Experience Lwc Runtime Observe

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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 lightning dev component --help

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:

Lightning Preview running at URL: <URL>

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:

frontdoor_env="$("<skill_dir>/scripts/open-frontdoor.sh" <orgAlias> <previewUrl>)" || exit 1
eval "$frontdoor_env"

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.