bench

A front-end performance measurement process for page speed and Core Web Vitals, which are browser metrics for loading, layout stability, and interaction response. It compares measurements with a target budget or an earlier baseline.

In plain words
What is it for?
Use it to measure LCP—the time until the main content appears—CLS—unexpected layout movement—and INP or related interaction timing, along with page-load and hydration timing.
Why use it?
It turns a page that merely feels slow into measurable evidence and helps detect regressions after a change.

Skill for Claude CodeCodex

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.

agentmods
npx agentmods add skills/mtthsnc/tempest/bench
Any agent
npx skills add mtthsnc/tempest --skill bench
Clone the repo
git clone --depth 1 https://github.com/mtthsnc/tempest

Made for: Claude Code, Codex.

Per session 91 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 959 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
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.

ModelPer sessionOnce invoked
Fable 5 $0.00091 $0.00959
Opus 5 $0.00046 $0.00479
Sonnet 5 $0.00018 $0.00192
Haiku 4.5 $0.00009 $0.00096

Measured 2d ago against content hash c93a697453e4, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

bench 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.

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.

plugins/tempest/skills/bench/SKILL.md · 69 lines

How it starts

The opening of the file, as written. The whole thing — 69 lines — stays where its author put it; the contents beside it link to each section on GitHub.

bench — measure Core Web Vitals against a budget

Overview

The performance gate. Turn "feels slow" into numbers: capture Core Web Vitals (LCP, CLS, INP/TBT) plus load and hydration timing for a page, then compare them against an explicit budget or a baseline so a regression is a fact, not a vibe.

This is a STARTER skill — a generic spine. Rewrite the procedure and thresholds below to encode your own perf budgets (your LCP/CLS/INP targets, the build you measure, the device profile).

Measurement runs through agent-browser's vitals capability. Use whichever invocation path your harness offers:

  • On pi: prefer the native agent_browser tool from the pi-agent-browser-native extension, passing raw args — { "args": ["vitals", "<url>"] }, or { "args": ["open", "<url>"] } then { "args": ["vitals"] }.
  • On Claude Code (or any harness without that tool): drive the same CLI via Bash — agent-browser vitals <url>.

Prefer the native tool when present, else the CLI; the vocabulary is identical. Setup: the agent-browser CLI must be on PATH (and on pi, the pi-agent-browser-native extension installed). If it is missing, run the repo's scripts/doctor.sh (or pi-agent-browser-doctor) before measuring.

Procedure

1. Decide what to measure and against what

  • Pick the exact URL/page under test.
  • Measure a production-like build, never a dev build — dev bundles are unminified and slow, so their numbers are meaningless.
  • Name the comparison target up front: the perf budget (e.g. LCP ≤ 2.5s, CLS ≤ 0.1, INP ≤ 200ms) or a baseline run from before the change. Without one, "slow" has no meaning.

2. Capture vitals

  • Run agent-browser vitals <url> — native tool args on pi, the CLI on Claude Code.
  • Collect LCP, CLS, INP (or TBT as its proxy), and load/hydration timing from the output.

3. Take multiple samples

  • A single run is noisy. Run several (e.g. 5) and use the median, not the best or first.
  • Note the variance / spread across runs so you can tell a real change from jitter.

Read the full file on GitHub · 69 lines

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.

  1. 2d ago First seen · 69 lines · 91 tokens per session scan A c93a697453e4

Subscribe to this mod's changes

bench is a skill published in the GitHub repository mtthsnc/tempest (2 stars, last pushed 2mo ago), licensed MIT. It adds 91 tokens to every session and 959 once invoked, about $0.0005 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 2026-08-31.

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