Test Impact Analyzer

Determine which tests need to run for a given code change — trace file dependencies, map source-to-test relationships, identify untested changes, and prioritize test execution order for faster CI feedback.

Don't run all tests for every change. Analyze which source files changed, trace their dependencies, find the corresponding tests, and produce a targeted test execution plan. Faster CI, focused testing, immediate feedback.

Use when: "which tests should I run", "what does this change affect", "test impact analysis", "optimize CI test time", "what tests cover this file", "skip unrelated tests", or speeding up CI pipelines.

Commands

1. affected — Find Tests Affected by Changes

Given a set of changed files (from git diff), find all tests that should run.

bash

# Get changed files (compared to main/master)
BASE_BRANCH="${1:-main}"
CHANGED_FILES=$(git diff --name-only "$BASE_BRANCH"...HEAD 2>/dev/null || git diff --name-only HEAD~1 2>/dev/null)

if [ -z "$CHANGED_FILES" ]; then
  echo "No changed files detected. Specify base branch or ensure you're on a feature branch."
  exit 0
fi

echo "Changed files:"
echo "$CHANGED_FILES" | sed 's/^/  /'
echo ""

Step 1: Direct Test Matches

bash

echo "=== Direct Test Matches ==="
echo "$CHANGED_FILES" | while read f; do
  # Skip non-source files
  echo "$f" | grep -qE '\.(ts|js|tsx|jsx|py|go|rs|java)$' || continue
  # Skip test files themselves
  echo "$f" | grep -qE '\.(test|spec)\.' && continue

  BASE=$(basename "$f" | sed 's/\.[^.]*$//')
  DIR=$(dirname "$f")

  # Find corresponding test file
  for pattern in "${BASE}.test.ts" "${BASE}.test.js" "${BASE}.test.tsx" "${BASE}.test.jsx" \
                  "${BASE}.spec.ts" "${BASE}.spec.js" "${BASE}.spec.tsx" \
                  "test_${BASE}.py" "${BASE}_test.py" "${BASE}_test.go"; do
    FOUND=$(find "$DIR" -maxdepth 2 -name "$pattern" -not -path '*/node_modules/*' 2>/dev/null | head -1)
    if [ -n "$FOUND" ]; then
      echo "  $f → $FOUND"
      break
    fi
  done
done

Step 2: Import Chain Analysis

bash

echo ""
echo "=== Import Chain (files that import changed files) ==="
echo "$CHANGED_FILES" | while read f; do
  echo "$f" | grep -qE '\.(ts|js|tsx|jsx)$' || continue
  echo "$f" | grep -qE '\.(test|spec)\.' && continue

  BASE=$(basename "$f" | sed 's/\.[^.]*$//')
  DIR=$(dirname "$f")

  # Find files that import this module
  # Match relative imports like: from './module' or from '../utils/module'
  IMPORTERS=$(rg -l "from ['\"]\..*/${BASE}['\"]|from ['\"]\./${BASE}['\"]|require\(['\"]\..*/${BASE}['\"]\)" \
    -g '*.{ts,js,tsx,jsx}' -g '!node_modules' -g '!dist' 2>/dev/null)

  if [ -n "$IMPORTERS" ]; then
    echo "  $f is imported by:"
    echo "$IMPORTERS" | while read imp; do
      IMP_BASE=$(basename "$imp" | sed 's/\.[^.]*$//')
      # Is the importer a test file?
      if echo "$imp" | grep -qE '\.(test|spec)\.'; then
        echo "    🧪 $imp (test — should run)"
      else
        # Check if the importer has its own test
        TEST=$(find "$(dirname "$imp")" -maxdepth 2 \
          -name "${IMP_BASE}.test.*" -o -name "${IMP_BASE}.spec.*" 2>/dev/null | head -1)
        if [ -n "$TEST" ]; then
          echo "    📄 $imp → 🧪 $TEST (transitive)"
        else
          echo "    📄 $imp (no test found)"
        fi
      fi
    done
  fi
done

Step 3: Collect All Tests to Run

bash

echo ""
echo "=== Test Execution Plan ==="

