CDP method Every method takes a single object argument matching the CDP wire params; it resolves to the typed return value (no result envelope, no id correlation — handled for you). Connecting Default: just call session.connect() with no args. It auto detects running Chromium based browsers (Chrome, Chromium, Edge, Brave, Arc, Vivaldi, Opera, Comet, Canary) by scanning OS specific profile dirs for a DevToolsActivePort file, ordered by most recently launched, and picks the first one whose WebSocket accepts. OS agnostic — works on macOS, Linux, Windows. Use detectBrowsers() first if you want to see what's available (or let the user pick) before connecting: Explicit forms — use these only when auto detect picks the wrong browser, or when you already know where to connect: Form When to use { profileDir } Target a specific browser when several are running. Reads <profileDir /DevToolsActivePort directly. { wsUrl } You already have ws://…/devtools/browser/<uuid (e.g. piped from elsewhere). Profile paths by OS — use these with { profileDir } : macOS: ~/Library/Application Support/<Browser (e.g. Google/Chrome , Comet , BraveSoftware/Brave Browser , Arc/User Data ) Linux: ~/.config/<browser (e.g. google chrome , chromium , BraveSoftware/Brave Browser ) Windows: %LOCALAPPDATA%\<Browser \User Data (e.g. Google\Chrome , Microsoft\Edge , BraveSoftware\Brave Browser ) Per candidate WS open timeout defaults to 5s — live browsers answer with open/close within ~100ms, so 5s is already generous. The only case where 5s is too short is when Chrome is showing the Allow popup and waiting on the user to click. If you expect that, pass timeoutMs: 30000 : If you see No detected browser accepted a connection — the browsers have DevToolsActivePort files but none are currently serving WS. Most common cause: remote debugging is enabled but the user hasn't clicked Allow on the prompt yet. Tell them to click Allow, then retry (or bump timeoutMs ). Picking a target (tab) After connect() , call session.use(targetId) once; subsequent page level calls (Page/DOM/Runtime/Network/etc.) auto route to that target's sessionId. Browser. and Target. calls always hit the browser endpoint. listPageTargets() uses CDP's Target.getTargets (not /json ), so it works on Chrome 144+ too. It already filters out chrome:// and devtools:// URLs. Equivalent raw call: To switch tabs: session.use(otherTargetId) . To detach: session.setActiveSession(undefined) . Events Persisting state across calls Each snippet runs inside its own async wrapper, so its let / const declarations vanish when it returns. To carry data forward, attach to globalThis : session itself, the active sessionId, and event subscribers are already preserved by the server — globals are only needed for ad hoc data. Connecting to a running Chrome (chrome://inspect flow) When attaching to the user's already running browser: 1. Try await session.connect() first (no args) — auto detect handles every Chromium based browser via DevToolsActivePort . If it returns, you're done. 2. If auto detect fails with No running browser with remote debugging detected , the user needs to turn it on. Open the inspect page: Only macOS's AppleScript path avoids the profile picker; Linux/Windows may prompt the user to pick a profile first. 3. Tick "Discover network targets" in chrome://inspect, then click Allow when Chrome prompts. 4. If auto detect picks the wrong browser (multiple running, you want a specific one): list them with await detectBrowsers() , then await session.connect({ profileDir: <the one you want }) . 5. If session.connect() returns No detected browser accepted a connection , the user has remote debugging on but hasn't clicked Allow yet. Tell them to click it and retry, or pass timeoutMs: 30000 to wait for the click. Working with targets (tabs) Filter Chrome internals. listPageTargets() already drops chrome:// and devtools:// URLs. If you call Target.getTargets() directly, filter manually. CDP target order ≠ visible tab strip order. When the user says "the first tab I can see", use a screenshot or page title to identify it — Target.activateTarget only switches to a known targetId. Looking up a method The full typed surface is in <skill dir /sdk/generated.ts (~655 KB, only loaded if you read it). Each method has its CDP description as a JSDoc comment plus typed Params / Return interfaces in per domain namespaces. Regenerating the SDK When the upstream protocol JSONs change, replace sdk/browser protocol.json and/or sdk/js protocol.json and re run: Files All paths are relative to <skill dir (the install path — see top of this doc). /usr/local/bin/browser harness js → <skill dir /sdk/browser harness js (the CLI) sdk/repl.ts — HTTP server ( Bun.serve on 127.0.0.1:9876 ) sdk/session.ts — Session class (transport, connect, target routing, events) sdk/generated.ts — codegen output: every CDP method as a typed wrapper sdk/gen.ts — codegen script sdk/{browser,js} protocol.json — upstream protocol (vendored)