Imported from jamesaphoenix/tx-agent-kit (.codex/skills/design-doc/SKILL.md). Install upstream with npx skills add jamesaphoenix/tx-agent-kit --skill design-doc. Copyright stays with the author.

Generate Design Document

Create a comprehensive technical design document using the tx doc primitive. Design docs specify HOW the system implements requirements, with traceable invariants and a concrete testing strategy.

Design Doc + PRD are companions. A PRD defines WHAT and WHY. A design doc defines HOW. If a PRD exists for this feature, the design doc reads it automatically and maps every must-priority EARS requirement to an invariant + verification entry.

Workflow State Machine

START
  │
  ▼
┌─────────────────────────────────────────────────────┐
│ Step 0: PLAN GATE                                    │
│                                                      │
│ Is there an active plan in this conversation?        │
│                                                      │
│ ├─ YES → Save plan to `~/.codex/plans/<name>.md` if not       │
│ │        already saved. Set `plan: ~/.codex/plans/<name>.md`  │
│ │        in frontmatter.                             │
│ │        → Continue to Step 0.5                      │
│ │                                                    │
│ └─ NO  → Tell the user to run /plan first.           │
│          If enough detail provided, generate plan,   │
│          save to `~/.codex/plans/<name>.md`.                  │
│          → Continue to Step 0.5                      │
└─────────────────────────────────────────────────────┘
  │
  ▼
┌─────────────────────────────────────────────────────┐
│ Step 0.5: CHECK FOR COMPANION PRD                    │
│                                                      │
│ Run: tx doc list --kind prd                          │
│                                                      │
│ ├─ PRD exists for this feature?                      │
│ │   → tx doc show <prd-name> --md                    │
│ │   → Extract ALL EARS requirements                  │
│ │   → Each `must` EARS req MUST get an invariant     │
│ │     and a verification entry in this design doc    │
│ │   → Set `implements: <prd-name>` in frontmatter   │
│ │   → Continue to Step 1                             │
│ │                                                    │
│ └─ No companion PRD?                                 │
│     → Continue to Step 1 (design doc stands alone)   │
│     → Suggest creating PRD after: /prd <name>        │
└─────────────────────────────────────────────────────┘
  │
  ▼
┌─────────────────────────────────────────────────────┐
│ Step 1: SCAFFOLD via tx                              │
│                                                      │
│ tx doc add design <name> --title "<title>"           │
│ ├─ SUCCESS → Continue to Step 2                      │
│ └─ FAIL (exists) → Edit existing doc                 │
└─────────────────────────────────────────────────────┘
  │
  ▼
┌─────────────────────────────────────────────────────┐
│ Step 2: DEEP CONTEXT GATHERING                       │
│                                                      │
│ Read: ARCHITECTURE.md, QUALITY.md, CLAUDE.md,        │
│       domain code, schema.ts, effect-schemas,        │
│       API routes, workflows, activities,             │
│       existing designs (tx doc list --kind design)   │
│ → Continue to Step 3                                 │
└─────────────────────────────────────────────────────┘
  │
  ▼
┌─────────────────────────────────────────────────────┐
│ Step 3: FILL DOCUMENT                                │
│                                                      │
│ Write `# Plan` first (reference to plan file from     │
│ Step 0).                                             │
│ Then fill all sections from plan + PRD + codebase.   │
│                                                      │
│ MINIMUM THRESHOLDS:                                  │
│   - Invariants: ≥ 5                                  │
│   - Failure modes: ≥ 3                               │
│   - Verification entries: ≥ 5                        │
│   - Integration test files: ≥ 2 (HARD REQUIREMENT)   │
│   - Unit test files: ≥ 1 (recommended, not hard)     │
│   - Sequence diagrams: ≥ 2 (happy + error)           │
│   - Design decisions: ≥ 1                            │
│                                                      │
│ Integration tests are the primary verification       │
│ mechanism. Unit tests complement but do not replace   │
│ integration tests.                                   │
│                                                      │
│ RULE: No section may be left as a template/stub.     │
│                                                      │
│ COMPREHENSIVENESS: The design doc must cover EVERY   │
│ item from the plan. Every implementation step,       │
│ constraint, risk, and decision in the plan must      │
│ appear in the design doc with full technical detail.  │
│ The design doc is the single source of truth for HOW │
│ the feature is built — it should be detailed enough  │
│ that an engineer can implement from it alone.        │
└─────────────────────────────────────────────────────┘
  │
  ▼
┌─────────────────────────────────────────────────────┐
│ Step 4: SELF-AUDIT                                   │
│                                                      │
│ Check:                                               │
│ ├─ Every plan item captured in a section?            │
│ ├─ Every PRD `must` EARS req has an invariant?       │
│ ├─ Every invariant has a verification entry?         │
│ ├─ Minimums met?                                     │
│ ├─ No stubs/placeholders?                            │
│ ├─ All diagrams complete?                            │
│ └─ Plan file exists at frontmatter path and is       │
│    consistent with doc sections?                     │
└─────────────────────────────────────────────────────┘
  │
  ▼
┌─────────────────────────────────────────────────────┐
│ Step 5: VALIDATE                                     │
│                                                      │
│ tx spec lint                                         │
│ ├─ PASS → Continue to Step 6                         │
│ └─ WARN/FAIL → Fix, re-validate                     │
└─────────────────────────────────────────────────────┘
  │
  ▼
┌─────────────────────────────────────────────────────┐
│ Step 5.5: SYNC PLAN FILE                             │
│                                                      │
│ Read the plan file from frontmatter `plan:` path.    │
│ Compare with what the doc now contains.              │
│ UPDATE the plan file to incorporate:                 │
│   - Architecture decisions, component inventory      │
│   - Interface contracts, data model details          │
│   - Invariants, failure modes, error handling        │
│   - Implementation sequence, testing strategy        │
│ The plan file must reflect the FULL current state    │
│ of the feature — not just the initial draft.         │
│ This is a MANDATORY step, not optional.              │
└─────────────────────────────────────────────────────┘
  │
  ▼
┌─────────────────────────────────────────────────────┐
│ Step 6: DISCOVER + LINK + REPORT                     │
│                                                      │
│ tx spec discover --doc <name>                        │
│ tx doc link <prd> <design> (if PRD exists)           │
│ tx doc show <name>                                   │
│ tx spec gaps --doc <name>                            │
│ Print summary                                        │
└─────────────────────────────────────────────────────┘
  │
  ▼
DONE

