Agent Skills: Test-Driven Development

Test-Driven Development

Description

TDD methodology for development phase. Focus on testing what you build.

Details

Core principle: Write tests for the code you write.

Follow Red/Green/Verify cycle:

1. Red: Write a failing test first

• Write the test code that calls the new function
• Add function declaration to header file
• Add stub implementation that compiles but does nothing (e.g., return OK(NULL);)
• A compilation error is NOT a failing test - you need a stub that compiles and runs
• Verify the test actually fails with assertion failures

2. Green: Write minimal code to make the test pass

• Implement ONLY what the test requires
• STOP immediately when the test passes
• DO NOT write "helper functions" before they're called
• DO NOT write code "because you'll need it later"

3. Verify: Run quality checks

• check-unit --file=PATH - Test must pass
• check-filesize --file=PATH - File size under 16KB
• check-complexity --file=PATH - Function complexity under threshold

Development Phase Focus

Test what you build. During development:

• Write tests for the happy path
• Write tests for obvious error cases
• Cover the main functionality thoroughly
• Keep momentum - don't get stuck on edge cases

Coverage gaps are OK during development. They will be closed in a dedicated coverage phase before release.

The test should come first - but don't let perfect coverage slow down feature development.

If writing a helper function, ask: "Does a passing test call this right now?" If no, DELETE IT.

Coverage Phase (Strict Mode)

ABSOLUTE RULE: NEVER WRITE CODE BEFORE YOU HAVE A TEST THAT NEEDS IT.

When closing coverage gaps, the verify step adds:

• check-coverage — 90% coverage is MANDATORY

Hunt every branch:

• Test all error paths (use mocks to trigger failures)
• Test assertion violations (tcase_add_test_raise_signal with SIGABRT)
• Test edge cases (empty, null, boundary conditions)
• Only use LCOV_EXCL markers when genuinely untestable (OOM, impossible states)

If a branch is uncovered, ask: "Can I write a test for this?" If yes, WRITE IT.