Inspecting the live Hermes desktop DOM

Overview

When you are developing apps/desktop and the user is running that same app (hgui / npm run dev), you can read the live rendered DOM of the window they are looking at — computed styles, geometry, which CSS rule actually won, console output — instead of inferring it from .tsx and being wrong.

Dev-server runs open a Chrome DevTools Protocol port on 127.0.0.1:9222 automatically. The renderer is a Chromium page, so everything DevTools can read, a script can read.

This does not replace looking at it. CDP answers factual questions ("what is the computed padding", "did this element render", "which selector matches"). It cannot tell you whether the result looks good. Colour balance, spacing feel, and "is this ugly" still need the user's eyes or a screenshot. Answer facts with CDP; hand aesthetics to the user.

When to Use

• Verifying a UI change actually took effect in the running app
• "Why is this element still X?" — find the winning rule before editing anything
• Locating a stable selector for a component you're about to change
• Checking a design token's computed value on a real node
• Reading renderer console errors the user mentions but can't copy out

Don't use for: perf profiling or heap work (node-inspect-debugger, debugging-hermes-desktop), or anything where the real question is "does this look right".

The port

Open on 127.0.0.1:9222 for any dev-server run. Closed in exactly two cases (apps/desktop/electron/dev-cdp.ts):

• packaged builds — always, and no environment value overrides it;
• no HERMES_DESKTOP_DEV_SERVER — an unpackaged electron . against dist/ is how the packaged app gets smoke tested, so it behaves like one.

HERMES_DESKTOP_CDP_PORT moves the port (=9333) or disables it (=off).

Check before doing anything else:

curl -s --max-time 3 http://127.0.0.1:${HERMES_DESKTOP_CDP_PORT:-9222}/json/version

Empty → no port. Do not guess another port silently.

Never relaunch the user's app to get a port. That destroys their session and their state. Launch your own isolated instance instead (below).

Reading the DOM

apps/desktop/scripts/eval.mjs is the one-liner:

cd apps/desktop
node scripts/eval.mjs "document.querySelectorAll('[data-slot]').length"

For multi-step work use the shared client — it has target discovery and promise-aware eval:

import { CDP, SELECTORS } from './scripts/perf/lib/cdp.mjs'

const cdp = await CDP.connect({ port: 9222, match: '5174' })
const out = await cdp.eval(`JSON.stringify({
  radius: getComputedStyle(document.documentElement).getPropertyValue('--radius-scalar').trim(),
  composer: !!document.querySelector('[data-slot="composer-rich-input"]')
})`)
cdp.close()

SELECTORS in scripts/perf/lib/cdp.mjs holds the stable data-slot hooks (composer, thread viewport, assistant message, turn pair, profile rail). Prefer them over inventing a querySelector — they are updated as a unit when components move.

The question this is best at: which rule won?

Editing every call site because a style "isn't applying" is the classic waste. Read the real node first:

const el = document.querySelector('[data-slot="aui_assistant-message-root"] a')
JSON.stringify({
  ownClasses: el.className,
  weight: getComputedStyle(el).fontWeight,
  parents: (() => {
    const out = []
    let n = el
    while ((n = n.parentElement) && out.length < 6) out.push(n.className)
    return out
  })()
})

If the node carries no class of its own, the value is inherited — sweeping call sites will not fix it, and you need the ancestor rule. A plugin stylesheet (e.g. @tailwindcss/typography's prose a { font-weight: 500 }) routinely beats a utility class; override on the shared class, not at each usage.

Your own isolated instance

When there is no port, or you must not disturb the user's window:

cd apps/desktop
HERMES_HOME=/tmp/cdp-probe-home \
HERMES_DESKTOP_DEV_SERVER=http://127.0.0.1:5174 \
HERMES_DESKTOP_CDP_PORT=9333 \
  npx electron . --user-data-dir=/tmp/cdp-probe-userdata

The separate --user-data-dir dodges Electron's single-instance lock, so it cannot collide with a running hgui; the separate HERMES_HOME keeps it away from real sessions. Pick a port other than 9222 for the same reason. Run it in the background and kill it when done.

npm run perf:serve does the same with a temp HERMES_HOME baked in, if you also want the perf harness.

Pitfalls

• Never kill the user's dev server or app to "free" anything. A mid-serve kill nukes Chromium's socket pool, and the resulting ERR_NETWORK_CHANGED gets blamed on whatever you just changed.
• A throwaway HERMES_HOME has no backend. The app logs ECONNREFUSED for hermes:api and may exit on its own. The renderer still mounts and the DOM is readable — read promptly, and don't mistake a self-exited probe for a broken port. Chromium logs DevTools listening on ws://127.0.0.1:<port>/… when it binds; that line is the proof the port opened.
• Poll, don't probe once. A just-launched app needs a second or two before the port answers.
• Never dump the whole DOM. The desktop renders hundreds of nodes and outerHTML will bury your context. Project down to a small JSON object inside the evaluated expression.
• Pass match to CDP.connect. Without it you may attach to the pet overlay, quick-entry window, or a devtools target instead of the main window.
• cdp.eval returns the value; raw Runtime.evaluate double-nests it (.result.result.value). Use the wrapper.
• import.meta.env.DEV is true under vite dev in this repo. The note in apps/desktop/scripts/profile-typing-lag.md claiming otherwise is stale.