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alexjudaalexjuda/alexjuda-skills/skills/tdd/SKILL.md

tdd

Skill0 starsChanged 8 months agoApache-2.0View on GitHub

Apply Test-Driven Development — red/green/refactor cycle to drive code and design

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

4 rules. 0 are common, 3 are distinctive, and 1 is 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.

  • 1

    Very specific

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

  • Common
  • Distinctive
  • Very specific
  1. Distinctive:

    Read the codebase to understand the structure.

    line 9
  2. Distinctive:

    Ask the human for input on genuinely ambiguous decisions (e.g., "should X be a unit test or integration test?").

    line 15
  3. Very specific:

    Before entering the red/green/refactor cycle, write out an initial list of test cases for the feature:

    line 19
  4. Distinctive:

    Not asking for feedback before continuing

    line 55

“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

Step 0: Plan the Approach

Read the codebase to understand the structure. Then:

  • Identify what modules/classes need unit tests
  • Decide where integration tests are warranted
  • List test scenarios before writing any code (see below)
  • Determine testing priority

Ask the human for input on genuinely ambiguous decisions (e.g., "should X be a unit test or integration test?"). Propose alternatives and tradeoffs.

List Test Scenarios First

Before entering the red/green/refactor cycle, write out an initial list of test cases for the feature:

  • What's the happy path?
  • What edge cases exist?
  • What error conditions should be handled?
  • What if a service times out? What if data doesn't exist?

This is not a complete plan—it's a starting point. As you implement and learn, add missing cases to the list. Don't try to envision the whole implementation upfront; that would be waterfall. Start with the simplest case to establish the basic interface, then add progressively more complex scenarios.

The Cycle (One Test at a Time)

For each element in the plan:

  1. Red — Write a SINGLE failing test. One behavior, one assertion.
  2. Green — Write minimal code to make it pass.
  3. Refactor — Clean up while keeping tests green.
  4. Ask for feedback — Wait for human input before proceeding to the next test.

Repeat for each test case.

Principles

  • Tests drive design: hard to test = design problem
  • Minimal implementation: only what's needed now
  • Fast feedback: run tests every few minutes
  • One test at a time: write one, not multiple

Connecting to Test Desiderata

See test-desiderata skill. TDD works best with tests that are isolated, fast, readable, behavioral, and specific.

Common Mistakes

  • Skipping the planning step
  • Writing multiple tests at once (the main TDD failure mode)
  • Skipping red (tests after implementation defeats TDD)
  • Green-slopping (ugly code that passes, no refactor)
  • Not asking for feedback before continuing

When to Load

  • New code or features
  • Fixing bugs (test first to reproduce)
  • Refactoring (tests provide safety)

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