Step 0 — Plan Gate

The plan is the primary input. Check for plan content in the conversation:

• The plan is saved as a standalone file at ~/.codex/plans/<name>.md (relative to repo root).
• If a plan already exists in the conversation, write it to that file.
• If a plan file already exists at that path, read it instead.
• If no plan and vague request, ask user to run /plan first.

The doc's frontmatter gets plan: ~/.codex/plans/<name>.md and the # Plan section contains a reference link + brief summary, not the full verbatim content.

Step 0.5 — Check for Companion PRD

tx doc list --kind prd

If a PRD exists for this feature:

1. Read it: tx doc show <prd-name> --md
2. Extract every EARS requirement
3. Every must-priority EARS requirement MUST become:
   • An invariant in invariants: YAML block
   • A verification entry in verification: YAML block
4. Set implements: <prd-name> in frontmatter

If no PRD exists, the design doc stands alone. Suggest creating one after.

Step 1 — Scaffold via tx

tx doc add design $ARGUMENTS --title "<Human-Readable Title>"

Creates specs/design/<name>.md. If exists, edit instead.

Step 2 — Deep Context Gathering

Read these files:

• docs/ARCHITECTURE.md — architecture + DDD structure
• docs/QUALITY.md — all invariants, governance rules
• CLAUDE.md — stack, conventions
• Companion PRD (from Step 0.5)
• Domain code: packages/core/src/domains/
• Database schema: packages/infra/db/src/schema.ts
• Effect schemas: packages/infra/db/src/effect-schemas/
• API routes: apps/api/src/
• Workflows: apps/worker/src/workflows.ts
• Activities: apps/worker/src/activities.ts
• Existing designs: tx doc list --kind design

Step 3 — Fill the Document

Required Frontmatter (already generated by tx)

---
kind: spec
spec_type: design
name: <name>
title: "<title>"
status: draft
version: 1
owners:
  - <team-or-person>
summary: Technical approach for <title>
domain: <product-area>
tags:
  - design
depends_on: []
supersedes: []
implements: <prd-name-or-null>
last_reviewed_at: <YYYY-MM-DD>
plan: ~/.codex/plans/<name>.md
---

Update owners, summary, domain, tags, depends_on, implements.

Body Structure — ALL sections MUST have real content

# Plan comes first (as a reference to the plan file). Then all technical sections. No section may be a stub.

If a companion PRD exists, every must EARS requirement maps to an invariant + verification entry.

# Plan

> Full plan: [~/.codex/plans/<name>.md](../~/.codex/plans/<name>.md)

<2-3 sentence summary of what the plan covers. The full plan lives in the file referenced above.>

# Summary

2-3 sentences on design approach and key technical decisions.

# Architecture

## System Context

Where this feature sits in the existing system. Reference `docs/ARCHITECTURE.md`.

