Nw Distill

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.

LANGUAGE CONVENTION FRAME (read FIRST — overrides all examples below)

Code examples in this skill use Python syntax for illustration only. They are NOT prescriptive about target language. nWave is language-agnostic per the "genericity and agnosticism" mandate (2026-05-24).

Before authoring ATs, detect the target project's language from these manifest files (in order):

• package.json → TypeScript / JavaScript (jest, vitest, cucumber-js, playwright)
• Cargo.toml → Rust (cargo test, proptest, cucumber-rust)
• go.mod → Go (testing, ginkgo, godog)
• pyproject.toml / setup.py / Pipfile → Python (pytest, pytest-bdd, hypothesis)
• pom.xml / build.gradle → Java / Kotlin (JUnit5, Cucumber-JVM, jqwik)
• *.csproj / *.fsproj → C# / F# (xUnit, SpecFlow, FsCheck)
• Gemfile → Ruby (RSpec, Cucumber-Ruby)
• Package.swift → Swift (XCTest, swift-testing)

When the target language is NOT Python:

1. Adapt EVERY code example to the target language's conventions (naming, imports, type system, test-framework idioms, file extensions).
2. Replace Python-specific imports (from pytest_bdd import ..., from hypothesis import ..., import dataclasses) with target-language equivalents (import { Given, When, Then } from '@cucumber/cucumber', import * as fc from 'fast-check', etc.).
3. Replace Python type hints (def f(x: int) -> str) with target-language type syntax.
4. Replace Python directory conventions (tests/, __init__.py) with target conventions (test/, __tests__/, no init files for TS/JS).
5. Replace Python class/function syntax (class Customer:, def given_port():) with target equivalents.

Project conventions ALWAYS WIN over examples below. If the user's repo has 50 TS files using describe()/it() blocks and zero Python files, ATs MUST be TypeScript with describe()/it() — never Python pytest-bdd regardless of how authoritative this skill's examples look.

Empirical anchor: skill examples being Python-only caused LLM to infer Python conventions universal, leading to Python code emitted in greenfield TS project. Connects [[feedback_language_adapter_plugin_architecture_2026_05_24]] (genericity mandate) + F-LANGUAGE-ADAPTER-PLUGIN-INFRASTRUCTURE epic.

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 | DDR | 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
• DDR: Design Decision Record reference that authorizes any change (e.g., DDR-3) or n/a / (none) when not applicable. (Renamed from DDD to avoid colliding with Domain-Driven Design — issue #50. The legacy DDD column and DDD-N references are still accepted during the deprecation window.)
• 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, DDR, Impact) in any order, case-insensitive. The legacy DDD header is also accepted.

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_behavior}"

Translating Gherkin to Property-Based Tests (state-delta + Universe)

Layer constraint (per nw-test-design-mandates Mandate 9): this recipe applies ONLY to layers 1-2 (unit, in-memory acceptance with in-memory doubles). For subprocess / FS acceptance and integration tests (layers 3+), use example-only with assert_state_delta for the universe guard (Mandate 8) — sad paths stay enumerated, never PBT-generated (Mandate 11).

Per the Paradigm Mandate (PBT + state-delta is the default for unit + in-memory acceptance), DISTILL outputs Property: framings, not classic Scenario: examples, whenever the spec is quantifiable AND the test runs at layer 1-2. Single-example Scenario: is FALLBACK — use when the property cannot be expressed (one-off regression repro) OR when the scenario runs at layer 3+ (real adapter, subprocess, integration, WS).

