tdd

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.

Test-Driven Development

Project context

This is a .NET solution tested with xUnit. A typical, well-structured test surface uses:

• xUnit as the test framework ([Fact] / [Theory]), with an assertion library (e.g. Shouldly or FluentAssertions).
• A mocking library (e.g. NSubstitute or Moq) used only at system boundaries.
• Snapshot testing (e.g. Verify) for asserting on rich output, with .verified.* files committed alongside the tests.
• HTTP-layer fakes (e.g. RichardSzalay.MockHttp) when a component takes an HttpClient.

Tests are conventionally named {Class}Should.{Behavior}. Reusable test helpers/builders wire up the standard system-boundary fakes (file system, clock, external clients) so handlers can be driven end-to-end without real I/O.

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. A command-handler test is the model — it calls the public entry point (e.g. RunAsync) and asserts on the observable result (the file/content/response produced, captured via a faked IFileSystem or an in-memory sink). It describes what the system does, not how.

Bad tests are coupled to implementation. They mock internal collaborators (private parsers, helpers), assert on Received(n) call counts for things that aren't true boundaries, or independently invoke an internal component to verify a handler.

See tests.md for good vs bad examples and mocking.md for where the system boundaries actually are (IFileSystem, a clock abstraction, HttpClient, an external-service client interface).

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, method 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 (e.g. a new command handler, a new service, or a new parser)
• Confirm with user which behaviors to test (prioritize)
• Identify opportunities for deep modules (small interface, deep implementation)
• Design interfaces for testability
• 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

For a new command handler, the tracer bullet is usually the happy-path "does the work and produces the expected output" test, verified by driving the public entry point and asserting on the result (e.g. via a snapshot of the produced content).

Verify each RED→GREEN cycle with a targeted filter, not the full suite:

dotnet test --no-build --filter "FullyQualifiedName~MyTestMethodName"

Run the full suite only once — before committing — and only when shared infrastructure changed (see AGENTS.md §Test speed for when it is safe to skip the full suite).

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:

• Extract duplication (often into a shared extensions/helpers type)
• Deepen modules (move complexity behind simple interfaces)
• Apply SOLID principles where natural
• Consider what new code reveals about existing code
• Run dotnet test --filter "FullyQualifiedName~<affected class>" after each refactor step; full suite only if shared helpers changed

Never refactor while RED. Get to GREEN first.

Checklist Per Cycle

[ ] Test describes behavior, not implementation
[ ] Test uses public interface only (e.g. RunAsync, Parse)
[ ] Test would survive internal refactor
[ ] Code is minimal for this test
[ ] No speculative features added