DISTILL Methodology: Acceptance Test Creation

This skill provides the acceptance designer's methodology for creating acceptance tests. The orchestrator controls the overall flow (agent dispatch, review gate, handoff) -- this skill focuses on HOW to create good acceptance tests.

ADR-025 (2026-05-07) — DISTILL is canonical AT author

DISTILL produces ALL acceptance tests as scaffolded RED (skip/pending markers). DELIVER's 3-phase cycle (RED / GREEN / COMMIT, per ADR-025) does NOT re-author ATs in RED — it only unskips the scaffolds and writes PBT unit tests. Wave separation: DISTILL = "what should the system do" (ATs), DELIVER = "how" (PBT unit + impl). The pre-DELIVER fail-for-right-reason gate (described in this skill) becomes the RED phase entry/exit gate in DELIVER per ADR-025 D2.

Output Tiers (per D2)

Provenance: feature lean-wave-documentation — D2 (schema-typed sections), D10 (one-line expansion descriptions). Tier-1 [REF] sections (always emitted) + Tier-2 EXPANSION CATALOG items (lazy, on-demand) are the two output bands. The .feature file remains the SSOT for scenarios; the wave-delta sections are pointers + structured summaries. Full contract: nWave/skills/nw-density-resolution-contract/SKILL.md.

Tier-1 [REF] — always emitted

Under ## Wave: DISTILL / [REF] <Section> headings:

• Scenario list with tags — table of scenario titles + tags (@walking_skeleton, @US-N, @real-io, @in-memory, @error, @property)
• WS strategy — A/B/C/D selection per Mandate 5 with one-line justification
• Adapter coverage table — per Mandate 6, every driven adapter mapped to at least one @real-io scenario
• Scaffolds — list of RED-ready scaffold files created (per Mandate 7) with __SCAFFOLD__ markers
• Test placement — tests/{path}/ directory choice with one-line precedent justification
• Driving Adapter coverage — every CLI/endpoint/hook in DESIGN mapped to at least one subprocess/HTTP/hook scenario
• Pre-requisites — DESIGN driving ports + DEVOPS environment matrix the scenarios depend on

Tier-2 EXPANSION CATALOG — lazy, on-demand (per D10)

Rendered under ## Wave: DISTILL / [WHY|HOW] <Section> only when requested via --expand <id> (DDD-2), the wave-end menu (expansion_prompt = "ask"), mode = "full" auto-expansion, or an ad-hoc user request mid-session.

Density resolution (per D12)

Call resolve_density(global_config) from scripts/shared/density_config.py after reading ~/.nwave/global-config.json (missing/malformed = empty dict). Returns mode ("lean" | "full") + expansion_prompt ("ask" | "always-skip" | "always-expand" | "smart") per the D12 cascade (resolver-internal, DDD-5 — do NOT replicate locally). Branch on density.mode for what to emit; branch on density.expansion_prompt at wave end for menu behaviour. Full cascade detail, branch semantics, ad-hoc override workflow: nWave/skills/nw-density-resolution-contract/SKILL.md.

Telemetry (per D4 + DDD-6)

Every expansion choice emits a DocumentationDensityEvent (dataclass at src/des/domain/telemetry/documentation_density_event.py) via event.to_audit_event() → JsonlAuditLogWriter().log_event(...). Schema fields per D4: feature_id, wave, expansion_id, choice, timestamp. For this wave the schema declares "wave": "DISTILL". Use helper scripts/shared/telemetry.py:write_density_event(...) — do NOT write JSONL directly.

Wave-specific signal: DELIVER consuming a lean DISTILL feature-delta — downstream --expand for fixture-design or edge-case enumeration indicates the [REF] baseline plus the .feature file was insufficient for the crafter. Full emission rules: nWave/skills/nw-density-resolution-contract/SKILL.md.

Feature-Delta Schema (US-01, US-02)

Provenance: unified-feature-delta US-01 (scaffold command) and US-02 (E1+E2 validator rules).

Every feature-delta.md is a Markdown document with ## Wave: <NAME> sections. The canonical table format must be used in every ### [REF] Inherited commitments block.

Scaffold command

nwave-ai init-scaffold --feature <feature-name>

Creates docs/feature/<feature-name>/feature-delta.md with three pre-populated wave sections (DISCUSS, DESIGN, DISTILL), each containing a ready-to-fill commitments table. The scaffold passes the E1+E2 validator immediately.

Canonical table format

Every ### [REF] Inherited commitments block MUST have exactly four columns in this order:

## Wave: DISCUSS

### [REF] Inherited commitments

| Origin | Commitment | DDD | Impact |
|--------|------------|-----|--------|
| n/a | <commitment text> | n/a | <impact text> |

Column semantics:

• Origin: wave and row reference of the upstream commitment (e.g., DISCUSS#row1) or n/a for root commitments
• Commitment: the specific commitment inherited or newly introduced in this wave
• DDD: Design Decision Document reference that authorizes any change (e.g., DDD-3) or n/a / (none) when not applicable
• Impact: substantive description (>=10 words or a consequence verb from the verb list) of the commitment's effect on the system

Validator rules (E1+E2)

• E1 (SectionPresent): every ## Wave: <NAME> heading must match the canonical pattern. Known wave names: DISCOVER, DISCUSS, DESIGN, DEVOPS, DISTILL, DELIVER. Near-misses get a did-you-mean suggestion.
• E2 (ColumnsPresent): every ### [REF] Inherited commitments block must have a header row with the four required columns (Origin, Commitment, DDD, Impact) in any order, case-insensitive.

Incremental authoring

Sections for waves not yet authored may be omitted entirely. The validator does not require all six wave sections to be present. An incremental feature-delta with only DISCUSS is valid. Missing future-wave sections are never flagged.

Acceptance Criteria: Port-to-Port Principle

Every AC MUST name the driving port (entry point) through which the behavior is exercised. This enables port-to-port acceptance tests that make TBU (Tested But Unwired) defects structurally impossible.

Each AC includes:

1. Observable outcome: what the user/system sees
2. Driving port: the entry point that triggers the behavior (service, handler, endpoint, CLI command)

Without the driving port, a crafter can write correct code that is never wired into the system.

Features: "When user {action} via {driving_port}, {observable_outcome}" Bug fixes: "When {trigger}, {modified_code_path} produces {correct_outcome} instead of {current_broken_