Recipe

1. Take the Scenario:: identify pre-condition, action, post-condition.
2. Identify the Universe (layer-specific — see nw-tdd-methodology Layered test discipline matrix):
   • Acceptance: use-case observable outcomes at driving port (events emitted, state on driven-port double, error class)
   • Walking Skeleton: user-visible end-to-end output (stdout, exit code, FS side-effects)
3. Quantify the precondition: from "a feature task exists" → forall task in tasks where task.type in {feature, fix} via Hypothesis @given(strategy).
4. Express the invariant: instead of "the row's DELIVER cell shows [in-progress] phase: GREEN", state "for every task entered into DELIVER GREEN phase, the row's DELIVER cell renders status=in-progress with phase name visible".
5. Frame as Property: in the .feature file:

   @property @driving_port @us-XX
   Property: Operator sees in-progress phase for every task entered into a wave
     Given a workflow definition with named phases
     When for every task that emits PhaseEntered with phase=P, wave=W via the driving port
     Then the operator sees a row whose W cell renders status=in-progress with phase=P
     And the cell shape is invariant across {feature, fix, spike} task types
   

6. Build the step-defs with @given Hypothesis strategies + assert_state_delta(before, after, universe, expected) — universe entries are port-exposed names, never internal fields.

Example (good vs bad Universe)

# GOOD — port-exposed observable Universe
universe = {
    "events.PhaseEntered.emitted_count",
    "board.rows[task_id].cells[wave].status",
    "board.rows[task_id].cells[wave].phase",
}

# BAD — internal-field Universe (refactor breaks)
universe = {
    "BoardProjection._rows_cells_dict",
    "BoardProjection._rows_workflow_columns",
}

The bad Universe couples the test to private mutation details. Renaming _rows_cells_dict to _cells_by_task reds the test for an implementation rename — a refactoring-hostile signal.

Walking Skeleton vs general acceptance

• 1-2 @walking_skeleton @wiring_e2e scenarios per slice, via subprocess + real I/O, prove wiring once.
• The rest of Property: scenarios run via driving-port direct invocation with in-memory doubles for driven ports (~10ms each). Fast feedback for the use-case logic.

State-machine PBT trigger (Hebert ch.11)

State machine properties are for when the model itself is a state machine, not the system. If you can describe the SUT's behaviour by a state machine model with command/postcondition pairs, use stateful PBT. If not, use regular FORALL with property-based assertions.

This sharpens the earlier heuristic ("users perceive distinct states"): the trigger is about the model shape you can write, not the user's perception. A circuit-breaker policy is a state-machine model (ok / tripped / blocked + transitions); a sort function is not, even if its internal state has phases.

Negative testing workflow (Hebert ch.6)

Hebert ch.6 — to surface under-specification, deliberately RELAX assumptions in the test (e.g., remove a precondition, widen the input domain). If the property still holds, you've over-specified. If it fails on inputs you'd expect to be valid, the spec is incomplete. Apply when an existing property never fails — the property may be vacuously true.

Workflow:

1. Start with a happy-path property suite (positive tests).
2. Pick one assumption the suite relies on (e.g. "prices are numeric", "items list is non-empty").
3. Write a new property that relaxes that assumption.
4. Run. A crash signals (a) a real bug, (b) an under-specified contract that should fail deliberately with a clear error, or (c) a place the spec needs tightening.
5. Repeat per assumption.

The negative-testing pattern is the property-level instrument for the "inputs validated at boundaries" mandate from the production-grade quality bar.

Architecture of Reference (ports & adapters — project-level defaults)

Three classes of ports, each with a default test treatment. This table is PROJECT-LEVEL, decided once per project (typically during DESIGN of the first feature, or at framework adoption time). It is NOT renegotiated per feature. The agent applies these defaults; the per-feature decision is the MECHANISM (see Project Infrastructure Policy below), not the treatment.

This table replaces the earlier per-feature Walking Skeleton Strategy A/B/C/D choice. The decision is structural — port CLASS implies port TREATMENT — and the per-project Infrastructure Policy specializes the mechanism for each treatment.

If a port cannot be classified by the agent, ask the user with a soft prompt — do not improvise the classification.

Project Infrastructure Policy

The Architecture of Reference fixes the port class → test treatment defaults. The Project Infrastructure Policy specializes those defaults with the concrete mechanism used in THIS codebase (Testcontainers vs dedicated env vs in-memory; which fake class). The decision is made once per project, not per feature.

File location and structure

Lives at docs/architecture/atdd-infrastructure-policy.md (project-local). Three tables, one per port class, columns: Port | Mechanism | Note.

