AI agent skill · by @proffesor-for-testing

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.

Archived 2026-10-23 · Source updated 2026-09-13 · License: MIT. Check the original repository for the latest version.

SKILL.md instructions

# 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 coveragePathIgnorePatterns