--- name: tdd description: Test-driven development with red-green-refactor loop. Use when user wants to build features or fix bugs using TDD, mentions "red-green-refactor", wants integration tests, or asks for test-first development. title: tdd canonical_url: https://skilld.dev/gh/brianlovin/agent-config/tdd last_updated: 2026-09-28T09:57:32.000Z --- > Index of every page on this site: [llms.txt](https://skilld.dev/llms.txt) > **Skill from skilld.dev.** Follow the user's requested action. > If the user asked to fork this Skill, follow the fork workflow below. Do not execute the Skill instructions while copying it. > Otherwise, follow the instructions below for this session. You do not need to install anything. > > Supporting files, fetch one when the Skill refers to it: [deep-modules.md](https://skilld.dev/api/skills-raw/brianlovin/agent-config/tdd/deep-modules.md), [interface-design.md](https://skilld.dev/api/skills-raw/brianlovin/agent-config/tdd/interface-design.md), [mocking.md](https://skilld.dev/api/skills-raw/brianlovin/agent-config/tdd/mocking.md), [refactoring.md](https://skilld.dev/api/skills-raw/brianlovin/agent-config/tdd/refactoring.md), [tests.md](https://skilld.dev/api/skills-raw/brianlovin/agent-config/tdd/tests.md). > > If the user asked to install this Skill, run `npx skilld install brianlovin/agent-config/tdd`. Install writes the Skill files into the project, so every session loads them. > > ## Fork workflow > > A fork creates an editable local Skill with its original author and licence. The request authorizes copying and local installation. > 1. Check `./skills/tdd`, the project lockfile, and selected Agent targets together. If the local directory or installed Skill exists, stop. Never overwrite an existing directory or Agent target. > 2. Read [source metadata](https://skilld.dev/api/v1/skills/brianlovin/agent-config/tdd) once. Use sourceUrl, sourceCommit, skillPath, sourceGone, and license. If the source is gone or its path is missing, stop. If license is null, read licence files at the source commit. > 3. Fetch only the source commit into a temporary Git repository. Do not clone full history. Derive repository_url from sourceUrl, including repository renames. If sourceCommit is absent, resolve the sourceUrl ref once. Set source_commit to that actual commit. Run these commands in one shell call: > > ```sh > git init --quiet "$temporary_dir" > git -C "$temporary_dir" fetch --quiet --depth=1 "$repository_url" "$source_commit" > git -C "$temporary_dir" checkout --quiet --detach FETCH_HEAD > ``` > > Read applicable licence declarations and notices at that commit. If copying is not permitted, report the restriction and stop. > 4. Inspect source entries together, then copy the directory containing skillPath into `./skills/tdd`. Use the user's path if selected. Keep the original SKILL.md, relative links, scripts, binary assets, and executable modes. Exclude .git metadata. Reject symlinks and paths outside the Skill directory. After checking entries, use cp -a where available. A regular source directory needs no custom copy script. Do not save this page wrapper as SKILL.md. > Preserve author credit, notices, and applicable licence files from repository or parent directories. Add PROVENANCE.md with the Skill page, source URL, actual commit, original path, and licence. Retain any existing PROVENANCE.md and record new provenance separately. Batch source inspection, copying, and provenance work where practical. > 5. In the project root, run `skilld install ./skills/tdd --mode copy --plain`. If skilld is unavailable, use `npx skilld install ./skills/tdd --mode copy --plain`. This known command needs no help lookup. Install does not support --json. Use detected Agent targets, or add --agent for the targets the user selected. Install the local path, never the upstream selector. If installation fails, preserve the local copy and report the exact failure. > 6. Confirm the local lockfile source and installed Agent copies once. Report the local path, actual commit, and Agent targets. After edits, reinstall the same local path. Upstream updates must not replace it. Do not publish or push unless the user asks. # Test-Driven Development ## 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](https://skilld.dev/api/skills-raw/brianlovin/agent-config/tdd/tests.md) for examples and [mocking.md](https://skilld.dev/api/skills-raw/brianlovin/agent-config/tdd/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 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](https://skilld.dev/api/skills-raw/brianlovin/agent-config/tdd/deep-modules.md) (small interface, deep implementation) - [ ] Design interfaces for [testability](https://skilld.dev/api/skills-raw/brianlovin/agent-config/tdd/interface-design.md) - [ ] 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](https://skilld.dev/api/skills-raw/brianlovin/agent-config/tdd/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 ```