Test Coverage Analyzer Skill

Test Coverage Analyzer

Current State

!ls package.json pyproject.toml Cargo.toml go.mod 2>/dev/null || echo 'No project manifest found' !node -v 2>/dev/null || python3 --version 2>/dev/null || echo 'No runtime detected'

Overview

Analyze code coverage metrics to identify untested code paths, dead code, and coverage gaps across line, branch, function, and statement dimensions. Supports Istanbul/nyc (JavaScript/TypeScript), coverage.py (Python), JaCoCo (Java), and Go coverage tools.

Prerequisites

Coverage tool installed and configured (Istanbul/nyc, c8, coverage.py, JaCoCo, or go test -cover)
Test suite that can run with coverage instrumentation enabled
Coverage threshold targets defined (recommended: 80% lines, 70% branches)
Coverage output format set to JSON or LCOV for programmatic analysis
Git history available for coverage trend comparison

Instructions

Run the test suite with coverage instrumentation enabled:
- JavaScript: npx jest --coverage --coverageReporters=json-summary,lcov
- Python: pytest --cov=src --cov-report=json --cov-report=term-missing
- Go: go test -coverprofile=coverage.out ./...
- Java: Configure JaCoCo Maven/Gradle plugin with XML report output.
Parse the coverage report and extract per-file metrics:
- Line coverage percentage per file.
- Branch coverage percentage per file.
- Function coverage percentage per file.
- Uncovered line ranges (specific line numbers).
Identify critical coverage gaps by prioritizing:
- Files with coverage below the threshold (sort ascending by coverage %).
- Files with high complexity but low coverage (use cyclomatic complexity if available).
- Recently modified files with decreasing coverage trends.
- Public API functions and exported modules lacking any tests.
Analyze uncovered branches specifically:
- Find if/else blocks where only one branch is tested.
- Identify switch/case statements with missing case coverage.
- Locate error handling paths (catch, except) never exercised.
- Check || and && short-circuit conditions.
Generate a prioritized action plan:
- List top 10 files needing coverage improvement with specific line ranges.
- Suggest test scenarios for each uncovered branch.
- Estimate effort (small/medium/large) for each coverage improvement.
Compare current coverage against the previous commit or baseline:
- Calculate coverage delta per file.
- Flag files where coverage decreased.
- Verify new code added since baseline has adequate coverage.
Write coverage enforcement configuration (coverage thresholds in Jest config, .coveragerc, or CI checks).

Output

Coverage summary report with overall and per-file metrics
Prioritized list of uncovered code paths with file:line references
Coverage delta report comparing against baseline
Suggested test cases for top coverage gaps
Coverage threshold configuration file for CI enforcement

Error Handling

| Error | Cause | Solution | |-------|-------|---------| | Coverage report shows 0% | Tests ran but coverage instrumentation was not enabled | Verify --coverage flag is passed; check that source files are not excluded by config | | Coverage includes node_modules | Coverage collection scope too broad | Add collectCoverageFrom in Jest config; set --cov=src in pytest; exclude vendor dirs | | Branch coverage much lower than line coverage | Conditional logic is only partially tested | Write tests for both truthy and falsy conditions on each branch; focus on edge cases | | Coverage drops after refactor | New code added without corresponding tests | Set coverageThreshold with global minimums; add --changedSince for incremental checks | | Flaky coverage numbers between runs | Non-deterministic test execution skipping code paths | Sort tests deterministically; run coverage with --runInBand for consistent results |

Examples

Jest coverage configuration with thresholds:

{
  "jest": {
    "coverageThreshold": {
      "global": {
        "branches": 70,
        "functions": 80,
        "lines": 80,
        "statements": 80
      },
      "src/utils/": {
        "branches": 90,
        "lines": 95
      }
    },
    "collectCoverageFrom": [
      "src/**/*.{ts,tsx}",
      "!src/**/*.d.ts",
      "!src/**/index.ts"
    ]
  }
}

Coverage gap analysis output:

Coverage Gaps (sorted by impact):
1. src/auth/oauth.ts        Lines: 45%  Branches: 30%  [Lines 42-67, 89-103 uncovered]
   Suggestion: Add tests for token refresh failure and expired session handling
2. src/api/middleware.ts     Lines: 62%  Branches: 41%  [Lines 28-35, 71-80 uncovered]
   Suggestion: Test rate limiting edge cases and malformed header handling
3. src/utils/parser.ts       Lines: 71%  Branches: 55%  [Lines 112-130 uncovered]
   Suggestion: Test malformed input, empty string, and encoding edge cases

Resources

Istanbul/nyc: https://istanbul.js.org/
c8 (Node.js native coverage): https://github.com/bcoe/c8
coverage.py: https://coverage.readthedocs.io/
JaCoCo: https://www.jacoco.org/jacoco/
Go test coverage: https://go.dev/blog/cover
Code coverage best practices: https://testing.googleblog.com/2020/08/code-coverage-best-practices.html

Agent Skills: Test Coverage Analyzer

Install this agent skill to your local

Skill Files