Tdd

---
name: tdd
description: 'Use when asked to develop a fix or feature test-first, plan a multi-step TDD build, or work red-to-green in slices. Writes attacks before implementation, probes the suite with mutations, and advances one vertical slice at a time. Not for hardening existing code without a change; use tests-adversarial. Not for deleting weak tests; use tests-purge-unneeded.'
---

# Offensive-first TDD

Classic agent TDD rots in a known way: the red test is written as a polite specification, the implementation drifts, and the agent — lacking the metacognition to notice — bends fixtures, weakens assertions, and monkey-patches contracts until green appears. The tokens burn twice and the suite proves nothing. This skill inverts the stance. Red is an attack, not a description: before any implementation, write the tests that try to BREAK the contract — boundary raids, invariant violations, error-path probes, adversarial orderings. Green means the attacks failed to break the code, never that a fixture was satisfied.

## Contract

| Field | Bound contract |
|---|---|
| Trigger | Develop a fix or feature test-first, plan a multi-step TDD build from a spec, or work behaviour red-to-green in slices. |
| Authority | Reversible-local: write only named test and source files; rollback via VCS revert of the implementation commit. Tests and implementation never change in the same repair step. |
| Side effect | Adds an attack suite and the minimal implementation that survives it, one vertical slice at a time; multi-step work also writes a plan document. |
| Done | Per slice: every contract clause has an attack that failed red for the right reason and passes green unweakened, plus one mutation probe the suite caught. Named evidence: test paths, failing output, passing output, probe output. |

## Inputs

- Target and requirement (required): the behavior to build or fix, and the public seam where it is observed. A seam is confirmed with the user before any test exists at it.
- Contract source (required): spec, caller evidence, or known-good literals — the independent truth attacks are derived from. Never the implementation's own logic.
- Test framework (optional if discoverable): the project's runner. Stop and report if absent and not discoverable; never install one unasked.

## Refusals

- Never weaken an assertion, bend a fixture, skip a test, or loosen a contract to reach green. Any such edit during a green phase is a stop-the-line violation: revert it, then either fix the implementation or declare a contract change and re-attack from red.
- Never edit a test and its implementation in the same repair step. Test-fixing and code-fixing are separate cycles, each with a named justification.
- Never accept a tautological attack (expected value recomputed the way the code computes it) or an implementation-coupled one (mocks internals, tests private methods). Attacks fire at confirmed public seams only.
- Not for hardening existing code with no behavior change — use tests-adversarial. Not for deleting weak tests — use tests-purge-unneeded.

## Procedure

1. **State the contract under attack.** From the spec, callers, and known-good literals, write the clauses the code must honor: valid behavior, boundaries, invariants, error semantics. Confirm the seam with the user. Done when: clauses are listed with an independent truth source each, and the seam is confirmed.
2. **Attack first.** For the current slice, write the breaking suite before any implementation: for each clause, the boundary raid, the invalid input, the invariant violation, the error-path probe, the adversarial ordering that would expose a sloppy implementation. Run it. Every attack must fail red **for the right reason** — assert the failure mode, not just any failure; an attack that passes against absent code is tautological, discard and rewrite it. Done when: the suite is red with each failure matching its predicted mode.
3. **Implement until the attacks stop breaking it.** Minimal but real: only enough code to survive the suite, no speculative features, no refactor of unrelated code. Rerun until green. If green cannot be reached, revert the implementation and try a different minimal approach; the attacks do not move. Done when: the suite is green with tests byte-identical to their red state.
4. **Grill the green.** Diff every test, fixture, and contract file against its red-phase state. Any drift is a violation: revert it, classify the change (implementation bug → fix code; genuine contract change → declare it, update the clause list, return to step 2). Done when: the diff over test surfaces since red is empty or every change is a declared contract change with its own re-attack.
5. **Probe the suite's teeth.** Mutate the implementation once — flip the fix, invert a branch, off-by-one a bound — and confirm at least one attack catches it; then revert the mutation. A suite that survives its mutation probe proves nothing and returns to step 2 for a sharper attack. Done when: the probe was caught and the revert is clean.
6. **Advance by vertical slice.** Commit the slice (attack suite + implementation, one commit naming the behavior), then take the next clause or seam from step 2. Multi-step work runs from a plan document listing one attack-entry per requirement, ordered by dependency; the plan's only execution mode is offensive-first — there is no implementation-first option. Done when: all agreed seams are exhausted and every test in the tree maps to a confirmed seam.

## Failure and recovery

| Failure class | Rule |
|---|---|
| Green by test-bending | The signature drift this skill exists to kill. Revert the test edit, keep the red, fix the code or declare the contract change out loud. |
| Attack cannot be written | Missing prerequisite (unclear requirement, unconfirmed seam, no framework). Stop and report; never proceed to implementation on an unattacked contract. |
| Implementation cannot survive within the slice | Revert the implementation, keep the attacks. Try a different minimal approach or stop and report; partial result = red suite + its output, never a weakened green. |
| Mutation probe survives | The suite is decorative. Sharpen the attack before touching more implementation. |
| Existing suite breaks | Revert; narrow the change until only the new slice's behavior moves. |

Partial-result rule: completed slices are retained; a blocked cycle never invalidates prior green slices and never claims the done predicate.

## Output

Per slice, in order: attack files, red run output (failure modes matched), green run output, mutation-probe evidence, implementation commit SHA. Multi-step work prepends the plan document path.