# ATDD Infrastructure Policy

## Driving
| Port | Mechanism | Note |
|---|---|---|
| HTTP API | WebApplicationFactory<Program> | |
| CLI | subprocess from tmp_path | |

## Driven internal (real)
| Port | Mechanism | Note |
|---|---|---|
| IUserRepository (MongoDB) | Testcontainers.MongoDb, fresh db per test class | |

## Driven external / non-deterministic (fake)
| Port | Fake | Note |
|---|---|---|
| IClock | FakeClock | manual advance |
| IEmailSender | FakeEmailSender | in-memory capture |

The Note column is optional — use only when the mechanism needs a one-line clarification.

Apply-if-exists / write-if-absent

1. File exists (default mode --policy=inherit): read the policy and apply recorded decisions. No port-by-port negotiation for ports already in the table.
2. Port in scope is missing from the policy: ask the user with a soft prompt (one row per missing port: which mechanism for {port}?), then append the row to the policy before generating scenarios. The policy grows by accretion.
3. File is absent: create an empty skeleton with the three section headers (use the policy-bootstrap-template expansion below), then treat every port in scope as missing (case 2).

The file is edited in place. No per-row versioning — git history is the audit trail.

--policy=fresh flag

When the user passes --policy=fresh:

• Ignore the existing file for this run.
• Treat every port in scope as missing (soft prompt per port).
• On completion, rewrite the file from scratch with the newly agreed decisions.

Use fresh for major refactors (stack swap, test strategy overhaul). In all other cases, inherit is the default.

Relationship to the Architecture of Reference

The Architecture of Reference answers: "what kind of treatment does this port class get?" (real vs fake). The Project Policy answers: "and which concrete implementation does this project use for that treatment?" (Testcontainers vs dedicated env, which fake class).

The policy CANNOT override the port class defaults: a driven-internal port cannot become a fake through the policy (that requires an explicit waiver documented in distill/wave-decisions.md). The policy only records the mechanism for each default treatment.

Wave-Decision Reconciliation HARD GATE (pre-scenario)

This is the ONLY hard gate before scenario writing. Execute it BEFORE any other DISTILL work:

1. Read all wave-decisions.md from prior waves: docs/feature/{feature-id}/discuss/wave-decisions.md, docs/feature/{feature-id}/design/wave-decisions.md, docs/feature/{feature-id}/devops/wave-decisions.md.
2. For each DISCUSS decision, check whether DESIGN or DEVOPS contradicts. Examples: DISCUSS "email notifications" but DESIGN "in-app only" = CONTRADICTION; DISCUSS "REST API" but DESIGN "gRPC" = CONTRADICTION; DISCUSS "single-tenant" but DEVOPS "multi-tenant" = CONTRADICTION.
3. If ANY contradiction → return {CLARIFICATION_NEEDED: true, questions: [{file, contradicting-decisions, ask-which-stands}]} and BLOCK.
4. If zero contradictions → log "Reconciliation passed — 0 contradictions" and proceed.

Do NOT silently pick one side of a contradiction. Do NOT write scenarios against ambiguous specifications. The cost of blocking is minutes; the cost of implementing the wrong behavior is hours.

Graceful Degradation Matrix (warn vs block)

Missing artifacts trigger warnings, not failures — EXCEPT when the missing artifact makes a design mandate unverifiable (DESIGN for hexagonal boundary). In that case, BLOCK.

Two-Tier Acceptance Composition (Mandate 10 expanded for DISTILL)

Per nw-test-design-mandates Mandate 10, acceptance tests come in two tiers. DISTILL decides which tiers apply per feature.

Default — Tier A only: most features need only Tier A (Gojko-style, production composition root, 1-2 scenarios per journey).

Add Tier B when both conditions hold:

• Feature has a journey of ≥3 chained scenarios (Pillar 2 active — the Given of N reuses N-1's Given + When), AND
• Input space is domain-rich (emails, dates, payloads, free-text, IDs from a large set).

Skip Tier B when:

• Feature is config-shaped (single-shot installer config, schema validation, one-off CLI), OR
• Journey has 1-2 scenarios (Tier A example covers the space), OR
• The only observable is "did it crash" (no state mutation to model).

