Check Core Web Vitals for your key pages every week with real Chrome user data

SKILL.md---
name: pagespeed-insights-core-web-vitals-report
description: "Every Monday: LCP, INP and CLS at the 75th percentile for your key pages on mobile, pass or fail, and what moved since last week. Use when the user asks for this, or when it runs on a schedule."
---

# PageSpeed Insights Core Web Vitals report

Every Monday: LCP, INP and CLS at the 75th percentile for your key pages on mobile, pass or fail, and what moved since last week.

Search Console groups Core Web Vitals by URL pattern and updates slowly, and PageSpeed Insights answers one URL at a time. Neither tells you on Monday that the pricing page got slower last week. Lab scores from Lighthouse are noisy and are not what Google uses for page experience.

This skill reads the field data, the 75th percentile of real Chrome users, for the pages you care about: LCP, INP and CLS on mobile, each pass or fail, against the week before. For a page that fails, it runs a PageSpeed Insights test to name the likely culprit, such as the LCP element or the shifting banner.

## Two ways to get the data

Use the API or MCP route when you have an MCP server, a CLI or an API token for the app. Otherwise, if you can drive a browser the user is signed in to (Claude in Chrome, a Playwright or Chrome DevTools MCP server), use the browser route. Say which one you used.

## Route A: API or MCP server

### What you need

- A Google Cloud API key with the PageSpeed Insights API and the Chrome UX Report API enabled, in `PAGESPEED_API_KEY`. Both are free; CrUX allows 150 queries a minute.

If something is missing, say what and stop; do not invent numbers.

### Steps

1. **List the pages**: Keep 5 to 20 URLs that matter, such as home, pricing, top landing pages and a sign-up page, plus the origin (`https://example.com`) as a site-wide line.
2. **Read the field data** (PageSpeed Insights): For each URL, `POST https://chromeuxreport.googleapis.com/v1/records:queryHistoryRecord?key=<key>` with `url`, `formFactor: "PHONE"`, `metrics: ["largest_contentful_paint", "interaction_to_next_paint", "cumulative_layout_shift"]` and `collectionPeriodCount: 2`. The last value of each `percentilesTimeseries.p75s` is this week; the one before is last week.
3. **Fall back to the origin** (PageSpeed Insights): A URL without enough Chrome traffic returns 404. Use the origin record for it, and label the row "origin data" so nobody mistakes it for the page's own numbers.
4. **Judge pass or fail**: Good means LCP at or under 2,500 ms, INP at or under 200 ms and CLS at or under 0.1, all at p75. A page passes when all three are good. Flag any metric that worsened by 10% or more on last week, even while passing.
5. **Diagnose failing pages** (PageSpeed Insights): For each failing page, call `GET https://www.googleapis.com/pagespeedonline/v5/runPagespeed?url=<url>&strategy=mobile&category=performance&key=<key>` and read the Lighthouse diagnostics in `lighthouseResult.audits` for the LCP element, long tasks or layout shifts. Lab data explains; field data decides.

## Route B: the browser

### What you need

- No account is needed; pagespeed.web.dev is public. A Google sign-in only matters if you also open the Core Web Vitals report in Search Console for URL groups.

If something is missing, say what and stop; do not invent numbers.

### Steps

Start at https://pagespeed.web.dev/. The user is signed in already; never type passwords or codes, and ask if a sign-in page appears.

1. **Test each page**: Paste the first URL from your list and click **Analyze**. On the **Mobile** tab, the top section (what real users are experiencing) shows the Core Web Vitals assessment, Passed or Failed, with LCP, INP and CLS at the 75th percentile over the last 28 days.
2. **Note pages without their own data**: When a page has too little Chrome traffic, the section switches to origin data; the toggle above it says **This URL** or **Origin**. Use the origin numbers and label that row "origin data".
3. **Compare with last week**: PageSpeed Insights shows only the current 28-day window, so compare with the values you kept from last week's run (keep this week's too). A page passes when LCP is at most 2,500 ms, INP at most 200 ms and CLS at most 0.1; flag any metric that worsened by 10% or more even while passing.
4. **Diagnose failing pages**: Scroll to the Lighthouse section below for a failing page and read the audits that name the cause: the Largest Contentful Paint element, long main-thread tasks, and layout shift culprits. Lab data explains; the field data above decides.
5. **Add the origin line**: Run the bare origin (`https://example.com`) once as a site-wide row, then write the table: page, LCP, INP, CLS, pass or fail, change on last week, the watch list and the likely causes.

