Ctrl+C Ctrl+V — TDD Extension

You are the same engineer. Same Friday flight to the Alps. Same system. You learned one thing the hard way: the code that gets you called back on Saturday is never the code you wrote — it is the code you forgot to test at the boundary.

So you built a second pattern library. Tests, like code, are copyable. A test for "rejects bad input" is the same shape whether the input is an email or a date. You copy, change the assertion, change the fixture, ship. You do not write every test. You write tests where it matters: at boundaries where someone else's code meets yours.

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§ T0 — When TDD activates

The question is never "should I follow TDD?" — it is "who calls this?"

Caller of the code under change	Tests?
Only this file (private helper)	NO
Only this module (internal use)	NO unless logic is non-trivial
Other modules in this project	YES — contract test
External processes, jobs, CLI users	YES — contract + boundary
Third-party API or DB (you call out)	YES — effect test (mock the boundary)
HTTP / RPC client (you are callee)	YES — contract + boundary + effect
Pure UI rendering, no logic	NO
UI with state machine or input validation	YES — contract on the logic

Two-question fallback when the table feels ambiguous:

1. If this returns the wrong value silently, who notices? "Nobody for a week" → write the test.
2. If someone refactors this next month, what tells them they broke it? "Nothing" → write the test.

§ T1 — TDD integrates with task sizing

Same SMALL / MEDIUM / LARGE. TDD slots into each.

• SMALL — no tests. § 6 self-review is the gate.
• MEDIUM — § T0 decides. If YES: copy test pattern → RED → implement → GREEN → § 6 → § 7.
• LARGE — tests required and written FIRST. Active spec lists test cases as part of Plan. Boundary tests all RED before any implementation. Each task in Tasks: turns specific tests GREEN.
• REFACTOR — characterization tests written GREEN first (Feathers WELC Ch.13; aka pinning / golden master / approval); STAY GREEN through extract. See § T7.

When in doubt, the § T0 table decides — not vibes.

§ T2 — Find before you write (test version)

Same hierarchy as § 1. Different library.

0. ~/.claude/patterns/tests/INDEX.md → match? → copy that .md
1. THIS PROJECT     grep "TEST PATTERN:" in tests/
2. FRAMEWORK        does the test framework provide this fixture?
3. APPROVED DEPS    approved test helper / mock library?
4. TEMPLATES        tests/templates/ or example test files
5. OFFICIAL DOCS    framework's docs → copy example
6. FROM SCRATCH     last resort → save to patterns/tests/

Step 6 has the same tax as § 5: tag, save, index. Details → @${CLAUDE_SKILL_DIR}/playbooks/test-search.md

§ T3 — Test the boundary, not the implementation

Three categories. Pick by what the code does, not by framework.

Category	Tests this question	Example shape
Input boundary	Does it reject bad input?	empty / null / oversize / wrong type / malicious
Contract	Does it return what callers expect?	shape, field names, error types, success vs failure
Effect	Does it produce the right side effect?	DB row, API call, file write, event emitted

Write ONE of each. Not three of the same kind. Per-category templates → @${CLAUDE_SKILL_DIR}/playbooks/test-boundaries.md

§ T4 — Tests are also patterns

# TEST PATTERN: contract — pure function, happy + 2 errors
# USE WHEN: input → output mapping, no side effects
# COPY THIS: change function name, fixture, expected output, error cases

Tag, copy, save. Identical mechanics to § 3 and § 5. The library at ~/.claude/patterns/tests/ is your second library — grepped together with code patterns, not separately.

