Sdd Spec

---
name: sdd-spec
description: "Write SDD delta specs with requirements and scenarios. Trigger: orchestrator launches spec work for a change."
disable-model-invocation: true
user-invocable: false
license: MIT
metadata:
  author: gentleman-programming
  version: "2.0"
  delegate_only: true
---

> **ORCHESTRATOR GATE**: If you loaded this skill via the `skill()` tool, you are
> the ORCHESTRATOR — STOP. Do NOT execute these instructions inline. Delegate to
> the dedicated `sdd-spec` sub-agent using your platform's delegation primitive
> (e.g., `task(...)`, sub-agent invocation, etc.). This skill is for EXECUTORS
> only.

## Purpose

You are a sub-agent responsible for writing SPECIFICATIONS. You take the proposal and produce delta specs — structured requirements and scenarios that describe what's being ADDED, MODIFIED, or REMOVED from the system's behavior.

## What You Receive

From the orchestrator:
- Change name
- Artifact store mode (`engram | openspec | hybrid | none`)

## Execution and Persistence Contract

> Follow **Section B** (retrieval) and **Section C** (persistence) from `skills/_shared/sdd-phase-common.md`.

- **engram**: Read `sdd/{change-name}/proposal` (required). If specs span multiple domains, concatenate into a single artifact with domain headers. Save as `sdd/{change-name}/spec`.
- **openspec**: Read and follow `skills/_shared/openspec-convention.md`.
- **hybrid**: Follow BOTH conventions — persist to Engram (single concatenated artifact) AND write domain files to filesystem.
- **none**: Return result only. Never create or modify project files.

## What to Do

### Step 1: Load Skills
Follow **Section A** from `skills/_shared/sdd-phase-common.md`.

### Step 2: Identify Affected Domains

Read the proposal's **Capabilities section** — this is your primary contract:

```
FOR EACH entry under "New Capabilities":
├── This becomes a NEW full spec: openspec/specs/<capability-name>/spec.md
└── Write a complete spec (not a delta) — no existing behavior to reference

FOR EACH entry under "Modified Capabilities":
├── This becomes a DELTA spec: openspec/changes/{change-name}/specs/<capability-name>/spec.md
└── Read existing openspec/specs/<capability-name>/spec.md first — your delta modifies it
```

If the proposal has no Capabilities section (older format), fall back to inferring from "Affected Areas". But always prefer the explicit Capabilities mapping when present.

### Step 3: Read Existing Specs

**IF mode is `openspec` or `hybrid`:** If `openspec/specs/{domain}/spec.md` exists, read it to understand CURRENT behavior. Your delta specs describe CHANGES to this behavior.

**IF mode is `engram`:** Existing specs were already retrieved from Engram in the Persistence Contract. Skip filesystem reads.

**IF mode is `none`:** Skip — no existing specs to read.

### Step 4: Write Delta Specs

**IF mode is `openspec` or `hybrid`:** Create specs inside the change folder:

```
openspec/changes/{change-name}/
├── proposal.md              ← (already exists)
└── specs/
    └── {domain}/
        └── spec.md          ← Delta spec
```

**IF mode is `engram` or `none`:** Do NOT create any `openspec/` directories or files. Compose the spec content in memory — you will persist it in Step 5.

#### MODIFIED Requirements Workflow (CRITICAL — read before writing deltas)

When writing a `## MODIFIED Requirements` section, follow this exact workflow:

```
1. Locate the requirement in openspec/specs/{domain}/spec.md
2. COPY the ENTIRE requirement block — from `### Requirement:` through ALL its scenarios
3. PASTE it under `## MODIFIED Requirements`
4. EDIT the copy to reflect the new behavior
5. Add "(Previously: {one-line summary of what changed})" under the requirement text

Why copy-full-then-edit?
→ The archive step REPLACES the requirement in main specs with your MODIFIED block
→ If your block is partial, the archive will lose scenarios you didn't copy
→ Common pitfall: only writing the changed scenario and losing the rest
→ If adding NEW behavior WITHOUT changing existing behavior, use ADDED instead
```

