experience-lwc-runtime-observe

Previewing LWC Runtime — Local Dev + DOM Inspection

Guides the agent through two sequential jobs that typically run together:

1. Start a Lightning Preview session with the Salesforce CLI so an LWC app or component renders in a local dev environment.
2. Navigate to that preview URL and extract the runtime DOM subtree so the agent (or user) can reason about what actually rendered.

Preview is the prerequisite for DOM inspection. Keep them in this order.

When to Use This Skill

• The user wants to preview an LWC app or component locally before deploying (sf lightning dev app / sf lightning dev component).
• The user wants the rendered HTML subtree for a specific component at runtime — for a diff, a screenshot pipeline, or to reason about shadow DOM.
• The user asks why the template content "isn't in the DOM" — almost always a shadow-DOM misunderstanding handled in Step 2.

Prerequisites

• Salesforce CLI (sf) installed. Check with sf version.
• @salesforce/plugin-lightning-dev installed. Check with sf plugins.
• An authenticated org (sf org list shows at least one connected org).
• For DOM extraction: browser automation available in the agent harness (Playwright, Puppeteer, Chrome DevTools Protocol — whatever is wired up).

Step 1 — Start the Lightning Preview

Ask the user (or infer from the request) whether the scope is app or component.

1a. Verify the toolchain

Ask the user for (or infer from the request) the org alias the preview should target. Run the bundled verification script with that alias and surface any error it emits — do not re-invoke sf version / sf plugins / sf org display in prose. The script checks CLI presence, @salesforce/plugin-lightning-dev installation, and that the specific target alias is authenticated and Connected, and exits nonzero with an actionable message on failure:

"<skill_dir>/scripts/verify-toolchain.sh" <orgAlias>

If it reports that the target alias is not authenticated or not Connected, direct the user to Enable Local Dev and have them run sf org login web --alias <orgAlias>, then re-run the verification script before proceeding.

1b. Read the subcommand's flags

# For app scope
sf lightning dev app --help
 
# For component scope
sf

Note required vs optional flags. If a required flag is ambiguous, ask the user before running.

1c. Launch the preview

Prefix every preview command with OPEN_BROWSER=false — the CLI opens a browser window by default, which we do not want when an agent is driving. (This env var is undocumented in --help; rely on it unless the user explicitly asks for a browser window.)

When the preview is up, capture the URL and log it in exactly this format:

If any sub-step fails, stop and surface the error to the user — don't try to paper over auth or plugin-install problems.

Step 2 — Extract the Runtime DOM

Only run this step once Step 1 has produced a live preview URL.

2a. Get an authenticated front-door URL — without surfacing the token to agent context

The front-door URL returned by sf org open --url-only --json contains a short-lived OTP that authenticates the session. Never run sf org open directly — its output would land in the agent tool-output stream and leak an auth token into the transcript, violating agent-safety standard S1. The bundled helper is the only supported way to obtain the URL: it runs sf org open internally with stdout redirected into a chmod 600 tempfile and stderr suppressed, so no URL fragment or token substring ever reaches agent context.

Instead, use the bundled helper script, which writes the URL to a chmod 600 tempfile and prints only export assignments (no token material). Capture the helper output first so a nonzero exit status aborts the caller — eval "$(...)" alone masks failures, letting the agent proceed with an unset $FRONTDOOR_URL_FILE:

The helper suppresses sf and python3 stderr as well as stdout, so partial URLs and progress noise never reach the agent transcript.

The browser driver must:

1. Read the URL from $FRONTDOOR_URL_FILE.
2. Navigate to it to establish a session.
3. Delete the tempfile immediately after use.
4. Never cat or otherwise echo the file contents.

OTP tokens expire fast. Bring up the browser driver first, then invoke the helper just before navigating — otherwise the token goes stale.

2b. Navigate and wait

1. Open the preview URL from Step 1 in the automation browser.
2. Wait for the network to go idle and the initial render to complete.
3. If the expected element isn't there, back off briefly and retry before declaring failure — local dev pages sometimes hydrate in two phases.

2c. Extract the subtree

Two deterministic string operations — turning a component name into its c-<kebab> selector and wrapping the extracted HTML in the exact DOM_OUTPUT_START / DOM_OUTPUT_END markers — are handled by <skill_dir>/scripts/extract-dom.sh. Do NOT re-derive the selector or compose the markers in prose; call the script.

Actual DOM traversal must be run by the browser-automation driver (Playwright / Puppeteer / CDP); the script does not drive the browser.

If scope is component

Compute the selector deterministically:

Then, using the driver, pierce the lwr_dev-preview-container shadow root, locate the first visible element matching $selector, and read its subtree HTML (via element.shadowRoot.innerHTML). Pipe that HTML into the wrap step:

If scope is app

Using the driver, identify the primary app root (prefer document.body or the main app container — not document.documentElement), exclude <head> and non-rendered nodes, and read the subtree HTML. Pipe it into the wrap step:

Emit the script's stdout unchanged as the DOM output.

2d. About LWC Shadow DOM (the #1 gotcha)

Lightning Web Components use Shadow DOM. The template content is not directly queryable on the host element:

If the browser driver's DOM query doesn't pierce shadow roots by default, walk shadowRoot explicitly. Most modern automation drivers have a shadow-piercing query helper — use it.

Examples

For a worked end-to-end example (preview a component, capture the front-door URL safely, and extract its DOM), load examples/component-preview-and-dom.md.

Verification Checklist

• <skill_dir>/scripts/verify-toolchain.sh <orgAlias> exited zero for the target alias.
• Preview command ran with OPEN_BROWSER=false.
• Preview URL logged in the Lightning Preview running at URL: <URL> format.
• Front-door URL fetched via <skill_dir>/scripts/open-frontdoor.sh (never sf org open directly).
• DOM extraction used shadowRoot traversal (not innerHTML on the host).
• DOM output was produced by <skill_dir>/scripts/extract-dom.sh wrap … (script owns the markers).

Troubleshooting

• verify-toolchain.sh reports "Target org alias is not authenticated or not Connected" — run sf org login web --alias <orgAlias> (the alias must match the one you passed to verify-toolchain.sh and the -o value the preview command uses).
• verify-toolchain.sh reports the plugin is missing — install it with sf plugins install @salesforce/plugin-lightning-dev.
• Preview opens a browser window — you forgot OPEN_BROWSER=false.
• Empty DOM on extraction — you're looking at innerHTML on a host. Use element.shadowRoot.innerHTML instead. Also verify you pierced the lwr_dev-preview-container shadow root.
• Front-door token fails — it's expired. Re-invoke <skill_dir>/scripts/open-frontdoor.sh and navigate within ~60 seconds.