SDD Design Engine

This skill consolidates the entire design phase into a unified, friction-free flow. It transforms a structured request.md (produced by sdd-request-engine) into precise technical specifications through an automated pipeline with built-in Ambiguity Resolution.

Core Responsibilities

1. Unified Design Flow: Seamlessly transitions from Requirements Analysis → System Architecture → Object Design → Interface & Contract Design.
2. Ambiguity Resolution: Surface concerns, ask clarifying questions, and converge on precise specs before proceeding.
3. Continuous Guardrails: Automatically invokes sdd-guardrails at every sub-stage to ensure consistency.
4. Auto-Persistence: Automatically saves state to sdd-knowledge-base—no manual commit steps required.
5. Drift Management: Handles feedback from implementation via /sdd-spec-update.

Commands

• /sdd-design: Main entry point. Intelligently determines the next design step based on context.json.current_stage.
  • If no active feature: Suggest running /sdd-request first to create a feature and produce request.md.
  • If stage is design and request.md exists: Starts Requirements Analysis.
  • If requirements exist: Proceed to Architecture.
  • If architecture exists: Proceed to Object Design.
  • If object design exists: Proceed to Interface & Contract Design.
• /sdd-design-requirements: Force entry into Requirements Analysis.
• /sdd-design-architecture: Force entry into Architecture Design.
• /sdd-design-objects: Force entry into Object Design.
• /sdd-design-interfaces: Force entry into Interface & Contract Design.
• /sdd-spec-update: Adjust spec based on "drift" detected during implementation.

JSON Writing Rule

When generating any JSON artifact (requirements.json, architecture.json, data_api.json, concerns.json), all string values MUST have special characters properly escaped (\", \\, \n, \t, control chars). Validate JSON is well-formed before writing to disk. If validation fails, fix escaping issues before saving.

Feature-Scoped Output

All spec artifacts are written to .sdd/spec/<feature-id>/:

• requirements.json
• architecture.json
• object_design.json
• openapi.yaml — if feature involves HTTP APIs
• data_api.json — if feature involves persistent data
• interface_contract.json — if feature has other interface boundaries (CLI, SDK, events, IPC, etc.)
• diagrams/*.mmd (component, sequence, class diagrams)
• concerns.json (clarification history)

The <feature-id> is read from context.json.current_feature. If null, prompt the user for a feature name before proceeding.

Ambiguity Resolution Protocol

Between every sub-stage, the agent runs a clarification loop before writing final output:

Step 1: Analyze and Score

After analyzing user intent or input artifacts, the agent assigns a confidence assessment to each generated item and produces a concerns.json:

{
    "feature": "user-auth",
    "stage": "requirements",
    "concerns": [
        {
            "id": "C-001",
            "category": "BLOCKING",
            "question": "Authentication via JWT or Session-based?",
            "context": "Both are viable but affect architecture significantly.",
            "answer": null,
            "resolved": false
        },
        {
            "id": "C-002",
            "category": "WARNING",
            "question": "Assuming minimum password length is 8 characters. OK?",
            "context": "No explicit requirement stated.",
            "answer": null,
            "resolved": false
        },
        {
            "id": "C-003",
            "category": "INFO",
            "question": "Per project_rules.md, using Clean Architecture.",
            "context": "Matches architecture_style in context.json.",
            "answer": null,
            "resolved": false
        }
    ]
}

Step 2: Categorize Concerns

Category	Meaning	Behavior
BLOCKING	Must clarify before proceeding	Agent stops and asks the user
WARNING	Can assume but needs user confirmation	Agent states assumption and asks for confirmation
INFO	Informational, no action needed	Agent informs and proceeds

Step 3: Resolve and Iterate

1. STOP and Ask User: If ANY BLOCKING concerns exist, you MUST present them to the user and WAIT for their response. DO NOT proceed. DO NOT makeup answers.
2. Collect Answers: Record user answers in concerns.json.
3. Re-incorporate: Re-incorporate answers into the spec.
4. Re-run Analysis: If new concerns arise, repeat.
5. Proceed: Only proceed to the next sub-stage when ALL BLOCKING items are resolved.

Post-step: Feedback Capture (MANDATORY)

After presenting any design artifact to the user, if the user requests changes or corrections:

1. Apply the requested changes to the design artifact.
2. Immediately write a lesson to .sdd/knowledge/lessons/ capturing:
   • What was originally generated vs what the user corrected.
   • Why the correction was needed (infer from context or ask the user).
   • Tags for future retrieval (feature name, design stage, domain keywords).
3. If the correction reveals a reusable pattern (e.g., a preferred architectural style, a standard API convention), also save to .sdd/knowledge/patterns/.

This is NOT optional. Every user correction during design is a gap between expectation and reality — it MUST be recorded as a lesson.

Design Pipeline

Pre-step: Knowledge Lookup (MANDATORY — Index-Based)

Before generating any design artifact (requirements, architecture, or API), the engine MUST:

1. Read .sdd/knowledge/index.json (the lightweight index).
2. Filter patterns entries whose tags overlap with the current feature's domain keywords.
3. Filter lessons entries whose tags overlap OR whose trigger matches "designing-*".
4. Load ONLY the matched files (via the file path in each index entry). Do NOT scan the full patterns/ or lessons/ directories.
5. Summarize relevant findings and incorporate them into the design output.
6. If no matches found in the index, proceed normally without loading any knowledge files.
7. Output the knowledge match results before proceeding to the design pipeline:

📚 **Knowledge Loaded** (stage: design)

| Type | ID | Matched Tags | Summary |
|------|----|-------------|---------|
| <type> | <id> | `<tag1>`, `<tag2>` | <summary> |

> No knowledge matched. (if empty)

1. Requirements (formerly sdd-requirements-engine)

• Input: .sdd/spec/<feature-id>/request.md (produced by sdd-request-engine).
• Action: Transform structured user stories and acceptance criteria into technical requirements. Assign confidence_score to each requirement.
• Clarify: Run Ambiguity Resolution Protocol. Resolve all BLOCKING concerns.
• Output: .sdd/spec/<feature-id>/requirements.json.
• Guardrail: Check for ambiguity and potential conflicts with project_rules.md.