#### Delta Spec Format

```markdown
# Delta for {Domain}

## ADDED Requirements

### Requirement: {Requirement Name}

{Description using RFC 2119 keywords: MUST, SHALL, SHOULD, MAY}

The system {MUST/SHALL/SHOULD} {do something specific}.

#### Scenario: {Happy path scenario}

- GIVEN {precondition}
- WHEN {action}
- THEN {expected outcome}
- AND {additional outcome, if any}

#### Scenario: {Edge case scenario}

- GIVEN {precondition}
- WHEN {action}
- THEN {expected outcome}

## MODIFIED Requirements

### Requirement: {Existing Requirement Name}

{Full updated requirement text — replaces the existing one entirely}
(Previously: {what it was before, in one line})

#### Scenario: {Unchanged scenario — keep if still valid}

- GIVEN {precondition}
- WHEN {action}
- THEN {outcome}

#### Scenario: {Updated or new scenario}

- GIVEN {updated precondition}
- WHEN {updated action}
- THEN {updated outcome}

## REMOVED Requirements

### Requirement: {Requirement Being Removed}

(Reason: {why this requirement is being deprecated/removed})
```

#### For NEW Specs (No Existing Spec)

If this is a completely new domain, create a FULL spec (not a delta):

```markdown
# {Domain} Specification

## Purpose

{High-level description of this spec's domain.}

## Requirements

### Requirement: {Name}

The system {MUST/SHALL/SHOULD} {behavior}.

#### Scenario: {Name}

- GIVEN {precondition}
- WHEN {action}
- THEN {outcome}
```

### Step 5: Persist Artifact

**This step is MANDATORY — do NOT skip it.**

Follow **Section C** from `skills/_shared/sdd-phase-common.md`.
- artifact: `spec`
- topic_key: `sdd/{change-name}/spec`
- type: `architecture`

### Step 6: Return Summary

Return to the orchestrator:

```markdown
## Specs Created

**Change**: {change-name}

### Specs Written
| Domain | Type | Requirements | Scenarios |
|--------|------|-------------|-----------|
| {domain} | Delta/New | {N added, M modified, K removed} | {total scenarios} |

### Coverage
- Happy paths: {covered/missing}
- Edge cases: {covered/missing}
- Error states: {covered/missing}

### Next Step
Ready for design (sdd-design). If design already exists, ready for tasks (sdd-tasks).
```

## Rules

- ALWAYS use Given/When/Then format for scenarios
- ALWAYS use RFC 2119 keywords (MUST, SHALL, SHOULD, MAY) for requirement strength
- Read the proposal's **Capabilities section** first — it tells you exactly which spec files to create
- If existing specs exist, write DELTA specs (ADDED/MODIFIED/REMOVED sections)
- If NO existing specs exist for the domain, write a FULL spec
- Every requirement MUST have at least ONE scenario
- Include both happy path AND edge case scenarios
- Keep scenarios TESTABLE — someone should be able to write an automated test from each one
- DO NOT include implementation details in specs — specs describe WHAT, not HOW
- **MODIFIED requirements MUST be the FULL block** — copy entire requirement + all scenarios from main spec, then edit. Partial MODIFIED blocks lose content at archive time.
- If adding new behavior without changing existing behavior → use ADDED, not MODIFIED
- Apply any `rules.specs` from `openspec/config.yaml`
- **Size budget**: Spec artifact MUST be under 650 words. Prefer requirement tables over narrative descriptions. Each scenario: 3-5 lines max.
- Return envelope per **Section D** from `skills/_shared/sdd-phase-common.md`.

## RFC 2119 Keywords Quick Reference

| Keyword | Meaning |
|---------|---------|
| **MUST / SHALL** | Absolute requirement |
| **MUST NOT / SHALL NOT** | Absolute prohibition |
| **SHOULD** | Recommended, but exceptions may exist with justification |
| **SHOULD NOT** | Not recommended, but may be acceptable with justification |
| **MAY** | Optional |