--- name: tdd description: Test-driven development. Use when the user wants to build features or fix bugs test-first, mentions "red-green-refactor", or wants integration tests. category: "development" risk: "safe" source: "community" source_repo: "mattpocock/skills" source_type: "community" date_added: "2026-06-19" author: "Matt Pocock" license: "MIT" license_source: "https://github.com/mattpocock/skills/blob/main/LICENSE" tags: - engineering - workflow - coding-agents tools: - claude-code - codex-cli - cursor --- # Test-Driven Development ## When to Use Use when this workflow matches the user request: Use this skill for its documented workflow. _Source: [mattpocock/skills](https://github.com/mattpocock/skills) (MIT)._ ## Philosophy **Core principle**: Tests should verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't. **Good tests** are integration-style: they exercise real code paths through public APIs. They describe _what_ the system does, not _how_ it does it. A good test reads like a specification - "user can checkout with valid cart" tells you exactly what capability exists. These tests survive refactors because they don't care about internal structure. **Bad tests** are coupled to implementation. They mock internal collaborators, test private methods, or verify through external means (like querying a database directly instead of using the interface). The warning sign: your test breaks when you refactor, but behavior hasn't changed. If you rename an internal function and tests fail, those tests were testing implementation, not behavior. See [tests.md](tests.md) for examples and [mocking.md](mocking.md) for mocking guidelines. ## Anti-Pattern: Horizontal Slices **DO NOT write all tests first, then all implementation.** This is "horizontal slicing" - treating RED as "write all tests" and GREEN as "write all code." This produces **crap tests**: - Tests written in bulk test _imagined_ behavior, not _actual_ behavior - You end up testing the _shape_ of things (data structures, function signatures) rather than user-facing behavior - Tests become insensitive to real changes - they pass when behavior breaks, fail when behavior is fine - You outrun your headlights, committing to test structure before understanding the implementation **Correct approach**: Vertical slices via tracer bullets. One test → one implementation → repeat. Each test responds to what you learned from the previous cycle. Because you just wrote the code, you know exactly what behavior matters and how to verify it. ``` WRONG (horizontal): RED: test1, test2, test3, test4, test5 GREEN: impl1, impl2, impl3, impl4, impl5 RIGHT (vertical): RED→GREEN: test1→impl1 RED→GREEN: test2→impl2 RED→GREEN: test3→impl3 ... ``` ## Workflow ### 1. Planning When exploring the codebase, read `CONTEXT.md` (if it exists) so that test names and interface vocabulary match the project's domain language, and respect ADRs in the area you're touching. Before writing any code: - [ ] Confirm with user what interface changes are needed - [ ] Confirm with user which behaviors to test (prioritize) - [ ] Identify opportunities for deep modules (small interface, deep implementation) — run the `/codebase-design` skill for the vocabulary and the testability checks - [ ] List the behaviors to test (not implementation steps) - [ ] Get user approval on the plan Ask: "What should the public interface look like? Which behaviors are most important to test?" **You can't test everything.** Confirm with the user exactly which behaviors matter most. Focus testing effort on critical paths and complex logic, not every possible edge case. ### 2. Tracer Bullet Write ONE test that confirms ONE thing about the system: ``` RED: Write test for first behavior → test fails GREEN: Write minimal code to pass → test passes ``` This is your tracer bullet - proves the path works end-to-end. ### 3. Incremental Loop For each remaining behavior: ``` RED: Write next test → fails GREEN: Minimal code to pass → passes ``` Rules: - One test at a time - Only enough code to pass current test - Don't anticipate future tests - Keep tests focused on observable behavior ### 4. Refactor After all tests pass, look for [refactor candidates](refactoring.md): - [ ] Extract duplication - [ ] Deepen modules (move complexity behind simple interfaces) - [ ] Apply SOLID principles where natural - [ ] Consider what new code reveals about existing code - [ ] Run tests after each refactor step **Never refactor while RED.** Get to GREEN first. ## Checklist Per Cycle ``` [ ] Test describes behavior, not implementation [ ] Test uses public interface only [ ] Test would survive internal refactor [ ] Code is minimal for this test [ ] No speculative features added ``` ## Limitations - Requires the upstream tool, account, API key, or local setup when the workflow names one. - Does not authorize destructive, production, paid, or external-message actions without explicit user approval. - Validate generated artifacts or recommendations against the user's real sources before treating them as final. --- Source attribution: mattpocock/skills contributors, via Agentic Awesome Skills Original project: https://github.com/mattpocock/skills License: MIT + CC-BY-4.0 License information: https://creativecommons.org/licenses/by/4.0/ Original upstream instructions; downloaded from Dave AI. ## Upstream license notice MIT License Copyright (c) 2026 Matt Pocock Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. # Dave AI resource metadata Creator / publisher: mattpocock/skills contributors, via Agentic Awesome Skills Original project: https://github.com/mattpocock/skills Attribution basis: Recorded upstream repository Fields of work: Development & Engineering Roles: Software Developer, QA / Test Engineer Specialties: Specialty not specified Tools: Tool not specified Country applicability: Country not specified Classification basis: Dave AI task classification from package title, description, and declared metadata Instruction SHA-256: 0eb6630a004ebf6124bc052f2e66d1eea489dedecfbab698bf87c576971c8e20 Metadata reviewed: 2026-10-27 This metadata supersedes earlier Dave AI catalog attribution. Original instruction and supporting files are preserved.