File layout when both tiers are emitted

tests/{test-type-path}/{feature-id}/acceptance/
  {feature}.feature                       # Tier A — Gherkin scenarios (production DI)
  steps/
    conftest.py
    steps_{feature}.py                    # Tier A step-methods (production composition root)
  tier_b/
    test_{feature}_state_machine.py       # Tier B — RuleBasedStateMachine
    in_memory_composition.py              # InMemoryComposition (same interfaces, in-memory doubles)

Shared vocabulary contract

Both tiers invoke the same step-method names (Given_<precondition>, When_<action>, Then_<outcome>). Tier A wires them through the production composition root; Tier B wires them through InMemoryComposition. The step-method NAMES are the contract — DRY across tiers.

When DISTILL emits Tier B, it MUST verify each @rule-decorated method invokes a step-method that already exists in the Tier A steps_{feature}.py. New step-method names introduced only in Tier B are a smell — they hint the journey was modeled differently for in-memory exploration than for production wiring.

Wave: DISTILL / [HOW] Expansion Templates (lazy)

These templates are inline so the skill ships with the bootstrap snippets. They are emitted into the wave's feature-delta.md only when rendered as Tier-2 expansions (per the Density Resolution + Expansion Catalog above).

Expansion policy-bootstrap-template

Emitted on first DISTILL in a project (file absent at docs/architecture/atdd-infrastructure-policy.md):

# ATDD Infrastructure Policy

Per `nw-distill` § Project Infrastructure Policy. One file per project. Apply-if-exists; write-if-absent; rewrite with `--policy=fresh`. Git history is the audit trail.

## Driving
| Port | Mechanism | Note |
|---|---|---|

## Driven internal (real)
| Port | Mechanism | Note |
|---|---|---|

## Driven external / non-deterministic (fake)
| Port | Fake | Note |
|---|---|---|

Expansion tier-b-state-machine-template (Python pilot)

Reference shape for tests/{feature-id}/acceptance/tier_b/test_{feature}_state_machine.py. Other host languages add their own template lazily.

# tests/<path>/tier_b/test_<feature>_state_machine.py
import hypothesis.strategies as st
from hypothesis.stateful import RuleBasedStateMachine, rule, precondition, invariant, initialize

from nwave_ai.state_delta import assert_state_delta, set_to, unchanged, appended_with

# Import shared step-method vocabulary (same as Tier A .feature steps)
from tests.<path>.acceptance.steps.steps_<feature> import (
    Given_<precondition>,
    When_<action>,
    Then_<outcome>,
    # ...
)

# In-memory composition root (Tier B difference from Tier A's production DI)
from tests.<path>.acceptance.tier_b.in_memory_composition import InMemoryComposition


class <Feature>Journey(RuleBasedStateMachine):
    @initialize()
    def setup(self):
        self.composition = InMemoryComposition()
        Given_<precondition>(self.composition)
        # ... initial state flags as instance attributes for @precondition use

    @rule(<input_strategies>)
    def <action>(self, ...):
        before = self.composition.capture_universe()
        When_<action>(self.composition, ...)  # SAME step-method as Tier A
        after = self.composition.capture_universe()
        assert_state_delta(
            before=before,
            after=after,
            universe={
                "<port_exposed_name_1>",
                "<port_exposed_name_2>",
            },
            expected={
                "<port_exposed_name_1>": set_to(<value>),
                "<port_exposed_name_2>": unchanged(),
            },
        )

    @invariant()
    def <invariant_name>(self):
        # universe-bound cross-rule invariant
        Then_<outcome>(self.composition)


Test<Feature>Journey = <Feature>Journey.TestCase

InMemoryComposition.capture_universe() returns a dict whose keys are the port-exposed observable names declared in the universe set. Tier A steps_*.py and Tier B @rule methods agree on these names — they are the shared contract.

Expansion domain-language-fact-to-step-table

Soft-gate table proposed to the user BEFORE step-methods are generated. One row per Given/When/Then surface used in the planned scenarios. User approval is a quick exchange, not a formal blocking gate — but renaming an established step-method is expensive, so the agent surfaces the names early.

