lighthouse

name	lighthouse
description	Lighthouse audit + improvement loop until targets met. Triggers "lighthouse", "performance audit", "page speed", "improve scores", "LCP", "CLS", "INP", "core web vitals".
context	fork
argument-hint	[url]
allowed-tools	["Bash","Read","Write","Edit","Grep","Glob","LS","mcp__chrome-devtools__navigate_page","mcp__chrome-devtools__take_screenshot","mcp__chrome-devtools__take_snapshot","mcp__chrome-devtools__lighthouse_audit"]
requires	[{"command":"lighthouse","install":"npm i -g lighthouse (CLI, used for batched 3x3 averaged audits)"},{"mcp":"chrome-devtools","install":"chrome-devtools MCP — provides on-demand audits + visual regression screenshots"}]

Lighthouse Optimization Loop

Method: 3 mobile + 3 desktop runs per audit, averaged for reliability. After each code change, re-audit AND visually verify the page with chrome-devtools MCP to catch regressions.

---

Setup

1. Parse URL from $ARGUMENTS. If no URL, ask the user. Default: http://localhost:3000

2. Verify prerequisites:

   lighthouse --version    # CLI must be installed for the batched 3x3 protocol
   

   If lighthouse is missing: npm install -g lighthouse The chrome-devtools MCP is shipped by default; if missing, see mcp-configs/recommended.json.

3. Create results directory:

   mkdir -p ~/.claude/tmp/lighthouse
   

4. Take baseline screenshots before any changes:

   • mcp__chrome-devtools__navigate_page (type: "url", url: <url>)
   • mcp__chrome-devtools__take_screenshot

   Describe the current layout, key elements, and visual state. This is your visual baseline — you will compare against it after every change to catch regressions.

5. Confirm with user: Show the URL, confirm the dev server is running, ask if there are specific pages or routes to audit beyond the main URL.

---

Audit Protocol

Each audit consists of 3 mobile + 3 desktop runs, averaged per category.

Run the audits

# Mobile runs (Lighthouse default is mobile)
for i in 1 2 3; do
  lighthouse <url> \
    --output=json \
    --output-path=~/.claude/tmp/lighthouse/mobile-$i.json \
    --chrome-flags="--headless --no-sandbox" \
    --only-categories=performance,accessibility,best-practices,seo \
    --quiet \
    2>/dev/null
done

# Desktop runs
for i in 1 2 3; do
  lighthouse <url> \
    --output=json \
    --output-path=~/.claude/tmp/lighthouse/desktop-$i.json \
    --chrome-flags="--headless --no-sandbox" \
    --preset=desktop \
    --only-categories=performance,accessibility,best-practices,seo \
    --quiet \
    2>/dev/null
done

Extract scores

For each JSON result file:

cat ~/.claude/tmp/lighthouse/mobile-1.json | \
  jq '{
    performance: (.categories.performance.score * 100),
    accessibility: (.categories.accessibility.score * 100),
    bestPractices: (.categories["best-practices"].score * 100),
    seo: (.categories.seo.score * 100)
  }'

Compute averages

Average the 3 runs per category for both mobile and desktop. Report as:

## Audit Results

| Category | Mobile (avg) | Desktop (avg) |
|----------|-------------|---------------|
| Performance | XX | XX |
| Accessibility | XX | XX |
| Best Practices | XX | XX |
| SEO | XX | XX |

Extract failing audits

From the JSON, find specific audits that failed or scored poorly:

cat ~/.claude/tmp/lighthouse/mobile-1.json | \
  jq '.audits | to_entries[] | select(.value.score != null and .value.score < 0.9) | {id: .key, score: .value.score, title: .value.title, description: .value.displayValue}'

Sort by impact (lowest scores first). These drive the improvement loop.

---

Improvement Loop

Autonomous mode: to drive this loop turn-by-turn without re-prompting, set /goal mobile and desktop scores in all four categories meet their targets, or stop after 20 rounds. A goal evaluator (Haiku by default) reads the audit table after each turn and decides whether to continue. See /goal docs.

LOOP until all scores >= 90 or user interrupts:

  1. IDENTIFY the lowest-scoring category and its top failing audits
     - Read the Lighthouse audit details for specific recommendations
     - Cross-reference with the project's performance rules

  2. PLAN one targeted fix
     - Focus on the highest-impact failing audit
     - One fix at a time — never batch multiple unrelated changes
     - Common fixes by audit:
       • render-blocking-resources → async/defer scripts, inline critical CSS
       • largest-contentful-paint → priority attribute, preload, optimize image
       • cumulative-layout-shift → explicit dimensions, font-display
       • unused-javascript → dynamic imports, code splitting
       • uses-responsive-images → srcSet + sizes, `next/image` (satus) or `<picture>`/`vite-imagetools` (novus), proper dimensions
       • uses-text-compression → verify gzip/brotli enabled
       • image-size-responsive → width/height attributes
       • unminified-javascript → check build config
       • dom-size → reduce DOM nodes, virtualize lists
       • third-party-summary → defer/lazy-load third-party scripts
       • font-display → font-display: swap or optional
       • offscreen-images → loading="lazy" (NOT on above-fold/LCP images)