# Collect unique test files
TESTS_TO_RUN=$(mktemp)

echo "$CHANGED_FILES" | while read f; do
  echo "$f" | grep -qE '\.(test|spec)\.' && echo "$f" >> "$TESTS_TO_RUN"

  BASE=$(basename "$f" | sed 's/\.[^.]*$//')
  DIR=$(dirname "$f")

  # Direct test matches
  find "$DIR" -maxdepth 2 \( -name "${BASE}.test.*" -o -name "${BASE}.spec.*" -o -name "test_${BASE}.*" -o -name "${BASE}_test.*" \) \
    -not -path '*/node_modules/*' 2>/dev/null >> "$TESTS_TO_RUN"

  # Tests from importers
  IMPORTERS=$(rg -l "from ['\"]\..*/${BASE}['\"]|from ['\"]\./${BASE}['\"]" \
    -g '*.{test.*,spec.*}' -g '!node_modules' 2>/dev/null)
  echo "$IMPORTERS" >> "$TESTS_TO_RUN" 2>/dev/null
done

UNIQUE_TESTS=$(sort -u "$TESTS_TO_RUN" | grep -v '^$')
TOTAL=$(echo "$UNIQUE_TESTS" | grep -c "." 2>/dev/null || echo "0")
ALL_TESTS=$(find . -type f \( -name "*.test.*" -o -name "*.spec.*" -o -name "test_*" \) \
  -not -path '*/node_modules/*' 2>/dev/null | wc -l)

echo "Tests to run: $TOTAL / $ALL_TESTS total ($(echo "scale=0; $TOTAL * 100 / ($ALL_TESTS + 1)" | bc)%)"
echo ""
echo "$UNIQUE_TESTS" | sed 's/^/  /'

rm -f "$TESTS_TO_RUN"

2. map — Source-to-Test Mapping

Build a complete map of which source files are covered by which tests.

bash

echo "=== Source → Test Map ==="

find . -type f \( -name "*.ts" -o -name "*.js" -o -name "*.tsx" -o -name "*.jsx" -o -name "*.py" -o -name "*.go" \) \
  -not -path '*/node_modules/*' -not -path '*/vendor/*' -not -path '*/dist/*' \
  -not -name '*.test.*' -not -name '*.spec.*' -not -name 'test_*' 2>/dev/null | sort | while read f; do
  BASE=$(basename "$f" | sed 's/\.[^.]*$//')
  DIR=$(dirname "$f")

  TEST=$(find "$DIR" -maxdepth 2 \( -name "${BASE}.test.*" -o -name "${BASE}.spec.*" -o -name "test_${BASE}.*" -o -name "${BASE}_test.*" \) \
    -not -path '*/node_modules/*' 2>/dev/null | head -1)

  if [ -n "$TEST" ]; then
    echo "✅ $f → $TEST"
  else
    echo "�� $f → (no test)"
  fi
done

Summary stats:

text

Coverage map: X/Y files have corresponding tests (Z%)
Untested critical files (by size):
  1. src/core/engine.ts (450 lines) — NO TEST
  2. src/api/handlers.ts (320 lines) — NO TEST

3. gaps — Find Untested Code Paths

Identify source files without tests, prioritized by risk.

bash

echo "=== Untested Files (by risk) ==="

python3 -c "
import subprocess, os

# Get all source files
src = subprocess.run(
    ['find', '.', '-type', 'f', '(', '-name', '*.ts', '-o', '-name', '*.js', '-o', '-name', '*.py', ')',
     '-not', '-path', '*/node_modules/*', '-not', '-path', '*/dist/*',
     '-not', '-name', '*.test.*', '-not', '-name', '*.spec.*', '-not', '-name', 'test_*'],
    capture_output=True, text=True
).stdout.strip().split('\n')

untested = []
for f in src:
    if not f: continue
    base = os.path.splitext(os.path.basename(f))[0]
    d = os.path.dirname(f)

