/inspecting-hermes-desktop-dom

name: inspecting-hermes-desktop-dom
description: "Read the live Hermes desktop DOM/CSS over CDP."
version: 1.0.0
author: Hermes Agent
license: MIT
platforms: [linux, macos, windows]
metadata:
  hermes:
    tags: [desktop, electron, cdp, dom, ui-verification, self-inspection]
    related_skills: [node-inspect-debugger, systematic-debugging, dogfood]

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.v