  3. IMPLEMENT the fix
     - Edit the relevant source files
     - Keep changes minimal and focused

  4. VERIFY BUILD
     - Run the project build to ensure no compilation errors
     - If TypeScript project: `tsc --noEmit` first

  5. VISUAL REGRESSION CHECK
     - `mcp__chrome-devtools__navigate_page` to the same URL
     - `mcp__chrome-devtools__take_screenshot`
     - Compare against the baseline screenshot:
       • Layout intact? (same general structure, no collapsed/missing sections)
       • Content visible? (text, images, interactive elements still present)
       • Styling correct? (colors, spacing, typography not broken)
       • Functionality preserved? (interactive elements still look clickable)
     - If regression detected: REVERT the change immediately and try a different approach
     - Also check critical user flows if the change affects interactive elements:
       - `mcp__chrome-devtools__take_snapshot` (a11y tree — confirms interactive elements are present, returns `uid`s)
       - `mcp__chrome-devtools__take_screenshot` (visual verification)

  6. RE-AUDIT
     - Run full audit protocol again (3 mobile + 3 desktop)
     - Compare against previous scores

  7. LOG RESULTS
     - Append to ~/.claude/tmp/lighthouse/results.tsv:
       round	mobile_perf	desktop_perf	mobile_a11y	desktop_a11y	status	description
     - Status: "kept" (scores improved), "reverted" (regression or no improvement)

  8. REPORT
     - Show score delta: "Performance: 72 → 85 (+13)"
     - Show what was changed and why
     - Show the current failing audits for the next round

  9. CONTINUE to next round

---

Visual Regression Protocol

This is the critical safety net. Performance changes MUST NOT break the UI.

After every code change:

1. Navigate: mcp__chrome-devtools__navigate_page (type: "url", url: <url>)
2. Screenshot: mcp__chrome-devtools__take_screenshot
3. Compare against baseline:
   • Is the page layout the same structure?
   • Are all visible elements still present?
   • Is text readable and properly styled?
   • Are images displaying correctly?
   • Are interactive elements (buttons, forms, nav) visually intact?

Regression = immediate revert

If any visual regression is detected:

1. git checkout -- <changed-files> to revert
2. Log status as "reverted (visual regression)" in results.tsv
3. Try an alternative approach to the same audit issue
4. NEVER accept a performance improvement that breaks the UI

Multi-page checks

If the user specified multiple URLs/routes, check ALL of them after each change. A fix that improves the homepage but breaks a subpage is still a regression.

---

Targets

Default targets (override by telling the agent different ones):

Category	Mobile	Desktop
Performance	>= 90	>= 95
Accessibility	>= 95	>= 95
Best Practices	>= 95	>= 95
SEO	>= 95	>= 95

The loop continues until ALL categories on BOTH mobile and desktop meet their targets, or the user interrupts.

---

Core Web Vitals Focus

When Performance score is low, prioritize these metrics:

Metric	Target	What to Fix
LCP < 2.5s	Optimize largest content element (usually hero image or heading). Use priority, fetchpriority="high", preload, optimize image format/size.
INP < 200ms	Reduce JavaScript execution time. Debounce handlers, use startTransition, yield to main thread with scheduler.yield().
CLS < 0.1	Set explicit dimensions on images/video/ads/embeds. Use font-display: optional. Reserve space for dynamic content.
TTFB < 800ms	Server-side: check caching, CDN, database queries. Use streaming SSR with Suspense.

---

Dashboard

After each round, write ~/.claude/tmp/lighthouse/dashboard.md:

# Lighthouse Optimization: <url>
Updated: <timestamp>

## Current Scores
| Category | Mobile | Desktop | Target | Status |
|----------|--------|---------|--------|--------|
| Performance | XX | XX | 90/95 | pass/fail |
| Accessibility | XX | XX | 95/95 | pass/fail |
| Best Practices | XX | XX | 95/95 | pass/fail |
| SEO | XX | XX | 95/95 | pass/fail |

## Progress (baseline → current)
| Category | Mobile | Desktop |
|----------|--------|---------|
| Performance | 62 → 91 (+29) | 78 → 96 (+18) |
| ... |

## Changes Applied
| Round | Fix | Mobile Perf Delta | Visual QA |
|-------|-----|-------------------|-----------|
| 1 | Added priority to hero image | +12 | pass |
| 2 | Deferred analytics script | +8 | pass |
| 3 | Added font-display: swap | +3 | pass |

## Remaining Issues
Top failing audits still to address...

---

Completion

When all targets are met:

1. Print final score summary with deltas from baseline
2. List all changes made (files modified and why)
3. Suggest running a final full visual QA: /qa <url>
4. Do NOT auto-commit — let the user review the changes first