coverage-drop-investigator
Use when test coverage has dropped and you need to find which changes caused it and what tests to add. Traces coverage regressions to specific commits and files.
Best use case
coverage-drop-investigator is best used when you need a repeatable AI agent workflow instead of a one-off prompt.
Use when test coverage has dropped and you need to find which changes caused it and what tests to add. Traces coverage regressions to specific commits and files.
Teams using coverage-drop-investigator should expect a more consistent output, faster repeated execution, less prompt rewriting.
When to use this skill
- You want a reusable workflow that can be run more than once with consistent structure.
When not to use this skill
- You only need a quick one-off answer and do not need a reusable workflow.
- You cannot install or maintain the underlying files, dependencies, or repository context.
Installation
Claude Code / Cursor / Codex
Manual Installation
- Download SKILL.md from GitHub
- Place it in
.claude/skills/coverage-drop-investigator/SKILL.mdinside your project - Restart your AI agent — it will auto-discover the skill
How coverage-drop-investigator Compares
| Feature / Agent | coverage-drop-investigator | Standard Approach |
|---|---|---|
| Platform Support | Not specified | Limited / Varies |
| Context Awareness | High | Baseline |
| Installation Complexity | Unknown | N/A |
Frequently Asked Questions
What does this skill do?
Use when test coverage has dropped and you need to find which changes caused it and what tests to add. Traces coverage regressions to specific commits and files.
Where can I find the source code?
You can find the source code on GitHub using the link provided at the top of the page.
SKILL.md Source
# Coverage Drop Investigator
Runbook-style skill for investigating coverage regressions. Identifies which changes caused coverage to drop and recommends targeted tests.
## Activation
```
/coverage-drop-investigator
```
## Investigation Flow
### Step 1: Measure Current Coverage
```bash
# Generate coverage report
npx jest --coverage --coverageReporters=json-summary
# View summary
cat coverage/coverage-summary.json | jq '.total'
```
### Step 2: Find When Coverage Dropped
```bash
# Compare coverage with main branch
git stash && npx jest --coverage --coverageReporters=json-summary
mv coverage/coverage-summary.json coverage/baseline.json
git stash pop && npx jest --coverage --coverageReporters=json-summary
# Compare
jq -s '.[0].total.statements.pct as $baseline | .[1].total.statements.pct as $current | {baseline: $baseline, current: $current, delta: ($current - $baseline)}' coverage/baseline.json coverage/coverage-summary.json
```
### Step 3: Identify Uncovered Files
```bash
# Find files with lowest coverage
cat coverage/coverage-summary.json | jq 'to_entries | map(select(.key != "total")) | sort_by(.value.statements.pct) | .[0:10] | .[] | {file: .key, statements: .value.statements.pct, branches: .value.branches.pct}'
```
### Step 4: Map to Recent Changes
```bash
# Find recently changed files with low coverage
git diff --name-only main...HEAD | while read file; do
jq --arg f "$file" '.[$f] // empty | {file: $f, statements: .statements.pct}' coverage/coverage-summary.json
done
```
### Step 5: Recommend Tests
For each uncovered file, identify:
1. **Uncovered functions** — need new test cases
2. **Uncovered branches** — need conditional test cases (if/else paths)
3. **Uncovered lines** — may indicate dead code or missing edge cases
### Step 6: Report
```markdown
## Coverage Drop Report
- **Current**: {{current_pct}}%
- **Baseline (main)**: {{baseline_pct}}%
- **Delta**: {{delta}}%
- **Files causing drop**:
| File | Coverage | Changed Lines | Tests Needed |
|------|----------|--------------|-------------|
| {{file}} | {{pct}}% | {{lines}} | {{count}} |
- **Recommended action**: {{write_tests / accept_drop / mark_as_excluded}}
```
## Composition
After investigation:
- **`/qe-test-generation`** — generate tests for uncovered files
- **`/mutation-testing`** — verify existing tests actually catch bugs
- **`/coverage-guard`** — prevent future drops
## Gotchas
- Coverage can drop because NEW code was added without tests, not because tests were removed
- 100% coverage is not always the goal — focus on critical paths, not getters/setters
- Branch coverage drops are more concerning than line coverage drops — branches indicate logic paths
- Coverage tools may count generated code or type definitions — exclude with coveragePathIgnorePatternsRelated Skills
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