---

name: implementing-specs description: > Implements features from technical specifications using TDD. Use when: repo contains /docs/specs/.md, task references acceptance criteria (AC-), .claude/current_task.json exists, or user mentions "implement the spec", "work on AC-", or "test first".

Implementing Specs

Quick Start

CopyTask Progress:
- [ ] Validate spec quality (see spec-checklist.md)
- [ ] Read spec and current_task.json
- [ ] For each AC: write test → implement → verify → commit
- [ ] Update current_task.json after each AC
- [ ] Create session log before ending

Files

If .claude/current_task.json doesn't exist, create it:

Copy{
  "spec": "/docs/specs/{feature}.md",
  "branch": "{current git branch}",
  "started_at": "{ISO 8601 timestamp}",
  "progress": { "acceptance_criteria": [] }
}

Spec Validation (Step 0)

Before implementing ANY spec, validate quality using spec-checklist.md.

Required gates (must pass):

• Every AC has Given/When/Then format
• Given clauses have concrete values (not "a user")
• Then clauses are testable (not "works correctly")
• No vague words: should, appropriate, reasonable
• Error cases documented

If validation fails: Stop and request spec revision. Do not implement low-quality specs.

Implementation Loop

For each acceptance criterion, copy and complete:

CopyAC-{N}:
- [ ] Read AC (Given/When/Then, constraints)
- [ ] Write test that fails (see Test Quality Rules below)
- [ ] Implement until test passes
- [ ] Add provenance comment
- [ ] Commit: [AC-{N}] {description}
- [ ] Update current_task.json status to "passed"

Test Quality Rules

A valid failing test must:

1. Reference the AC: Test name or comment includes AC-N
2. Exercise the code path: Actually call the function/endpoint being implemented
3. Assert the Then clause: Assertion directly maps to expected outcome from AC
4. Fail for the right reason: Failure message relates to missing implementation

Examples:

❌ Bad - Fake failing test:

Copytest('AC-1: validates email', () => {
  expect(true).toBe(false)  // Fails but tests nothing
})

❌ Bad - Too weak:

Copytest('AC-1: validates email', () => {
  const result = validateEmail('test@example.com')
  expect(result).toBeDefined()  // Passes even with stub
})

✅ Good - Specific assertion:

Copytest('AC-1: rejects invalid email format', () => {
  const result = validateEmail('not-an-email')
  expect(result.isValid).toBe(false)
  expect(result.error).toBe('INVALID_FORMAT')
})

Acceptable failure messages (before implementation):

• ✅ validateEmail is not defined - function doesn't exist yet
• ✅ expected false but got undefined - logic not implemented
• ✅ Cannot read property 'isValid' of undefined - return value missing
• ✅ NotImplementedError: validateEmail not yet implemented - explicit stub

Unacceptable failure reasons (fix test first):

• ❌ Syntax error in test code
• ❌ Import error unrelated to feature
• ❌ Test framework configuration issue
• ❌ Mock setup error

Before implementing: Verify test fails with acceptable message. If not, fix the test.

Provenance Comments

Every implementation file must reference its spec:

Copy// Implements: /docs/specs/password-reset.md#AC-1

Copy// Implements: /docs/specs/password-reset.md#AC-2
// Error: E001 (from spec error catalog)

Commit Format

Copy[AC-1] {what changed}

Examples:

• [AC-1] add validation schema
• [AC-1] implement endpoint
• [AC-1] fix timezone handling

When Blocked

CopyBlocked? →
├─ Can proceed with documented assumption? → Continue, flag for review
├─ Missing resource (mock, data, access)? → Skip AC, continue others
└─ Neither? → Stop. Document. Do not guess.

Record in session log:

Copyfailures:
  - type: spec_ambiguous|mock_missing|tool_failure|permission_denied
    detail: "{specific description}"
    resolution: "{what you did or 'blocked'}"

Failure types: See failure-types.md

Session End

Before ending, create .claude/sessions/{YYYY-MM-DD}-{4-random-chars}.yaml:

Copysession_id: {4 random chars}
timestamp: {ISO 8601}
spec: {path to spec}
branch: {git branch}

work_completed:
  - ac_id: AC-1
    outcome: passed  # or: failed, blocked, skipped
    attempts: 1

failures: []  # or list of {type, detail, resolution}

handoff: |
  {What's done}
  {What's in progress}
  {What to watch out for}

Commit the session log before ending.

Reference

• spec-checklist.md - Spec validation before implementation
• current-task-schema.json - Task file format
• session-schema.yaml - Session log format
• failure-types.md - Failure classification