tdd

Test-Driven Development

TDD is the red → green loop. This skill is the reference that makes that loop produce tests worth keeping: what a good test is, where tests go, the anti-patterns, and the rules of the loop. Every section applies on every cycle: consult them before and during the loop, not after.

When exploring the codebase, read GLOSSARY.md (if it exists) so test names and interface vocabulary match the project's domain language, and respect ADRs in the area you're touching.

What a good test is

Tests verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't. A good test reads like a specification: "user can checkout with valid cart" tells you exactly what capability exists, and it survives refactors because it doesn't care about internal structure.

See tests.md for examples and mocking.md for mocking guidelines.

Seams: where tests go

A seam is the public boundary you test at: the interface where you observe behavior without reaching inside. Tests live at seams, never against internals.

Test only at pre-agreed seams. Before writing any test, write down the seams under test and confirm them with the user. No test is written at an unconfirmed seam. You can't test everything, so agreeing the seams up front is how testing effort lands on the critical paths and complex logic instead of every edge case.

Ask: "What's the public interface, and which seams should we test?" Give each proposed seam a one-line note on what it catches and what it misses.

When the shape of that interface is itself in question (how deep the module is, where the seam belongs, what the interface should expose), read skills_read with skill="tdd", path="codebase-design.md" for the vocabulary. It is the shared source of the module, interface, depth, seam, adapter, leverage and locality terms, and it is a reference to consult, not a session to run.

Anti-patterns

• Implementation-coupled: mocks internal collaborators, tests private methods, or verifies through a side channel (querying the database instead of using the interface). The tell: the test breaks when you refactor but behavior hasn't changed.
• Tautological: the assertion recomputes the expected value the way the code does (expect(add(a, b)).toBe(a + b), a snapshot derived by hand the same way, a constant asserted equal to itself), so it passes by construction and can never disagree with the code. Expected values must come from an independent source of truth: a known-good literal, a worked example, the spec.
• Horizontal slicing: writing all tests first, then all implementation. Bulk tests verify imagined behavior: you test the shape of things rather than user-facing behavior, the tests go insensitive to real changes, and you commit to test structure before understanding the implementation. Work in vertical slices instead: one test → one implementation → repeat, each test a tracer bullet that responds to what the last cycle taught you.

Rules of the loop

• Red before green. Write the failing test first, then only enough code to pass it. Don't anticipate future tests or add speculative features.
• One slice at a time. One seam, one test, one minimal implementation per cycle.
• Refactoring is not part of the loop. It belongs to the review stage (review the agreed public interface and test results with the user), not the red → green implementation cycle.

Execute and prove each cycle

For an approved interface, first create one public test and the minimal failing implementation (for example a stub). Run that test with the offered shell tool and confirm its real failure before implementing the behavior. A test name containing 'red' is not evidence of a red run. Implement only that slice and run it again to prove green; then repeat for the remaining agreed behavior. Report actual failing and passing command results, never inferred outcomes or runs you did not perform.

Host capabilities

Use only tools offered in this conversation. Read supplied workspace files with read_file; create artifacts with write_file and edit existing files with apply_patch when offered. Do not invent a Skill tool. After creating any file, include its exact Markdown download link from the tool result in your answer, even when the rest of your answer asks the user a decision question. Never replace that link with a filename or a claim that the file exists. If a required file, workspace, executable, or tool is unavailable, explain the limitation and do not claim it ran. A user's already-agreed scope and test interface count as agreement; do not ask the same question again. Apply the workflow directly after reading it; keep progress narration brief.