What is webperf-core-web-vitals?

A browser-based toolkit for measuring Core Web Vitals, which are standard signals for how quickly a page loads, stays visually stable, and responds to input. It measures LCP, CLS, and INP and guides follow-up debugging.

Use it to audit Core Web Vitals, investigate slow Largest Contentful Paint (LCP), layout shifts, or interaction delays, and inspect loading details such as server response and rendering time.

It helps identify whether a website feels slow, shifts while loading, or responds late to clicks and other interactions. The guided checks help narrow down possible causes after a measurement.

webperf-core-web-vitals — Skill for Claude Code by nucliweb

About the project

WebPerf Snippets is a collection of JavaScript snippets that run in browser developer tools to measure and debug website performance. It helps developers investigate Core Web Vitals, loading, interaction responsiveness, images, video, scripts, and other performance issues. Its catalogue skills, setting, and instruction support agent-assisted use of the snippets.

nucliweb/webperf-snippets · 1,454 stars · on GitHub · webperf-snippets.net

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Any agent

npx skills add nucliweb/webperf-snippets --skill webperf-core-web-vitals

Clone the repo

git clone --depth 1 https://github.com/nucliweb/webperf-snippets

Made for: Claude Code.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/skills/nucliweb/webperf-snippets/webperf-core-web-vitals/github.svg)](https://agentmods.dev/skills/nucliweb/webperf-snippets/webperf-core-web-vitals)

Your own site

<a href="https://agentmods.dev/skills/nucliweb/webperf-snippets/webperf-core-web-vitals"><img src="https://agentmods.dev/badge/skills/nucliweb/webperf-snippets/webperf-core-web-vitals/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

Your own site · 80×15

<a href="https://agentmods.dev/skills/nucliweb/webperf-snippets/webperf-core-web-vitals"><img src="https://agentmods.dev/badge/skills/nucliweb/webperf-snippets/webperf-core-web-vitals.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 113 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 3,345 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe. Third-party audits

NVIDIA SkillSpector warn 7 Sept 2026

SkillSpector: 1 finding, up to medium

These are SkillSpector’s own severities. On a checked sample its high-severity flags on skills were ~96% false positives — a documented command, a public API, a “never do X” rule — so we show them as a caution to read, not a verdict. Why →

medium Excessive Agency · line 98
Skill enables autonomous high-impact decisions without human-in-the-loop verification. Critical operations (destructive commands, financial transactions, data deletion) should require explicit user confirmation.

Fix: Add human-in-the-loop confirmation for destructive, irreversible, or high-impact operations. Never auto-execute commands that modify files, send data, or alter system state.

How audits are shown

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.00113	$0.03345
Opus 5.5	$0.00045	$0.01338
Sonnet 5.5	$0.00023	$0.00669
Haiku 4.5	$0.00011	$0.00334

Measured 2d ago against content hash bf0dce224edc, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

webperf-core-web-vitals scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2d ago.

The scan reads SKILL.md. This mod also ships 7 executable files (scripts/CLS.js, scripts/INP.js, scripts/LCP-Image-Entropy.js, …), listed below but not scanned — reading those needs a real analyzer, not pattern matching.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

.claude/skills/webperf-core-web-vitals/SKILL.md · 243 lines

How it starts

The opening of the file, as written. The whole thing — 243 lines — stays where its author put it; the contents beside it link to each section on GitHub.

WebPerf: Core Web Vitals

JavaScript snippets for measuring web performance in Chrome DevTools.

Running a script

Run the script with mcp__chrome-devtools__evaluate_script and read the object it returns. The scripts have no console output, so the returned object is the result: script, status and the metric data (value, rating, details, items, issues), as described in references/schema.md.
When status is "tracking", the script keeps observing the page. After the interactions, call the function named in getDataFn with another mcp__chrome-devtools__evaluate_script call, for example getINP(), and read the object it returns.
When status is "error" or "unsupported", read error: the page may not have loaded yet, or the browser lacks the API.

Without Chrome DevTools MCP, run the same snippet with the webperf-snippets CLI, which loads the page in headless Chromium and prints the same result objects: npx webperf-snippets <url> --snippet <Name> --json, where <Name> is the script name without .js. It needs Playwright and Chromium (npm install --save-dev webperf-snippets playwright, then npx playwright install chromium), and tracking snippets take --interact-script <file> to drive the interactions.

Scripts

scripts/CLS.js — Cumulative layout shift (CLS)
scripts/INP.js — Interaction to next paint (INP)
scripts/LCP-Image-Entropy.js — LCP image entropy
scripts/LCP-Subparts.js — LCP subparts
scripts/LCP-Trail.js — LCP trail
scripts/LCP-Video-Candidate.js — LCP video candidate
scripts/LCP.js — Largest contentful paint (LCP)

Common Workflows

Complete Core Web Vitals Audit

When the user asks for a comprehensive Core Web Vitals analysis or "audit CWV":

LCP.js - Measure Largest Contentful Paint
CLS.js - Measure Cumulative Layout Shift
INP.js - Measure Interaction to Next Paint
LCP-Subparts.js - Break down LCP timing phases
LCP-Trail.js - Track LCP candidate evolution

Read the full file on GitHub · 243 lines

Files

What ships with it

9 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

2d ago Changed · -1 lines bf0dce224edc
5d ago Changed · +8 lines 75f624010104
6d ago Changed 7f4c2e13afe2
21d ago Changed e0688b2f14c7
22d ago Changed d8c066e29318
1mo ago First seen · 236 lines · 113 tokens per session scan A 6e492e43d268

Subscribe to this mod's changes

webperf-core-web-vitals is a skill published in the GitHub repository nucliweb/webperf-snippets (1,454 stars, last pushed today), licensed MIT. It adds 113 tokens to every session and 3,345 once invoked, about $0.0005 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

Analytics and metrics Debugging performance performance-analysis performance-metrics snippets webperf webperformance

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