Inspecting Hermes Desktop Dom

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 /…` 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.

## Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

- **Author:** [AtlasOmnia](https://github.com/AtlasOmnia)
- **Source:** [AtlasOmnia/hermes-custom-pack](https://github.com/AtlasOmnia/hermes-custom-pack)
- **License:** MIT

Install and usage instructions live in the source repository linked above.