skill-coverage-audit

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• SKILL.md

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# Test Coverage Audit

## Overview

Trace every codepath in a diff, map each path against existing tests, visualize coverage gaps, and auto-generate tests for uncovered paths.

**Core principle:** Trace codepaths in changed files -> Map against existing tests -> Score coverage quality -> Generate tests for gaps -> Report before/after counts.

---

## Caps and Limits

These hard limits prevent runaway analysis:

- **30 code paths max** per audit. If a diff yields more than 30, prioritize by complexity and risk (error paths, security-sensitive branches, public API surfaces first).
- **20 tests generated max** per audit. Focus on highest-impact gaps first.
- **2-minute per-test exploration cap.** If understanding a single test path takes longer than 2 minutes, mark it as "needs manual review" and move on.

---

## Phase 1: Codepath Tracing

### Step 1: Identify Changed Files

Determine the diff scope. Use the most relevant source:

```bash
# PR diff
git diff --name-only main...HEAD

# Staged changes
git diff --name-only --cached

# Last commit
git diff --name-only HEAD~1..HEAD
```

Filter to source code files only (exclude configs, docs, generated files).

### Step 2: Trace Data Flow Through Every Branch

For each changed file, you MUST trace:

1. **Conditionals** -- Every `if/else`, `switch/case`, ternary, and pattern match. Each branch is a separate codepath.
2. **Error paths** -- Every `catch`, `throw`, error return, validation failure, and early return with error. WHY: Error paths are the most common source of untested bugs.
3. **Function calls** -- Every function invoked from changed code. Trace one level deep into callees to identify integration boundaries.
4. **Loop boundaries** -- Empty collection, single item, and multi-item paths through loops.
5. **Guard clauses** -- Every early return, null check, and permission gate.

### Step 3: Build the Codepath Inventory

Produce a structured inventory:

```markdown
## Codepath Inventory: [filename]

| # | Path Description | Type | Risk |
|---|-----------------|------|------|
| 1 | validateUser() happy path | conditional | low |
| 2 | validateUser() missing email | error | medium |
| 3 | validateUser() invalid format | error | medium |
| 4 | processOrder() empty cart guard | guard | high |
| 5 | processOrder() payment timeout | error | high |
| 6 | processOrder() success | conditional | low |
```

**Type categories:** `conditional`, `error`, `guard`, `loop-boundary`, `integration`, `async`

**Risk assessment:** `high` = user-facing failure or data loss, `medium` = degraded behavior, `low` = cosmetic or logging

---

## Phase 2: Test Mapping and Quality Scoring

### Step 1: Search for Existing Tests

For each file in the diff, search the test directory for related tests:

```bash
# Find test files that reference the changed file or its exports
# Search by filename pattern
find tests/ -name "*[changed_file_stem]*" -type f

# Search by import/require of the changed module
grep -rl "import.*from.*[module_name]" tests/
grep -rl "require.*[module_name]" tests/

# Search by function name references
grep -rl "[function_name]" tests/
```

### Step 2: Score Test Quality

For each codepath, assess existing test coverage with this rubric:

| Rating | Meaning | Criteria |
|--------|---------|----------|
| ★★★ | Behavior + edge cases tested | Tests assert behavior AND cover boundary conditions, error cases, and edge inputs |
| ★★ | Happy path tested | Tests cover the success path but miss error branches or edge cases |
| ★ | Smoke test only | Test exists but only checks the function runs without error (no meaningful assertions) |
| ☆ | No test found | No test references this codepath at all |

### Step 3: Produce Coverage Map

Map each codepath to its test coverage:

```markdown
## Coverage Map: [filename]

| # | Codepath | Test File | Rating | Notes |
|---|----------|-----------|--------|-------|
| 1 | validateUser() happy path | test-user.sh:42 | ★★★ | Asserts valid + invalid inputs |
| 2 | validateUser() missing email | test-user.sh:58 | ★★ | Tests missing, not malformed |
| 3 | validateUser() invalid format | -- | ☆ | No test for format validation |
| 4 | processOrder() empty cart guard | -- | ☆ | Guard clause untested |
| 5 | processOrder() payment timeout | test-orders.sh:30 | ★ | Checks no crash, no assertions |
| 6 | processOrder() success | test-orders.sh:15 | ★★★ | Full integration test |
```

From mhdxbilal/Ai, MIT.

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