The table is emitted into feature-delta.md under ## Wave: DISTILL / [HOW] Domain language when the user requests this expansion (or when density.mode = "full").

DSL Emergence + SSOT via Types + Services (Mandate-12)

Mandate-12 — SSOT + Zero Duplication via Types + Services + DSL (2026-05-18, identity-essential; refined Opt 3 same day): domain concepts are expressed once via the type system; logic lives in services (composition root or driving-port methods) as single source of truth; step definitions, code, and tests reuse types and services to eliminate duplication. The DSL emerges from typed domain concepts — parameterized templates over enum-typed parameters, not 200+ unique step decorators. Domain types live in tests/{path}/acceptance/steps/domain_types.py (Python pilot); step methods invoke composition-root service methods, never inline business logic.

Four-criteria mechanical evidence (refined 2026-05-18)

Compliance is mechanical, not ratio-based:

1. Domain types module exists at tests/{path}/acceptance/steps/domain_types.py with typed enums / dataclasses / NewTypes for every domain noun used in Gherkin.
2. Composition methods consume typed parameters — service signatures use the enums from domain_types.py; no raw str parameters where a domain enum already exists.
3. No business logic in step bodies — AST mechanical check: each step function body has ≤2 statements, the final statement is composition.<service>.<method>(...), and the body contains no control flow (if/for/while/try).
4. Step-reuse-ratio reported as informational — total_step_invocations / unique_step_decorators measured per feature for natural-ceiling discovery. NOT a gate.

Natural-ceiling discovery (criterion 4): run the measurement formula below per feature and document the ratio in distill/wave-decisions.md alongside the feature shape. Config-shaped features (single-shot installer, schema validation) naturally cap below 4×; journey-rich features may exceed it. There is no target ratio — the empirical ceiling per feature is the data point. Substance is criteria (1)–(3); ratio is a symptom heuristic.

Empirical anchor: F-ENTERPRISE-RELEASE-READINESS DISTILL pre-refactor 1.13× (227 occurrences / 201 decorators) → post-refactor 1.43× natural ceiling with all four mechanical criteria met (14 typed enums, 45 composition methods, zero step-body logic). The 1.43× MISS vs the original ≥4× hard target concealed the substantive SUBSTANCE WIN — refined per source-vs-symptom discipline (memory feedback_lyra_failure_modes_2026_05_05).

Refactor pattern — parameterized templates over enum-typed parameters

# tests/{path}/acceptance/steps/domain_types.py
from enum import Enum
from dataclasses import dataclass

class PortClass(Enum):
    DRIVING = "driving"
    DRIVEN_INTERNAL = "driven_internal"
    DRIVEN_EXTERNAL = "driven_external"

class CommitStatus(Enum):
    PENDING = "pending"
    APPROVED = "approved"
    REJECTED = "rejected"

@dataclass(frozen=True)
class Customer:
    id: int
    email: str

# tests/{path}/acceptance/steps/steps_<feature>.py
from pytest_bdd import given, when, then, parsers
from tests.<path>.acceptance.steps.domain_types import PortClass, CommitStatus, Customer
from src.<your_app>.composition_root import build_app

@given(parsers.parse('a {port_class:PortClass} port for {port_name}'))
def given_port(port_class: PortClass, port_name: str):
    # Single decorator handles ALL port-class scenarios — no decorator-per-class duplication.
    # Body delegates to composition-root service method, NEVER inline business logic.
    app = build_app()
    app.register_port(port_class, port_name)

@when(parsers.parse('the operator submits a commit with status {status:CommitStatus}'))
def when_submit(status: CommitStatus):
    app = build_app()
    app.commit_service.submit(status)

@then(parsers.parse('the commit status becomes {status:CommitStatus}'))
def then_status(status: CommitStatus):
    app = build_app()
    assert app.commit_service.current_status() == status

Three Given/When/Then decorators cover the entire port_class × status cartesian — instead of 9 decorators (3 ports × 3 statuses), the DSL emerges from enum typing.

Anti-pattern — hard-coded literals + decorator proliferation

# BAD — one decorator per literal value (200+ decorators when scaled across the feature)
@given('a driving port for HTTP API')
def given_http_api(): ...

