Usage

Apply this skill when:

• A test suite exists and you need to find what is NOT tested

• A coverage report (coverage.py, Istanbul, JaCoCo) is provided for analysis

• A developer asks "what am I missing?" after initial test generation

• Pre-sprint planning requires coverage gap estimation

Coverage Types — Know All Four

Type What It Measures Target

Line Coverage Were these lines executed?

= 80%

Branch Coverage Were both sides of every if/else hit?

= 75%

Function Coverage Was every function called at least once?

= 90%

Statement Coverage Were all statements executed?

= 80%

Branch coverage is the most valuable — prioritize it.

Coverage Targets in Practice

A passing coverage percentage does not guarantee quality. Apply these rules:

• Critical paths (authentication, payments, data integrity, error handling) must reach 80%+ line and branch coverage — treat anything below as a HIGH gap

• Supporting utilities may have lower targets but must still cover error paths

• High coverage with weak assertions is worse than moderate coverage with strong ones

• Unit test coverage does not replace integration test coverage — note this distinction explicitly in every gap report when critical cross-service paths are involved

TDD and Coverage

In a TDD workflow, coverage gaps indicate missing Red-Green cycles — not just missing tests. Each uncovered branch represents a behavior that was never defined as a failing test first. Flag this in reports: coverage gaps in a TDD project suggest incomplete feature development, not just incomplete testing.

Analysis Workflow

Read references/index.md before executing any step.

Step 1 — Parse Existing Coverage Data

If a coverage report is provided (XML, JSON, HTML), extract:

• Overall coverage percentage per file

• Line numbers with 0 hits (never executed)

• Branch pairs where one side was never taken

• Functions with 0 calls

If no report exists, analyze source code structurally:

• Count all conditional branches (if, elif, else, switch cases, ternary)

• Count all exception handlers (try/except, try/catch)

• Count all public functions and methods

• Cross-reference against existing test file imports and call sites

Step 2 — Classify Each Gap by Severity

Severity Criteria Priority

CRITICAL Core business logic path untested Fix immediately

HIGH Error/exception handler untested Fix this sprint

MEDIUM Edge case or boundary value untested Fix next sprint

LOW Trivial getter/setter, logging line Acceptable gap

Step 3 — Produce the Gap Report

Output format:

Coverage Gap Report

Module: payment_processor.py Current Coverage: 64% line | 51% branch Target: 80% line | 75% branch Gap to Close: 16% line | 24% branch

Critical Gaps (must fix)

High Gaps (fix this sprint)

Recommended New Tests

1. test_process_partial_payment_when_valid_expects_success
2. test_check_refund_eligibility_when_ineligible_expects_false
3. test_parse_amount_when_invalid_string_expects_value_error

Step 4 — Estimate Effort to Close Gap

For each recommended test, estimate:

• Complexity: LOW | MEDIUM | HIGH (based on mocking needs and logic depth)

• Estimated lines of test code: ~5-20 per simple test, ~30-60 per complex mock test

• Total tests needed to reach target coverage

Coverage Tool Integration

Python — coverage.py

coverage run -m pytest coverage report --show-missing coverage json -o coverage.json # for programmatic analysis

JavaScript — Istanbul (nyc / c8)

npx c8 --reporter=json npm test

Outputs: coverage/coverage-final.json

Java — JaCoCo (Gradle)

./gradlew test jacocoTestReport

Outputs: build/reports/jacoco/test/jacocoTestReport.xml

C# — coverlet

dotnet test --collect:"XPlat Code Coverage"

Outputs: coverage.cobertura.xml

Coverage Anti-Patterns to Flag

• Tests that only cover happy paths (missing negative/error branches)

• Parametrized tests that share a single code path (not truly multi-branch)

• Mocks that are too permissive (mock returns True for everything — false coverage)

• Integration tests counted as unit test coverage (different scope)

Output Deliverables

• coverage_gap_report.md — prioritized gap analysis

• recommended_tests.md — specific test names and descriptions to write

• coverage_delta_estimate.md — projected coverage % after recommended tests are added