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:

# macOS (Apple Silicon + Homebrew)
command -v browser-harness-js >/dev/null || ln -sf <skill-dir>/sdk/browser-harness-js /opt/homebrew/bin/browser-harness-js

# macOS (Intel) / most Linux — may need sudo
command -v browser-harness-js >/dev/null || ln -sf <skill-dir>/sdk/browser-harness-js /usr/local/bin/browser-harness-js

# Linux without sudo (ensure ~/.local/bin is on PATH)
command -v browser-harness-js >/dev/null || { mkdir -p ~/.local/bin && ln -sf <skill-dir>/sdk/browser-harness-js ~/.local/bin/browser-harness-js; }

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.

browser-harness-js 'await session.connect()'
browser-harness-js 'await session.Page.navigate({url:"https://example.com"})'
browser-harness-js '(await session.Runtime.evaluate({expression:"document.title",returnByValue:true})).result.value'

Output is the raw result content — no {ok,result} envelope.

Errors go to stderr, exit code 1. The CDP error message and JS stack are printed verbatim, e.g.:

Error: CDP -32602: invalid params
    at _call (.../session.ts:117:33)
    ...

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.

browser-harness-js <<'EOF'
const tabs = await listPageTargets();
globalThis.tid = tabs[0].targetId;
await session.use(globalThis.tid);
return globalThis.tid;
EOF

CLI commands

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).

// no params
await session.DOM.enable()

// required params
await session.Page.navigate({ url: 'https://example.com' })

// all-optional params (object also optional)
await session.Page.captureScreenshot()
await session.Page.captureScreenshot({ format: 'png', quality: 80 })

// returns are stripped to the typed shape
const { root } = await session.DOM.getDocument()
const { nodeId } = await session.DOM.querySelector({ nodeId: root.nodeId, selector: 'h1' })

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.

await session.connect()   // auto-detect

Use detectBrowsers() first if you want to see what's available (or let the user pick) before connecting:

const found = await detectBrowsers()
// [{ name: 'Google Chrome', profileDir, port, wsPath, wsUrl, mtimeMs }, ...]

Explicit forms — use these only when auto-detect picks the wrong browser, or when you already know where to connect:

await session.connect({ profileDir: '/Users/<you>/Library/Application Support/Google/Chrome' })
await session.connect({ wsUrl: 'ws://127.0.0.1:9222/devtools/browser/<uuid>' })

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:

await session.connect({ profileDir: '/Users/<you>/Library/Application Support/Google/Chrome', timeoutMs: 30_000 })

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.

const tabs = await listPageTargets()                     // no args; uses the connected session
const sid  = await session.use(tabs[0].targetId)
await session.Page.enable()
await session.Page.navigate({ url: 'https://example.com' })

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:

const { targetInfos } = await session.Target.getTargets({})
const tabs = targetInfos.filter(t => t.type === 'page' && !t.url.startsWith('chrome://') && !t.url.startsWith('devtools://'))

To switch tabs: session.use(otherTargetId). To detach: session.setActiveSession(undefined).

Events

// Subscribe (returns an unsubscribe fn)
const off = session.onEvent((method, params, sessionId) => { ... })

// Or wait for a single matching event with optional predicate + timeout
await session.Network.enable()
const ev = await session.waitFor(
  'Page.frameNavigated',
  (p) => p.frame.url.includes('example.com'),
  10_000
)

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:

browser-harness-js '(await listPageTargets()).forEach((t,i)=>globalThis["tab"+i]=t.targetId)'
browser-harness-js 'await session.use(globalThis.tab0)'
browser-harness-js 'await session.Page.navigate({url:"https://example.com"})'

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:

   # macOS — prefer AppleScript over `open -a` (reuses current profile, avoids the profile picker)
   osascript -e 'open location "chrome://inspect/#remote-debugging"'
   
   # Linux
   google-chrome 'chrome://inspect/#remote-debugging'     # or: chromium, google-chrome-stable
   
   # Windows (PowerShell)
   Start-Process chrome 'chrome://inspect/#remote-debugging'

   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.

grep -n "navigate" <skill-dir>/sdk/generated.ts | head

Regenerating the SDK

When the upstream protocol JSONs change, replace sdk/browser_protocol.json and/or sdk/js_protocol.json and re-run:

cd <skill-dir>/sdk && bun gen.ts
browser-harness-js --restart   # pick up the new bindings

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)