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tdd

Skill0 starsChanged 3 months agoMITView on GitHub

Reference material for test-driven development discipline. Red before green, one slice at a time. Use as reference during implementation, not as a step-by-step procedure.

194 other projects use a version of this.

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The file

We split this skill into the rules it gives an agent

3 rules. 0 are common, 3 are distinctive, and 0 are very specific to this project.

  • 0

    Common

    Most of the 186 edited versions we read keep these.

  • 3

    Distinctive

    This author's own choices. Worth a look.

  • 0

    Very specific

    Tied to their project's names, files or tools. Probably skip.

  • Common
  • Distinctive
  • Very specific
  1. Distinctive:

    Write a failing test that expresses the expected behavior at a pre-agreed seam (public interface).

    line 18
  2. Distinctive:

    Write the smallest honest implementation that makes the test pass.

    line 26
  3. Distinctive:

    The test is the spec: If you can't write the test, the spec is unclear — go back to grilling, don't guess.

    line 37

“Common” is measured: kept by at least 6 in 10 of the 186 edited versions of this file we could read. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

Keep what fits and drop the rest. Your version, ready for your agents.
Files
SKILL.md

TDD

Reference material — not a procedure. This skill defines the discipline; implement defines the steps.

The loop

Red → Green. That's it. Refactoring belongs in the code-review pass, not interleaved with implementation.

Red

Write a failing test that expresses the expected behavior at a pre-agreed seam (public interface). Run it. Confirm it fails for the right reason — not a missing import, not a syntax error, not an unrelated failure. The failure message should describe the gap between current and desired behavior.

Green

Write the smallest honest implementation that makes the test pass. "Honest" means: no hardcoding expected output, no stubbing the behavior the test claims to verify. If the honest implementation is larger than expected, that's fine — the test defines correctness, not the estimate.

Principles

  • Tests verify behavior through seams — pre-agreed public interfaces and boundaries. Never test implementation internals.
  • One slice at a time. Finish red→green for one behavior before starting the next.
  • The test is the spec. If you can't write the test, the spec is unclear — go back to grilling, don't guess.

Anti-patterns

  • Implementation-coupled tests (testing private methods, internal state)
  • Tautological assertions (mock wired to return X, then assert X)
  • Tests that can't actually fail (always-true conditions)
  • Writing multiple tests before any green (batching red)

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