@given('a driving port for CLI')
def given_cli(): ...

@given('a driven_internal port for IUserRepository')
def given_user_repo(): ...

# BAD — step methods with inline business logic (logic SSOT violation)
@when('the operator submits a commit with status approved')
def when_submit_approved():
    # business logic inlined in step — duplicates production code,
    # diverges under refactor, breaks SSOT contract
    audit_log = []
    audit_log.append({"status": "approved", "timestamp": time.time()})
    assert audit_log[-1]["status"] == "approved"

Service consolidation guidance

• Composition-root service methods: business logic lives in src/<your_app>/<service>.py (or composition_root.py), invoked by step methods via the production DI container (Pillar 3).
• Step decorators delegate, never inline: the decorator body is ≤3 lines — fetch composition root, call service method, optionally capture observable for assertion.
• Domain types as decorator parameter coercers: parsers.parse (pytest-bdd) with an enum type converts the literal token to the typed value at parse time. The decorator's body sees port_class: PortClass, not a string.
• Tier B reuse: state-machine @rule methods import the SAME step methods from steps_<feature>.py — the domain types and service invocations are shared across Tier A (production composition) and Tier B (InMemoryComposition). Shared vocabulary contract per Mandate 10.

Empirical measurement (bash) — informational only

Used for criterion (4) above. The output is recorded as the per-feature natural ceiling, NOT compared against a target threshold.

TOTAL_OCCURRENCES=$(grep -cE '^\s*(Given|When|Then|And|But) ' tests/<path>/acceptance/*.feature | awk -F: '{sum+=$2} END {print sum}')
UNIQUE_DECORATORS=$(grep -cE '^@(given|when|then|step|when_then)' tests/<path>/acceptance/steps/steps_*.py)
RATIO=$(echo "scale=2; $TOTAL_OCCURRENCES / $UNIQUE_DECORATORS" | bc)
echo "step-reuse-ratio (informational): ${RATIO}x — natural ceiling for this feature shape"

Compliance is determined by criteria (1)–(3) (types module, typed signatures, no-logic-in-steps). The ratio informs natural-ceiling discovery; a low ratio on a config-shaped feature is expected and not a redesign signal.

Anti-pattern — forcing the ratio at the cost of Pillar 1

Collapsing readable Gherkin into one parameterized step that sacrifices domain coherence purely to raise the ratio. Example: merging Given the policy is approved and Given the artifact is published into Given the {noun:Object} is {verb:Action} to gain reuse — the resulting Gherkin is harder for a stakeholder to read and the domain language is degraded. Pillar 1 (Gherkin readability) outranks the ratio. If criteria (1)–(3) are met and the ratio is below 4×, the feature is compliant — that is the calibrated outcome.

Scope guard

Per [[feedback_target_machine_independence_2026_05_15]]: Mandate-12 applies to acceptance test infrastructure. Production code invariants live in core + plugin (src/des/, scripts/install/plugins/) and are out of scope for the step-reuse-ratio metric — they have their own SSOT discipline via hexagonal layering. Mandate-12 governs how tests EXPRESS contracts, not how production code STRUCTURES logic.

Retrofit scope

• Forward-only on new features: every new DISTILL session MUST meet the four mechanical criteria (types module, typed signatures, no-logic-in-steps, ratio measured + documented).
• Retrofit on slow tests: dogfooding scope per Ale 2026-05-18 — drastic suite-execution-time reduction goal. See backlog F-ATDD-MANDATE-12-SSOT-DUPLICATION + ADR-026.
• No retroactive enforcement on existing tests beyond the hot-path slow-test set. Audit logs of pre-2026-05-18 features remain valid; the mandate is a forward-looking quality bar.

Pre-DELIVER fail-for-the-right-reason gate

Before handing acceptance scenarios to DELIVER, run them once and verify each scenario fails for the right reason — the implementation is missing — not for setup error, fixture bug, import error, or test infrastructure problem.

