How to check core web vitals from the browser, a script or an agent

If you are working out how to check core web vitals, you need two readings: what real Chrome users experienced over the last 28 days, and what a controlled Lighthouse run finds when it loads the page now. For one URL, the quickest way to get both in one place is seogeoaeo.ai's Core Web Vitals Snapshot, which you can run from the browser or call from an agent over MCP or REST.

Below are the four ways developers usually check, what each one can and cannot see, and where each fits in a normal working day.

The three metrics you are checking

• LCP (Largest Contentful Paint): when the biggest piece of content finishes rendering. Good is 2.5 seconds or less.
• INP (Interaction to Next Paint): how long the page takes to respond to clicks, taps and key presses. Good is 200 ms or less. INP replaced First Input Delay as a stable Core Web Vital in 2024.
• CLS (Cumulative Layout Shift): how much visible content jumps around. Good is 0.1 or less.

Google judges each at the 75th percentile, split by mobile and desktop.

Four ways to check, compared

From an agent or a script

If you already work with Claude Code, Cursor or another MCP client, add a seogeoaeo.ai workspace API key and call the core-web-vitals-snapshot tool with a url and an optional strategy of mobile or desktop. The REST endpoint takes the same input. Each run costs 20 credits, the price is shown before it starts, and a failed run costs nothing.

This is the method to check a page's vitals from your editor before and after a change, without switching to a browser tab. The result keeps field and lab sections apart, so you do not mistake a lab number for what users saw.

In PageSpeed Insights

Paste the URL into PageSpeed Insights and pick mobile or desktop. The top section is CrUX field data for the page, or for the whole origin if the page lacks traffic. Below it is a Lighthouse lab run on a simulated mid-tier phone. If neither the page nor the origin has enough data, you only get the lab half.

In Chrome DevTools

Open the Performance panel, load the page and interact with it. DevTools measures LCP, CLS and INP for your own session, right next to the code you are changing. This is where you fix things. Your session is one fast machine on one network, so it is not proof of what users get.

In Search Console

The Core Web Vitals report groups URLs that behave alike and gives each group a Good, Need improvement or Poor status per device. After a fix, start validation and Google tracks the group for 28 days. You need a verified property and enough traffic for a group to appear.

A sensible order

1. Reproduce and fix in DevTools.
2. Confirm the single URL with the snapshot or PageSpeed Insights, lab section first, since field data takes weeks to move.
3. Watch Search Console for the site-wide picture.

When you need to watch every template on a large site, Search Console is the better tool. The snapshot checks one public URL per run and is not built for bulk lists.

FAQ

Can Lighthouse check INP? No. Lighthouse loads the page with no user, so it uses Total Blocking Time as a proxy. INP comes from field data or from interacting in DevTools.

Why did my score change between runs? Lab runs vary with network and server load. Field data moves slowly because it covers 28 days of visits.