# coverage-analysis

The skill reads coverage reports by risk, gates them in CI and keeps the number from sliding back. Useful when a green badge hides an untested payment module.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 1100 installs · ★ 211
- Author: kindlmann
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/petrkindlmann/qa-skills/tree/HEAD/skills/coverage-analysis/SKILL.md
- Updated: 2026-10-22
- HTML: https://affpapa.org/en/nevibecode/skills/kindlmann-coverage-analysis

## What it does

This skill pushes your agent past the vanity percentage. It gates on branch coverage, not just lines, and spells out why an executed line is not a tested one: `expect(true).toBe(true)` runs the function and asserts nothing. Mutation testing covers assertion quality.

Provider setup is part of it: `@vitest/coverage-v8`, `@vitest/coverage-istanbul`, `c8`, `nyc`, `pytest-cov`. The test runner decides the install. When V8 maps badly — uncovered lines land on blank lines, closing braces or decorators — the skill tells you to flip to Istanbul. Merging coverage from unit, integration and E2E runs is covered too, with the gate on the merged total rather than each suite alone.

The core mechanic is a ratchet: record current coverage as the floor, and every PR has to meet or beat it. New code gets a higher bar than legacy. A per-PR coverage diff shows reviewers what stayed untested. If you need the tests themselves written, that is the unit-testing skill; this one measures and gates.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add petrkindlmann/qa-skills --skill coverage-analysis -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/coverage-analysis/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/coverage-analysis/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add petrkindlmann/qa-skills --skill coverage-analysis -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/coverage-analysis/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add petrkindlmann/qa-skills --skill coverage-analysis -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/coverage-analysis/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install petrkindlmann/qa-skills coverage-analysis
```

2. **Manually: put the files in place**
- File: `.github/skills/coverage-analysis/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add petrkindlmann/qa-skills --skill coverage-analysis -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/petrkindlmann/qa-skills --path skills/coverage-analysis
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/coverage-analysis/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add petrkindlmann/qa-skills --skill coverage-analysis -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/petrkindlmann/qa-skills/tree/HEAD/skills/coverage-analysis/SKILL.md

## When to use it
- switch from line percentages to branch coverage
- stop coverage from regressing in CI with a ratchet
- show reviewers which lines in a PR have no tests
- find untested payment and auth logic
- check mutation score instead of executed lines

## Example prompts
- Configure coverage for Vitest and set a branch threshold in CI
- Show the coverage diff for this PR and sort uncovered files by risk
- Why does V8 mark blank lines as uncovered? What should I switch to?

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 95eb12b4c2f60869.

BEGIN THIRD-PARTY FILE 95eb12b4c2f60869
```text
---
name: coverage-analysis
description: >-
  Measure and improve test coverage meaningfully. Covers Istanbul/V8/coverage.py
  configuration, coverage gap analysis by risk, coverage-as-ratchet in CI (never let
  it decrease), PR coverage diff checks, mutation testing for assertion quality, and
  distinguishing meaningful from vanity coverage. Use when: "code coverage," "coverage
  gap," "Istanbul," "coverage threshold," "coverage report," "branch coverage."
  Not for: writing the tests that raise coverage — use unit-testing; coverage as a
  tracked KPI trend over time — use qa-metrics.
  Related: unit-testing, ci-cd-integration, qa-metrics, ai-qa-review.
license: MIT
metadata:
  author: kindlmann
  version: "2.0"
  category: metrics
---

<objective>
A suite at 90% line coverage with assertion-free tests catches zero bugs — line coverage
proves code ran, not that a regression would be caught. This skill measures the right
things (branch coverage, mutation score, critical-path coverage), gates them in CI with a
ratchet so coverage can only go up, and surfaces gaps by risk instead of chasing a vanity
number. It prevents the classic failure: a green coverage badge over a test suite that
never asserts anything meaningful, while the payment module sits at 30%.
</objective>

## Quick Route

| Situation | Go to |
|-----------|-------|
| Pick and configure a coverage provider | Coverage Tools → `references/tool-config.md` |
| Decide where to write tests next | Gap Analysis |
| Stop coverage from regressing in CI | Coverage as CI Gate → Ratchet Pattern |
| Show per-PR coverage to reviewers | Coverage as CI Gate → PR Diff |
| Tests run code but don't assert | Mutation Testing |
| Decide what to exclude / what target to set | Meaningful vs Vanity Coverage |

---

## Discovery Questions

Check `.agents/qa-project-context.md` first — if it exists, use it and skip anything already answered there. Then:

1. **What test runner and coverage tooling is configured?** Check for `vitest.config.*` (coverage block), `jest.config.*` (coverageProvider), `.nycrc`, `c8` in scripts, or `[tool.coverage]` in `pyproject.toml`. The runner decides the install — Vitest pulls `@vitest/coverage-v8`, not c8 (see Coverage Tools).
2. **What is the current coverage level?** Run the existing coverage command and note line, branch, and function percentages. This is the baseline for the ratchet.
3. **Is coverage gated in CI?** Check GitHub Actions / GitLab CI for `--coverage`, `coverageThreshold`, `fail_under`, or `--cov-fail-under`. No gate means coverage is decorative.
```
END THIRD-PARTY FILE 95eb12b4c2f60869

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