## Component Diagram

┌──────────────────────────────────────────────────────┐ │ API Layer │ │ ┌──────────────────────────────────────────┐ │ │ │ apps/api [MOD] │ │ │ │ ├── routes/.ts [NEW] │ │ │ └──────────────────┬───────────────────────┘ │ └─────────────────────┼────────────────────────────────┘ ▼ ┌──────────────────────────────────────────────────────┐ │ Domain Layer │ │ ┌──────────────────────────────────────────┐ │ │ │ packages/core/src/domains// [NEW]│ │ │ │ ├── domain/ [NEW] │ │ │ │ ├── ports/ [NEW] │ │ │ │ ├── application/ [NEW] │ │ │ │ └── adapters/ [NEW] │ │ │ └──────────────────┬───────────────────────┘ │ └─────────────────────┼────────────────────────────────┘ ▼ ┌──────────────────────────────────────────────────────┐ │ Infrastructure Layer │ │ ┌─────────────┐ │ │ │ infra/db │ │ │ │ [MOD] │ │ │ └─────────────┘ │ └──────────────────────────────────────────────────────┘

Mark: `[NEW]`, `[MOD]`, or unmarked.

## Component Inventory

| Component | Package/App | Responsibility | New/Modified |
|-----------|-------------|---------------|--------------|

