Agent Skills: Test-Driven Development

Language

All agent-authored prose MUST follow ASD-STE100 Simplified Technical English. Apply it to questions, explanations, recommendations, issue text, reports, handoffs, and procedures.

• Use short, complete sentences and active voice.
• Use one term for one meaning.
• Put conditions before commands.
• Use imperative sentences for procedures.
• Keep code, identifiers, commands, quoted text, product names, and exact domain terms unchanged.
• Use the iso-24495-1 skill for the full rule set.

Test-Driven Development

TDD is the red → green → refactor 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. The implementation loop stops at green. Refactoring happens during review unless the user asks for it during the cycle.

Before resolving domain artifacts, follow ALIGNMENT-ROOT.md.

When exploring the codebase, read CONTEXT.md from the CLI-reported ALIGNMENT_ROOT (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 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 — 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?"

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 (see the code-review skill), not the red → green implementation cycle.

Context resolution

Before reading or writing workflow or domain artifacts, use the agent-core:shared-context skill from the repository root. Use the root that the skill resolves as ALIGNMENT_ROOT. Use the repository root for source code and ordinary project files. Do not inspect the prompt or environment, or derive an alignment path by hand.

For a write under a shared ALIGNMENT_ROOT, use the agent-core:shared-context skill to resolve tracker, workflow, source, and ADR targets. Use that skill to update the source index only after a source directory changes and to apply the publishing procedure after the write. Follow ALIGNMENT-ROOT.md for the complete rule.