Copying a pattern → @${CLAUDE_SKILL_DIR}/playbooks/test-copy.md

When no pattern fits → @${CLAUDE_SKILL_DIR}/playbooks/test-scratch.md

§ T5 — Test self-review

□  Test name reads as a sentence: "rejects_empty_email"
□  One test, one assertion focus
□  No test depends on another's order
□  No real network / real DB (mock the boundary)
□  Test fails for the right reason: comment out impl → does it RED?
□  TEST PATTERN tag added if shape is reusable
□  Test file mirrors source (framework convention)
□  Mock 是 Test Spy (captures call args + asserted), not just a Stub
   configured to return a value (Meszaros 2007; Fowler "Mocks Aren't
   Stubs" 2007). Use autospec / spec_set / interface-based mocks
   (mockall / Mockito / gomock) so the type system enforces signature
   drift detection.
□  Dependency substitution happens at the **use site**, not the
   definition site. Python = `mock.patch("caller_module.dep")`;
   Java = @MockBean / @InjectMocks; Go / Rust = constructor DI of an
   interface / trait, with mock implementations from gomock / mockall.

Red flag: a test that still passes when you delete the implementation under test. That test is testing nothing.

Full checklist → @${CLAUDE_SKILL_DIR}/playbooks/test-review.md

§ T6 — LARGE tasks: spec drives tests drives code

LARGE task's Active spec gets one extra subsection:

### Test plan
- [ ] T1 boundary: rejects [bad inputs]
- [ ] T2 contract: returns [shape] when [condition]
- [ ] T3 effect: writes [row] to [table] on success
- [ ] T4 effect: does NOT call [external] when [condition]

All boundary tests RED before any implementation task starts. Each task maps to specific tests turning GREEN. "Verify" in Tasks checklist = "test Tn passes."

LARGE-specific rhythm → @${CLAUDE_SKILL_DIR}/playbooks/test-large.md

§ T7 — Characterization tests (refactor mode)

When the code under change is already working (LARGE refactor mode, ctrl-c-v § 9), TDD inverts: write tests that pass on the current monolith first (locking observed behavior), then refactor with those tests as the contract — any drift = revert.

Aliases (grep for all of these):

• characterization test — Feathers, WELC Ch.13 (canonical)
• pinning test — same shape, common in Java/Spring circles
• golden master — same, common in Approval Tests toolchain
• approval testing — Llewellyn Falco / Emily Bache framework family

Mnemonic: "GREEN-LOCK" — you lock the current GREEN state, refactor, stay GREEN. If a characterization test goes RED during refactor, you changed behavior; revert and split structural vs behavioral commits (Beck, Tidy First 2024).

Full mechanics + per-stack examples → @${CLAUDE_SKILL_DIR}/playbooks/test-refactor.md Execution sits at Step 2 of the extraction recipe → see ctrl-c-v/playbooks/refactor-extract.md

§ T8 — Adversarial test expansion (LARGE only)

After characterization tests are written GREEN but before the refactor starts, run an adversarial review of the test net itself: parallel LLM agents (or multiple human reviewers, or composed static tools — coverage diff + mutation testing + linters) hunt for must-add tests the spec missed.

Composes Fagan 1976 (inspection rigor) + Klein 2007 (pre-mortem timing) + LLM agent fan-out (cost). The LLM era reduces classical Fagan inspection from days-with-N-people to ~10 min, making pre-refactor review affordable for the first time.

Skip for SMALL / MEDIUM — ROI doesn't justify; the cost of the review exceeds the cost of the bug it would catch.

Full rhythm → @${CLAUDE_SKILL_DIR}/playbooks/test-refactor.md ("adversarial expansion" section)

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— Completion criteria —

Boundary coverage — every external caller has a contract test. Every effect has an effect test.

Pattern reuse — second similar test next month: copy, change 5 lines, ship.

Refactor safety — colleague rewrites your function next quarter. Tests catch the break before merge. You stay on the slopes.

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Source disclaimer: § T7 / § T8 derived from a single Python+FastAPI monolith characterization + extraction (gateway/server.py, 2026-06). Patterns generalized via Feathers/Beck/Meszaros/Fowler/Fagan/Klein classics. Non-Python stacks: § T7 generalizes cleanly; § T8 LLM-cost assumption holds in any framework; § T5 patch-where-used line has a Python-leaning example — see playbooks/test-refactor.md for Java/Go/ Rust/TS equivalents.

See you on the slopes. ⛷️