Procedure

1. Run the suite: pytest tests/{feature}/acceptance/. Capture failure output per scenario.
2. For each FAIL, classify the failure mode:
   • MISSING_FUNCTIONALITY: the assertion fires because behaviour is unimplemented (✅ correct RED)
   • IMPORT_ERROR / FIXTURE_BROKEN / SETUP_FAILURE: the test never reaches the assertion (❌ wrong RED — test bug)
   • WRONG_ASSERTION / OBSERVABLE_NOT_AT_PORT: assertion couples to internal struct (❌ wrong shape — fix Universe)
3. If any scenario is in category 2 or 3 → BLOCK handoff to DELIVER. Fix the test before crafter starts.

Why this gate matters

A scenario that fails for the wrong reason gives a false signal at GREEN: the crafter "fixes" the import error and the test goes green, but the feature was never tested. We have observed this class on 2026-05-06 (feedback_fixture_only_acceptance_hides_wiring_2026_05_06.md + feedback_layered_test_discipline_universe_per_layer_2026_05_06.md): a fixture-shape acceptance scenario passes against a wired-but-broken bridge because it never exercises the seam.

The gate output is a one-line classification per failing scenario, written to docs/feature/{feature-id}/distill/red-classification.md. DELIVER reads this file at PREPARE phase to confirm RED is genuine.

Prior Wave Reading

Before writing any scenario, read SSOT and feature delta artifacts.

READING ENFORCEMENT: You MUST read every file listed in steps 1-6 below using the Read tool before proceeding. After reading, output a confirmation checklist (+ {file} for each read, - {file} (not found) for missing). Do NOT skip files that exist.

1. Read Journeys — Read docs/product/journeys/{name}.yaml. Extract embedded Gherkin as starting scenarios, identify integration checkpoints and failure_modes per step. Gate: file read or marked missing.
2. Read Architecture Brief — Read docs/product/architecture/brief.md. Identify driving ports (from ## For Acceptance Designer section) for @driving_port tagged scenarios. Gate: file read or marked missing.
3. Read KPI Contracts — Read docs/product/kpi-contracts.yaml. Identify behaviors needing @kpi tagged scenarios (soft gate — warn if missing, proceed). Gate: file read or marked missing.
4. Read DISCUSS Artifacts — Read docs/feature/{feature-id}/discuss/user-stories.md (scope boundary and embedded acceptance criteria), story-map.md (walking skeleton priority and release slicing), and wave-decisions.md (quick check for upstream changes). Gate: files read or marked missing.
5. Read SPIKE Findings (if spike was run) — Read docs/feature/{feature-id}/spike/findings.md and docs/feature/{feature-id}/spike/wave-decisions.md. Check what assumptions were validated, what failed, performance measurements, and the promotion decision (PROMOTE / DISCARD / PIVOT). Update acceptance criteria if spike findings contradict DISCUSS. Gate: files read if present, marked as not found if absent. 5b. Read Walking Skeleton (only if SPIKE promoted a walking skeleton) — Read the existing tests/{test-type-path}/{feature-id}/acceptance/walking-skeleton.feature and the src/ modules it exercises. The walking skeleton is already committed and green — your job in DISTILL is to build additional scenarios and integration tests on top of it, not to rewrite it. Identify the driving adapter it uses, the e2e path it exercises, and the scenarios it does NOT yet cover (happy-path variants, error paths, adapter integration). Gate: walking-skeleton.feature read, scenario tagged @walking_skeleton confirmed green, or marked as not found.
6. Read DEVOPS Artifacts — Read docs/feature/{feature-id}/devops/wave-decisions.md. Check for infrastructure constraints affecting tests. Gate: file read or marked missing. 6b. Read Deliverable Type (ADR-PST-003 / DDD-6) — Read deliverable_type from the SAME .nwave/des-config.json the DES runtime gate uses — this is the single source of truth (DESConfig.deliverable_type precedence, ADR-PST-002): (1) declared project .nwave/des-config.json key deliverable_type if in the known set {application, plugin, skill}; (2) else global ~/.nwave/global-config.json defaults.deliverable_type; (3) else root-only FS detection; (4) a present-but-typo'd value resolves to the safe default (treated as application). Do NOT re-detect independently — read what the g

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