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.
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.
npx agentmods add skills/nucliweb/webperf-snippets/webperf-core-web-vitalsnpx skills add nucliweb/webperf-snippets --skill webperf-core-web-vitalsgit clone --depth 1 https://github.com/nucliweb/webperf-snippetsWrote 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.
[](https://agentmods.dev/skills/nucliweb/webperf-snippets/webperf-core-web-vitals)<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="Measured on agentmods" height="20"></a>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 | $0.00113 | $0.03077 |
| Opus 5 | $0.00056 | $0.01538 |
| Sonnet 5 | $0.00023 | $0.00615 |
| Haiku 4.5 | $0.00011 | $0.00308 |
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 5d ago.
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.
How it starts
The opening of the file, as written. The whole thing — 236 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. Execute with mcp__chrome-devtools__evaluate_script, capture output with mcp__chrome-devtools__get_console_message.
Scripts
scripts/CLS.js— Cumulative Layout Shift (CLS)scripts/INP.js— Interaction to Next Paint (INP)scripts/LCP-Image-Entropy.js— LCP Image Entropyscripts/LCP-Subparts.js— LCP Subpartsscripts/LCP-Trail.js— LCP Trailscripts/LCP-Video-Candidate.js— LCP Video Candidatescripts/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
LCP Deep Dive
When LCP is slow or the user asks "debug LCP" or "why is LCP slow":
- LCP.js - Establish baseline LCP value
- LCP-Subparts.js - Break down into TTFB, resource load, render delay
- LCP-Trail.js - Identify all LCP candidates and changes
- LCP-Image-Entropy.js - Check if LCP image has visual complexity issues
- LCP-Video-Candidate.js - Detect if LCP is a video (poster or video element)
CLS Investigation
When layout shifts are detected or the user asks "debug CLS" or "layout shift issues":
- CLS.js - Measure overall CLS score
- Layout-Shift-Loading-and-Interaction.js (pending — available in webperf-interaction skill)
- Cross-reference with webperf-loading skill:
- Find-Above-The-Fold-Lazy-Loaded-Images.js (lazy images causing shifts)
- Fonts-Preloaded-Loaded-and-used-above-the-fold.js (font swap causing shifts)
INP Debugging
When interactions feel slow or the user asks "debug INP" or "slow interactions":
- INP.js - Measure overall INP value; call
getINP()after interactions,getINPDetails()for full list - Interactions.js (pending — available in webperf-interaction skill)
- Input-Latency-Breakdown.js (pending — available in webperf-interaction skill)
- Long-Animation-Frames.js (pending — available in webperf-interaction skill)
- Long-Animation-Frames-Script-Attribution.js (pending — available in webperf-interaction skill)
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.
- references/schema.md 20 KB
- references/snippets.md 2.8 KB
- scripts/CLS.js 917 B runs code
- scripts/INP.js 3.5 KB runs code
- scripts/LCP-Image-Entropy.js 2.4 KB runs code
- scripts/LCP-Subparts.js 3.7 KB runs code
- scripts/LCP-Trail.js 2.2 KB runs code
- scripts/LCP-Video-Candidate.js 2.7 KB runs code
- scripts/LCP.js 1.6 KB runs code
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.
- 5d ago First seen · 236 lines · 113 tokens per session scan A 6e492e43d268
webperf-core-web-vitals is a skill published in the GitHub repository nucliweb/webperf-snippets (1,445 stars, last pushed 1mo ago), licensed MIT. It adds 113 tokens to every session and 3,077 once invoked, about $0.0006 per session on Opus 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.
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