2. Architecture (formerly sdd-architecture-system)

• Input: requirements.json.
• Action: Generate Mermaid diagrams (Component, Sequence) and architectural decisions.
• Clarify: Run Ambiguity Resolution Protocol (e.g., "Should User and Session be separate bounded contexts?").
• Output: .sdd/spec/<feature-id>/architecture.json + .sdd/spec/<feature-id>/diagrams/*.mmd.
• Guardrail: Ensure all user stories are covered by components. Validate architecture style compliance (see sdd-guardrails).

3. Object Design

• Input: architecture.json.
• Action:
  • Define core design units appropriate to the project — classes/interfaces (OOP), modules/functions (FP), components/hooks (UI), commands/handlers (CLI), resources/modules (IaC), etc.
  • Define abstractions — dependency inversion boundaries between layers.
  • Define relationships — inheritance, composition, dependency, association.
  • Generate Class / Module Diagram (Mermaid).
• Clarify: Run Ambiguity Resolution Protocol on structural decisions.
• Output: .sdd/spec/<feature-id>/object_design.json (see templates/object_design.json) + .sdd/spec/<feature-id>/diagrams/class.mmd.
• Guardrail: Validate layer boundaries per project_rules.md. Ensure all components from architecture.json have corresponding design units.

4. Interface & Contract Design

• Input: object_design.json + architecture.json.

• Action: Define the external-facing contracts and data schemas appropriate to the feature. Produce only the artifacts relevant to the feature's interface boundaries:

  Artifact	When to produce
  openapi.yaml	Feature exposes or consumes HTTP/REST APIs
  data_api.json	Feature involves persistent data (DB entities, schemas)
  interface_contract.json	Feature has non-HTTP interfaces (CLI args, SDK public API, event schemas, IPC, etc.)

  If the feature has no external interface boundaries (e.g., a pure refactoring or internal library), this stage may be skipped entirely.

• Clarify: Run Ambiguity Resolution Protocol on interface decisions.

• Output: Whichever artifacts from the table above are relevant, written to .sdd/spec/<feature-id>/.

• Guardrail: Validate consistency between produced interface specs and object_design.json; check for breaking changes.

Compounding Features

• Pattern Recognition: When generating architecture/interfaces, the engine queries sdd-knowledge-base for similar past patterns by tags to suggest proven designs.
• Lessons Learned: Checks sdd-knowledge-base for "avoid" lists before making decisions.

Stage Transitions

After completing each sub-stage successfully:

1. Auto-save artifacts to feature directory.
2. Update context.json.current_stage appropriately.
3. After all design sub-stages complete, set current_stage to "design-complete".

graph TD
    A[request.md] -->|/sdd-design| B(Analyze Requirements)
    B --> B1{Concerns?}
    B1 -->|BLOCKING/WARNING| B2[Ask User]
    B2 --> B3[User Answers]
    B3 --> B
    B1 -->|All Resolved| C{Guardrails Pass?}
    C -->|No| B
    C -->|Yes| D[Auto-Save requirements.json]
    D --> E(Generate Architecture)
    E --> E1{Concerns?}
    E1 -->|BLOCKING/WARNING| E2[Ask User]
    E2 --> E3[User Answers]
    E3 --> E
    E1 -->|All Resolved| F{Guardrails Pass?}
    F -->|No| E
    F -->|Yes| G[Auto-Save architecture.json]
    G --> OD(Object Design)
    OD --> OD1{Concerns?}
    OD1 -->|BLOCKING/WARNING| OD2[Ask User]
    OD2 --> OD3[User Answers]
    OD3 --> OD
    OD1 -->|All Resolved| ODF{Guardrails Pass?}
    ODF -->|No| OD
    ODF -->|Yes| ODG[Auto-Save object_design.json]
    ODG --> H(Interface & Contract Design)
    H --> H1{Concerns?}
    H1 -->|BLOCKING/WARNING| H2[Ask User]
    H2 --> H3[User Answers]
    H3 --> H
    H1 -->|All Resolved| I{Guardrails Pass?}
    I -->|No| H
    I -->|Yes| J[Finalize: Ready for Planning]

Example Usage

User: /sdd-design
Agent: Active feature: "001-user-auth". Found request.md.

       [Requirements] Analyzing request.md...
       > Generated 5 Technical Requirements from User Stories.
       > Confidence: REQ-001 (1.0), REQ-002 (0.8), REQ-003 (0.5)

       ⚠️ BLOCKING Concerns:
       - C-001: JWT or session-based tokens for social auth?

       ℹ️ INFO:
       - C-002: Per project_rules, using Clean Architecture.

User: JWT with refresh tokens.

Agent: > Resolved. Updating requirements...
       > All concerns resolved. Guardrails passed. Auto-saving...

       [Architecture] Generating...
       ...

       [Object Design] Analyzing architecture components...
       > Defined 4 Domain Classes: User, UserProfile, AuthToken, UserRepository.
       > Generated class diagram with relationships.

       ⚠️ BLOCKING Concerns:
       - C-004: Should UserService implement IAuthService or be a separate class?

User: Separate class. UserService handles profile, AuthService handles auth.

Agent: > Resolved. Auto-saving object_design.json + class.mmd...

       [Interface & Contract Design] Analyzing interface boundaries...
       ...