## Design Decisions

**MINIMUM: ≥ 1 decision with ADR-lite format.**

### Decision 1: <Title>

**Context:** Why needed.
**Options:**

| Option | Pros | Cons |
|--------|------|------|

**Decision:** Option X because <reasoning>.
**Consequences:** What follows.

# Interfaces

```yaml
interfaces:
  - name: <InterfaceName>
    type: api_endpoint | port | event_payload | schema
    definition: |
      // TypeScript/Effect Schema definition

API Endpoints

Request/Response Schemas

import { Schema } from 'effect'
export const Create<X>Request = Schema.Struct({ /* fields */ })
export const Create<X>Response = Schema.Struct({ /* fields */ })

Error Responses

Port Contracts

export interface <Domain>Repository {
  create(input: Create<X>Input): Effect.Effect<X, <Error>>
  findById(id: string): Effect.Effect<X | null, <Error>>
  findMany(filter: <Filter>): Effect.Effect<readonly X[], <Error>>
  update(id: string, input: Update<X>Input): Effect.Effect<X, <Error>>
  remove(id: string): Effect.Effect<void, <Error>>
}

Event Payloads (if applicable)

export interface <Domain><Verb>EventPayload { /* fields */ }
export const <Domain><Verb>EventPayloadSchema = Schema.Struct({ /* fields */ })

Data Model

New Tables

CREATE TABLE <table_name> (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
  updated_at TIMESTAMPTZ NOT NULL DEFAULT now()
);
CREATE INDEX idx_<table>_<column> ON <table_name>(<column>);

Modified Tables

ALTER TABLE <existing_table> ADD COLUMN <column> <type>;

Effect Schemas

export const <Table>Schema = Schema.Struct({ /* columns */ })

Factories

export const create<Table>Factory = (overrides?: Partial<Table>): Table => ({ /* defaults */ })

Entity Relationship Diagram

┌──────────────┐     ┌──────────────┐     ┌──────────────┐
│ organizations │────<│    teams      │────<│   <entity>   │
│              │  1:N │              │  1:N │              │
└──────────────┘     └──────────────┘     └──────────────┘

Invariants

MINIMUM: ≥ 5 invariants. If companion PRD exists, every must EARS req MUST map to an invariant.

invariants:
  - id: INV-<SCOPE>-001
    statement: <what must be true>
    enforcement: <lint script | test | pgTAP>
    traces_to: REQ-<SCOPE>-001  # link to PRD requirement if applicable

Failure Modes

MINIMUM: ≥ 3 failure modes.

failure_modes:
  - id: FM-<SCOPE>-001
    trigger: <what causes failure>
    impact: <user/system effect>
    detection: <logs, metrics, alerts>
    mitigation: <automatic response>
    recovery: <human intervention>

Verification

MINIMUM: ≥ 5 entries. Every invariant MUST have a verification entry.

verification:
  - invariant: INV-<SCOPE>-001
    test_file: <path>
    test_name: "description [INV-<SCOPE>-001]"
    type: unit | integration | lint | pgtap

Sequence Diagrams

MINIMUM: ≥ 2 (happy path + at least one error path).

Happy Path:

Client              API                   Core                  DB                   Worker
  │                  │                     │                    │                     │
  │ POST /api/<x>    │                     │                    │                     │
  │─────────────────▶│                     │                    │                     │
  │                  │ validate + auth      │                    │                     │
  │                  │ create<X>(input)     │                    │                     │
  │                  │────────────────────▶│                    │                     │
  │                  │                     │ BEGIN TXN           │                     │
  │                  │                     │───────────────────▶│                     │
  │                  │                     │ INSERT + event      │                     │
  │                  │                     │───────────────────▶│                     │
  │                  │                     │ COMMIT             │                     │
  │                  │                     │───────────────────▶│                     │
  │                  │◀────────────────────│                    │                     │
  │ 201 Created      │                     │                    │                     │
  │◀─────────────────│                     │                    │ poll + dispatch      │
  │                  │                     │                    │◀────────────────────│

Error Path:

Client              API
  │ POST /api/<x>    │
  │─────────────────▶│
  │                  │ <failure point>
  │ <status code>    │
  │◀─────────────────│

Testing Strategy

Integration tests are the primary verification mechanism and a HARD REQUIREMENT. Unit tests are recommended but secondary.

Integration Tests

HARD REQUIREMENT: ≥ 2 integration test files listed. Design docs MUST NOT be considered complete without integration tests.

Integration Test Patterns

describe('<Domain> API', () => {
  it('returns 401 for unauthenticated [INV-<SCOPE>-003]', async () => { /* ... */ })
  it('creates resource with valid input', async () => { /* ... */ })
})

Unit Tests (recommended, not hard requirement)

Unit Test Patterns

describe('<Entity>', () => {
  it('creates valid entity [INV-<SCOPE>-001]', () => { /* ... */ })
  // @spec REQ-<SCOPE>-001
  it('rejects invalid input', () => { /* ... */ })
})

Database Contract Tests (pgTAP)

Test Annotation Convention

it('description [INV-<SCOPE>-001]', () => { ... })  // preferred
// @spec INV-<SCOPE>-001                              // alternative

Migration Strategy

Database Migration

File: packages/infra/db/migrations/NNNN_<description>.sql

Rollback Plan

Security Considerations

Cross-Cutting Concerns

Observability

Data Retention

Implementation Sequence

Phase 1: Domain Foundation
  ├── <specific file paths>

Phase 2: Infrastructure
  ├── <specific file paths>

Phase 3: Application Layer
  ├── <specific file paths>

Phase 4: API Layer
  ├── <specific file paths>

Phase 5: Worker (if domain events)
  ├── <specific file paths>

Phase 6: Quality Gates
  ├── pnpm lint && pnpm type-check && pnpm test && pnpm test:integration
  ├── tx spec discover --doc <name>
  └── tx spec fci --doc <name>

Open Questions

• Questions requiring design input

## Step 4 — Self-Audit

Re-read the plan file and verify:
1. **Comprehensiveness check**: Read the plan file. For EVERY item (implementation steps, constraints, risks, decisions), confirm it has full technical detail in the design doc. The design doc must be detailed enough to implement from alone.
2. If PRD exists: every `must` EARS req has an invariant. Every invariant has a verification entry.
3. HARD: ≥2 integration test files (blocking requirement). SOFT: ≥1 unit test file (recommended).
4. Minimums met: ≥5 invariants, ≥3 failure modes, ≥5 verifications, ≥2 sequence diagrams, ≥1 decision.
5. No stubs, no empty tables, no "..." placeholders.
6. All diagrams complete.
7. Implementation sequence lists real file paths.
8. Verify plan file exists at the path in frontmatter and its content is consistent with the doc sections.

## Step 5 — Validate

```bash
tx spec lint

Step 5.5 — Sync Plan File (MANDATORY)

After filling and validating the doc, update the plan file at the plan: frontmatter path to reflect everything the design doc surfaced. The plan file must be the living source of truth — not a stale initial draft.

What to add to the plan file:

• Architecture decisions and their rationale
• Component inventory and file paths
• Interface contracts (API endpoints, port signatures, event payloads)
• Data model details (tables, indexes, constraints)
• Implementation sequence with specific file paths
• Invariants, failure modes, and error handling strategies
• Testing strategy and test file locations

Read the current plan file, merge in the new information, and write it back. Preserve the plan's structure but ensure it now covers the full technical design.

Step 6 — Discover, Link & Report

tx spec discover --doc <name>
tx doc link <prd> <design>       # if PRD exists
tx doc show <name>
tx spec gaps --doc <name>

After Generation

1. Print output path (specs/design/<name>.md).
2. Summarize: component count, invariant count, failure mode count, verification count, test file count.
3. If PRD exists, show EARS→invariant mapping coverage.
4. List open questions.
5. Run tx spec lint.
6. If the plan file is modified later, update the # Plan summary and derived sections in this doc. If this doc's scope changes, update the plan file to stay consistent.