    # Check for test file
    has_test = False
    for pattern in [f'{base}.test', f'{base}.spec', f'test_{base}', f'{base}_test']:
        result = subprocess.run(
            ['find', d, '-maxdepth', '2', '-name', f'{pattern}.*'],
            capture_output=True, text=True
        )
        if result.stdout.strip():
            has_test = True
            break

    if not has_test:
        # Get file size and git churn
        try:
            lines = sum(1 for _ in open(f))
        except:
            lines = 0

        try:
            churn = int(subprocess.run(
                ['git', 'log', '--since=90 days ago', '--oneline', '--', f],
                capture_output=True, text=True
            ).stdout.strip().count('\n')) + 1
        except:
            churn = 0

        risk = lines * 0.5 + churn * 10  # larger + more active = higher risk
        untested.append((risk, lines, churn, f))

untested.sort(reverse=True)
print(f'Untested files: {len(untested)} / {len(src)} ({len(untested)*100//max(len(src),1)}%)')
print()
print('Top 20 by risk (size × activity):')
for risk, lines, churn, f in untested[:20]:
    print(f'  [{lines:>4} lines, {churn:>2} commits] {f}')
" 2>/dev/null

4. order — Optimal Test Execution Order

Prioritize test execution for fastest feedback:

1. Tests for changed files — most likely to fail, run first
2. Tests for files importing changed files — transitive impact
3. Integration/E2E tests — broader coverage, run if unit tests pass
4. Everything else — only in full CI run

bash

echo "=== Recommended Test Order ==="
echo ""
echo "Phase 1 — Direct (run immediately, ~seconds):"
# Changed files' tests
echo "Phase 2 — Transitive (if Phase 1 passes, ~minutes):"
# Tests that import changed modules
echo "Phase 3 ��� Integration (if Phase 2 passes, ~minutes):"
# E2E/integration tests in affected areas
echo "Phase 4 — Full suite (nightly/merge, ~minutes-hours):"
# Everything

5. ci — Generate CI Test Commands

Output the exact commands to run only affected tests.

bash

# For Jest
JEST_TESTS=$(echo "$UNIQUE_TESTS" | grep -E '\.(test|spec)\.(ts|js|tsx|jsx)$' | tr '\n' ' ')
if [ -n "$JEST_TESTS" ]; then
  echo "Jest command:"
  echo "  npx jest --passWithNoTests $JEST_TESTS"
fi

# For pytest
PYTEST_TESTS=$(echo "$UNIQUE_TESTS" | grep -E '(test_.*\.py|.*_test\.py)$' | tr '\n' ' ')
if [ -n "$PYTEST_TESTS" ]; then
  echo "Pytest command:"
  echo "  pytest $PYTEST_TESTS"
fi

# For Go
GO_TESTS=$(echo "$UNIQUE_TESTS" | grep -E '_test\.go$' | xargs -I{} dirname {} | sort -u | tr '\n' ' ')
if [ -n "$GO_TESTS" ]; then
  echo "Go test command:"
  echo "  go test $GO_TESTS"
fi

Output Formats

• text (default): Human-readable with file tree
• json: {changed_files: [], affected_tests: [], untested_changes: [], execution_plan: {phases: []}}
• paths: Plain list of test file paths (pipe to test runner)

CI Integration

yaml

# GitHub Actions — run only affected tests
- name: Find affected tests
  id: tests
  run: |
    # Agent runs: test-impact-analyzer affected ${{ github.event.pull_request.base.ref }} --format paths > affected-tests.txt
    echo "count=$(wc -l < affected-tests.txt)" >> $GITHUB_OUTPUT

- name: Run affected tests
  if: steps.tests.outputs.count > 0
  run: npx jest $(cat affected-tests.txt | tr '\n' ' ')

- name: Full test suite
  if: steps.tests.outputs.count == 0
  run: npm test

Exit codes:

• 0: All changed code has test coverage
• 1: Some changed code is untested (lists the files)
• 2: No tests found at all

Notes

• Uses file naming conventions for test matching (file.test.ts, test_file.py, file_test.go)
• Import chain analysis is 1 level deep by default — use --depth 2 for transitive imports
• Does not parse code ASTs — uses grep/ripgrep patterns for speed
• Works best when test files are co-located or follow naming conventions
• For monorepos: respects workspace boundaries when tracing imports
• Not a replacement for code coverage tools — this is pre-execution analysis, not runtime coverage