Libretto Readonly

How Libretto Read-Only Works

• Use this skill when the browser session must stay strictly read-only.
• Libretto stores read-only vs write-access on the session itself.
• The primary inspection tools are snapshot and readonly-exec.
• readonly-exec reuses Libretto's normal execution pipeline, but it only exposes read-only helpers and denies mutating Playwright methods.
• Only a user can change the session mode for an existing session. Never change a session's mode on your own — the user must change it themselves manually.

Working Rules

• Announce which session you are using and what page you are inspecting.
• Do not use exec, run, or any direct Playwright action that could change browser or application state.
• Do not click, type, submit forms, navigate, upload files, dispatch DOM events, or send non-GET requests.
• Prefer snapshot first when the visible page state is unclear.
• Use readonly-exec for focused inspection: titles, HTML, locator text, counts, visibility checks, and GET requests.
• Keep snippets small and purpose-built. Do not run multiple readonly-exec commands at the same time.
• End with diagnosis and handoff guidance, not an attempted in-browser repair.

Commands

connect

• Use connect to attach to an existing CDP endpoint for a preserved browser session.
• Use --read-only when creating the Libretto session handle for a preserved browser session.
• Libretto read-only mode is enforced through Libretto commands; direct CDP clients that skip Libretto are outside this boundary.

npx libretto connect http://127.0.0.1:9222 --read-only --session failed-job-debug

pages

• Use pages when a popup, new tab, or second page exists.
• If readonly-exec or snapshot complains about multiple pages, list ids first and then pass --page.

npx libretto pages --session failed-job-debug

snapshot

• Use snapshot as the first high-level observation tool.
• Run snapshot <ref> to inspect a subtree from the latest full snapshot.

readonly-exec

• Use readonly-exec for narrow inspection code only.
• Denied operations fail with ReadonlyExecDenied: ....

Helpers

• page — a read-only Playwright Page proxy. Standard Playwright read methods work normally (url(), title(), content(), getByRole(), locator(), textContent(), isVisible(), count(), scrollIntoViewIfNeeded(), etc.). Anything that mutates the page (click, fill, goto, evaluate, keyboard, mouse) is blocked.
• state — the current Libretto session state object.
• get(url, options?) — HTTP client restricted to GET and HEAD requests. Replaces fetch, which is blocked in readonly mode. Any request with a body or a non-GET/HEAD method throws ReadonlyExecDenied.
• scrollBy(deltaX, deltaY) — scroll the viewport by pixel offset. Use this to inspect content below the fold without targeting a specific element.

Standard JS globals console, URL, Buffer, setTimeout, and setInterval are also available.

Examples

npx libretto readonly-exec "return page.url()" --session failed-job-debug
npx libretto readonly-exec "return await page.getByRole('heading').first().textContent()" --session failed-job-debug

# HTTP GET inspection
echo "const r = await get('https://api.example.com/status'); return await r.json()" \
  | npx libretto readonly-exec - --session failed-job-debug

# Scroll down to inspect below-the-fold content
npx libretto readonly-exec "await scrollBy(0, 500)" --session failed-job-debug

close

• Use close when the inspection session is no longer needed.

npx libretto close --session failed-job-debug