Implementer

Follow these phases in strict order. Do not skip phases. Do not proceed until the current phase's gate is satisfied.

This skill covers development only — no issue tracking, no commits, no pushes. The coordinator handles those.

Principles

• •Never silently work around problems. Throw errors for missing env vars, invalid state, missing dependencies.
• •Mock properly in tests. Do not add production fallbacks to make tests pass.
• •No as any or as unknown in production code.
• •No optional chaining on required properties.

Phase 1: Write Failing Tests

Write tests for the behavior you are about to change or add. Do this before touching any production code.

1. •Read the relevant production code to understand current behavior
2. •Write new test cases that describe the desired behavior after your change
3. •Run the tests:

npm test

Gate: Your new tests fail (or, for pure deletions/removals, you can write tests asserting the old behavior is gone — these will pass after implementation). If your new tests already pass, they are not testing anything new. Rewrite them.

Phase 2: Implement

Make the production code changes. Keep changes minimal and focused on the task.

Phase 3: Verify

Run quality gates:

npm test
npx tsc --noEmit
npm run lint

Gate: All three commands pass with zero errors. If any fails, fix the issues before proceeding.

Phase 4: Test Coverage Review

This is an audit, not a formality. Evaluate whether your tests actually cover the changes you made.

Step 1: List what changed

git diff --name-only

Separate the output into production files and test files.

Step 2: For each changed production file, evaluate

• •What behavior changed? (new feature, bug fix, removed feature, refactored logic)
• •What existing tests cover this file? Read the corresponding test file if one exists.
• •Are there gaps? Specifically:
  • •Happy path for new/changed behavior
  • •Error paths and edge cases
  • •Regression test if this is a bug fix (a test that would have caught the original bug)
  • •Boundary conditions

Step 3: Evaluate integration test needs

Integration tests are needed when changes affect:

• •Repository/persistence layer (database queries, data mapping)
• •API routes that combine multiple services
• •Auth flows or permission checks
• •Data flowing across multiple layers (API → service → repository)

If integration tests are needed, write them as *.integration.test.ts files.

Step 4: Evaluate E2E test needs

E2E tests are kept small and focused on critical workflows to minimize runtime. Check:

1. •Do any existing E2E tests cover workflows affected by your changes?
2. •If yes, do those E2E tests need updating to reflect your changes?

Do not add new E2E tests unless explicitly requested. Only update existing ones when the workflows they test have been modified.

Step 5: Fill gaps

Write any missing tests identified above. Then re-run quality gates:

npm test
npx tsc --noEmit
npm run lint

Gate: All tests pass, including your new coverage additions. If you identified no gaps in Steps 2-4, document your reasoning (e.g., "Changes were purely deletions; added regression tests in Phase 1 confirming removed elements no longer render").