# Sddevelopment Be Quickstart Agent Augmented Development Fix Bug > Fix Bug: Test-First Bug Fixing - Skill: `tomevault-io/sddevelopment-be-quickstart-agent-augmented-development-fix-` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add tomevault-io/sddevelopment-be-quickstart-agent-augmented-development-fix-` - Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/sddevelopment-be-quickstart-agent-augmented-development-fix-/raw - Safety review: pending (external: skill-scanner PASS, skillspector PASS) - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: AI & ML - Author: tomevault-io (https://skillmd.com/u/tomevault-io) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/tomevault-io/sddevelopment-be-quickstart-agent-augmented-development-fix- --- # Fix Bug: Test-First Bug Fixing Resolve software defects using a disciplined test-first approach. Write a failing test that reproduces the bug BEFORE modifying any production code. ## Instructions **Phase 1: Write a Failing Test (DO THIS FIRST)** 1. **Choose test level:** - Unit test (isolated component/function) - Integration test (component interaction) - Acceptance test (end-to-end behavior) 2. **Write test that reproduces the bug:** ```python # Python example def test_bug_description(): """Bug: [Describe what's wrong]""" # Arrange: Set up scenario that triggers bug input_data = create_bug_triggering_input() # Act: Execute the failing code result = system_under_test.process(input_data) # Assert: What SHOULD happen (test will FAIL) assert result == expected_correct_behavior ``` ```java // Java example @Test @DisplayName("Bug: [Describe what's wrong]") void reproduceBug() { // Arrange: Set up scenario that triggers bug var input = createBugTriggeringInput(); // Act: Execute the failing code var result = systemUnderTest.process(input); // Assert: What SHOULD happen (test will FAIL) assertThat(result).isEqualTo(expectedCorrectBehavior); } ``` 3. **Run the test - MUST FAIL:** - If test passes, it doesn't reproduce the bug - Adjust test until it fails 4. **Verify failure reason:** - Change assertion to expect WRONG behavior - Test should now PASS - Revert assertion back to correct behavior - This proves test reproduces the actual bug 5. **Commit the failing test:** ```bash git add tests/... git commit -m "test: reproduce bug - [description] Failing test demonstrates [bug behavior]. Expected: [correct behavior] Actual: [buggy behavior] Co-Authored-By: Claude Sonnet 4.5 " ``` **Phase 2: Fix the Code** 1. **Minimal change:** - Use test as guide for debugging - Make smallest change to pass test - Avoid "while I'm here" refactoring 2. **Run the test - MUST PASS:** - Test transitions from RED → GREEN - This proves bug is fixed 3. **Run ALL tests:** - Ensure no regressions - All existing tests must still pass 4. **Refactor if needed:** - Improve code quality - Keep tests passing **Phase 3: Commit the Fix** ```bash git add src/... git commit -m "fix: [description of bug fix] Resolves issue where [bug behavior] occurred. Root cause: [explanation] Fix: [what was changed] Test: tests/.../test_bug_reproduction.py Co-Authored-By: Claude Sonnet 4.5 " ``` ## Golden Rules ❌ **DO NOT:** - Make changes to production code before writing test - Run full application to debug - Create side-scripts to investigate - Deploy and test manually - Guess and check - Skip writing test because "I know what's wrong" ✅ **DO:** - Write failing test FIRST (no exceptions) - Verify test fails for the RIGHT reason - Make minimal change to pass test - Run all tests before committing - Commit test and fix together - Document root cause in commit message ## Why This Works **Traditional Approach (Anti-Pattern):** ``` 1. Guess where problem is 2. Change production code 3. Deploy and test manually 4. Still broken? Go to step 1 5. Eventually works (you think) 6. Hope you didn't break anything ``` **Problems:** - ❌ No proof bug is fixed - ❌ No proof nothing else broke - ❌ Wastes time on deployments - ❌ Bug can return unnoticed **Test-First Approach:** ``` 1. Write failing test (reproduces bug) 2. Fix production code 3. Test passes 4. Run all tests 5. Done - proven fixed ``` **Benefits:** - ✅ Proof bug existed - ✅ Proof bug is fixed - ✅ Proof nothing else broke - ✅ Fast feedback (no deployment) - ✅ Regression prevention - ✅ Documents the issue ## Example Workflow ``` User: Bug report - API returns 500 when input contains special characters Agent (Backend-Dev): Phase 1: Reproduce bug with test Created: tests/test_api_special_chars.py def test_api_handles_special_characters(): \"\"\"Bug: API returns 500 for input with special chars\"\"\" client = TestClient(app) response = client.post("/api/process", json={ "text": "Hello & friends!" }) # Should return 200, not 500 assert response.status_code == 200 assert response.json()["status"] == "success" Running test... ❌ FAILED AssertionError: assert 500 == 200 Good! Test reproduces the bug. Committed: git commit -m "test: reproduce API 500 with special chars" Phase 2: Fix the code Root cause: Input sanitization missing for HTML entities Fixed: src/api/processor.py - Added html.escape() before processing input Running test... ✅ PASSED Running all tests... ✅ 127/127 PASSED Phase 3: Commit fix git commit -m "fix: sanitize HTML entities in API input Resolves 500 error when input contains <, >, &, etc. Root cause: Missing input sanitization Fix: Added html.escape() in processor Test: tests/test_api_special_chars.py Co-Authored-By: Claude Sonnet 4.5 " ✅ Bug fixed with proof (test passes) ✅ Regression prevented (test stays in suite) ``` ## Test Level Selection Guide **Unit Test** - Choose when: - Bug in single function/method - Isolated component behavior wrong - Logic error in calculation - Example: "Function returns wrong value for edge case" **Integration Test** - Choose when: - Bug in component interaction - Data flow between modules broken - API contract violated - Example: "Service A sends wrong format to Service B" **Acceptance Test** - Choose when: - End-to-end user flow broken - Full feature behaves incorrectly - System-level behavior wrong - Example: "User cannot complete checkout process" **Start with smallest test that reproduces bug** (usually unit test). ## Common Pitfalls **Pitfall 1: "I'll write test after fixing"** - ❌ Never works - you forget or run out of time - ✅ Test FIRST ensures it gets written **Pitfall 2: "Test passes but bug still exists"** - ❌ Test doesn't reproduce actual bug - ✅ Verify test fails for RIGHT reason (change assertion to wrong value, should pass) **Pitfall 3: "Too hard to write test"** - ❌ Usually means code is hard to test (design smell) - ✅ Use this as opportunity to improve testability **Pitfall 4: "I fixed it manually, works now"** - ❌ No proof, bug can return - ✅ Capture fix in test for regression prevention ## Related Skills - `/iterate` - Use after fixing bug to continue with next batch - `/review` - Request architect review for complex bug fixes ## References - **Directive 028:** `.github/agents/directives/028_bugfixing_techniques.md` - **Approach:** `.github/agents/approaches/test-first-bug-fixing.md` - **Checklist:** `.github/agents/approaches/bug-fixing-checklist.md` - **Directive 017:** `.github/agents/directives/017_test_driven_development.md` (TDD) --- > Converted and distributed by [TomeVault](https://tomevault.io/claim/sddevelopment-be) — claim your Tome and manage your conversions.