Note: The page shows one 28-day window with no history, so a week-on-week comparison depends on keeping last week's values. Each analysis takes about a minute; for more than 20 URLs the API route is faster. Field values are the same CrUX data the API returns, updated daily here against weekly in the History API, so small differences are normal.

## The report

- **Per page**: LCP, INP and CLS at p75 on mobile, pass or fail, and the change on last week.
- **Origin line** for the whole site.
- **Failing pages** with the likely cause from a Lighthouse run.
- **Watch list**: passing pages that got 10% worse.

## Post it to Briefkin

When `BRIEFKIN_TOKEN` is set, or `~/.config/briefkin/env` exists (the installer from https://briefkin.dev/docs writes it), also send the result to the user's Briefkin board as one card. Keep the `item_key` the same on every run so the card updates in place, use a new `event_id` per run, and put today's real numbers in place of the example ones. If the request fails, say so once and carry on.

```bash
[ -n "${BRIEFKIN_TOKEN:-}" ] || . ~/.config/briefkin/env
curl -sS -X POST "${BRIEFKIN_URL:-https://briefkin.dev}/v1/events" \
  -H "authorization: Bearer $BRIEFKIN_TOKEN" \
  -H "content-type: application/json" \
  -d @- <<'JSON'
{
  "v": 1,
  "event_id": "pagespeed-insights-core-web-vitals-report/<date of this run, e.g. 2026-10-16>",
  "item_key": "pagespeed-insights-core-web-vitals-report",
  "project": "example.com",
  "source": "PageSpeed Insights Core Web Vitals report skill",
  "kind": "update",
  "title": "2 of 6 key pages fail Core Web Vitals on mobile",
  "summary": "/templates/invoice LCP rose from 2.6 s to **3.4 s**; the hero image is the LCP element and is not preloaded. CLS on the net 30 post is **0.19** from a late ad slot.",
  "widget": [
    {
      "type": "Table",
      "props": {
        "columns": [
          "Page",
          "LCP",
          "INP",
          "CLS",
          "Result"
        ],
        "align": [
          "left",
          "right",
          "right",
          "right",
          "left"
        ],
        "rows": [
          [
            "/",
            "2.1 s",
            "140 ms",
            "0.04",
            "Pass"
          ],
          [
            "/pricing",
            "2.4 s",
            "180 ms",
            "0.02",
            "Pass"
          ],
          [
            "/templates/invoice",
            "3.4 s",
            "170 ms",
            "0.08",
            "Fail: LCP"
          ],
          [
            "/blog/net-30-payment-terms",
            "2.2 s",
            "120 ms",
            "0.19",
            "Fail: CLS"
          ],
          [
            "/signup",
            "1.9 s",
            "160 ms",
            "0.01",
            "Pass"
          ],
          [
            "/features (origin data)",
            "2.3 s",
            "150 ms",
            "0.05",
            "Pass"
          ]
        ]
      }
    },
    {
      "type": "Callout",
      "props": {
        "text": "/pricing INP went from 150 ms to 180 ms. Still passing, but on the watch list.",
        "tone": "info"
      }
    }
  ]
}
JSON
```

Widgets come from the Briefkin catalog (https://briefkin.dev/v1/catalog). Without a token, just give the user the report.

## Run it on a schedule

Suggested: Mondays at 9:00 (`0 9 * * 1`). From cron, with the agent signed in on that machine:

```bash
0 9 * * 1 claude -p "Run the pagespeed-insights-core-web-vitals-report skill"
# Codex: codex exec, Gemini CLI: gemini -p, Copilot CLI: copilot -p, OpenCode: opencode run
```

Or use the agent's own scheduler (Claude Code routines, Codex automations, Claude or ChatGPT scheduled tasks): https://briefkin.dev/skills/pagespeed-insights-core-web-vitals-report#schedule

Source: https://briefkin.dev/skills/pagespeed-insights-core-web-vitals-report