Test Coverage Expander

Expand unit test coverage by targeting untested branches and edge cases.

When to Use

• User asks to "increase test coverage", "add more tests", "expand unit tests", or "cover edge cases"
• A CI pipeline reports low coverage and the user wants it improved
• A code review flags untested error paths or boundary conditions
• The user wants to identify and fill gaps in an existing test suite before a release

Instructions

1. Sync the branch with remote (see Repo Sync section below)
2. Create a feature branch for the new tests
3. Run the project's coverage tool to get a baseline report
4. Identify the lowest-coverage files and untested code paths
5. Write tests for error paths, boundary values, and missing branches
6. Re-run coverage to confirm improvement
7. Commit the new tests with a descriptive message

Repo Sync Before Edits (mandatory)

Before creating/updating/deleting files in an existing repository, sync the current branch with remote:

branch="$(git rev-parse --abbrev-ref HEAD)"
git fetch origin
git pull --rebase origin "$branch"

If the working tree is not clean, stash first, sync, then restore:

git stash push -u -m "pre-sync"
branch="$(git rev-parse --abbrev-ref HEAD)"
git fetch origin && git pull --rebase origin "$branch"
git stash pop

If origin is missing, pull is unavailable, or rebase/stash conflicts occur, stop and ask the user before continuing.

Workflow

0. Create Feature Branch

Before making any changes:

1. Check the current branch - if already on a feature branch for this task, skip
2. Check the repo for branch naming conventions (e.g., feat/, feature/, etc.)
3. Create and switch to a new branch following the repo's convention, or fallback to: feat/test-coverage

1. Analyze Coverage

Detect the project's test runner and run the coverage report:

• JavaScript/TypeScript: npx jest --coverage or npx vitest --coverage
• Python: pytest --cov=. --cov-report=term-missing
• Go: go test -coverprofile=coverage.out ./...
• Rust: cargo tarpaulin or cargo llvm-cov

From the report, identify:

• Untested branches and code paths
• Low-coverage files/functions (prioritize files below 60%)
• Missing error handling tests

2. Identify Test Gaps

Review code for:

• Logical branches (if/else, switch)
• Error paths and exceptions
• Boundary values (min, max, zero, empty, null)
• Edge cases and corner cases
• State transitions and side effects

3. Write Tests

Use project's testing framework:

• JavaScript/TypeScript: Jest, Vitest, Mocha
• Python: pytest, unittest
• Go: testing, testify
• Rust: built-in test framework

Target scenarios:

• Error handling and exceptions
• Boundary conditions
• Null/undefined/empty inputs
• Concurrent/async edge cases

4. Verify Improvement

Run coverage again and confirm measurable increase. Report:

• Before/after coverage percentages
• Number of new test cases added
• Files with the biggest coverage gains

Expected Output

After a successful run on a Python project, the final verification report shows:

Coverage before: 61% (47/77 statements)
Coverage after:  84% (65/77 statements)

New tests added: 9
Files improved:
  - src/parser.py        52% → 91%  (+7 tests: null input, empty string, unicode overflow)
  - src/auth.py          71% → 88%  (+2 tests: expired token, missing header)

All 56 tests passing. No regressions.

Acceptance Criteria

A run passes when all of the following are true:

• Coverage report exists from a runnable command for the detected stack (e.g., jest --coverage, pytest --cov, go test -cover).
• Post-run total coverage is strictly higher than the pre-run baseline — no test additions that fail to move the metric.
• New tests target previously-untested branches, error paths, or boundary values — not duplicates of existing assertions.
• The full test suite passes locally before committing (npm test, pytest, go test ./..., etc.).
• All new tests live on a feature branch (e.g., feat/test-coverage), never on main/master.
• Commit message records the before/after coverage percentages and the files newly covered.

Edge Cases

• No test framework detected: Skill checks package.json, pyproject.toml, Cargo.toml, or go.mod for test dependencies; if none found, asks the user which framework to use before writing any tests.
• Coverage tool not installed: Installs the appropriate tool (pytest-cov, nyc, cargo tarpaulin, etc.) and retries rather than failing silently.
• Existing tests are already failing: Does not add new tests until existing failures are resolved; reports the failing tests to the user first.
• 100% coverage already reached: Reports this to the user and exits — no tests are added unnecessarily.
• Generated code or vendored files in coverage report: Excludes auto-generated and third-party directories from analysis to avoid writing tests for code the project does not own.
• Async / concurrent code paths: Uses framework-appropriate async test utilities (e.g., pytest-asyncio, jest fakeTimers) rather than bare sync wrappers.

Step Completion Reports

After completing each major step, output a status report in this format:

◆ [Step Name] ([step N of M] — [context])
··································································
  [Check 1]:          √ pass
  [Check 2]:          √ pass (note if relevant)
  [Check 3]:          × fail — [reason]
  [Check 4]:          √ pass
  [Criteria]:         √ N/M met
  ____________________________
  Result:             PASS | FAIL | PARTIAL

Adapt the check names to match what the step actually validates. Use √ for pass, × for fail, and — to add brief context. The "Criteria" line summarizes how many acceptance criteria were met. The "Result" line gives the overall verdict.

Branch Setup phase checks: Feature branch created, Base coverage measured

Analysis phase checks: Coverage report parsed, Gaps identified, Priority ranked

Test Writing phase checks: Tests written, Edge cases covered, Framework conventions followed

Verification phase checks: Tests pass, Coverage improved, No regressions

Error Handling

No test framework detected

Solution: Check package.json, pyproject.toml, Cargo.toml, or go.mod for test dependencies. If none found, ask the user which framework to use and install it.

Coverage tool not installed

Solution: Install the appropriate coverage tool (nyc, pytest-cov, etc.) and retry.

Existing tests failing

Solution: Do not add new tests until existing failures are resolved. Report failing tests to the user first.

Guidelines

• Follow existing test patterns and naming conventions
• Place test files alongside source or in the project's existing test directory
• Group related test cases logically
• Use descriptive test names that explain the scenario
• Do not mock what you do not own — prefer integration tests for external boundaries