analyzing-test-coverage

name	analyzing-test-coverage
description	Analyze code coverage metrics and identify untested code paths. Use when analyzing untested code or coverage gaps. Trigger with phrases like "analyze coverage", "check test coverage", or "find untested code".
allowed-tools	Read, Write, Edit, Grep, Glob, Bash(test:coverage-*)
version	1.24.0
author	Jeremy Longshore <[e-mail]>
license	MIT
tags	["testing","analyzing-test"]
compatibility	Designed for Claude Code

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

1. 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.
2. 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).
3. 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.
4. 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.
5. 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.
6. 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.
7. 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