long-task-design

LANGUAGE RULE: You MUST respond to the user in Chinese (Simplified). All generated documents, reports, and user-facing output must be written in Chinese. Skill names, code identifiers, and JSON field names remain in English.

Design Document Generation

Take the approved SRS as input. Propose implementation approaches, get section-by-section design approval, and produce a design document that answers HOW — while the SRS answers WHAT.

Anti-Pattern: "The SRS Is Detailed Enough To Start Coding"

The SRS describes WHAT the system must do. The design document describes HOW. Even when requirements are crystal clear, the implementation approach (architecture, data model, tech stack choices) needs explicit decisions and user approval. Skipping design causes mid-session corrections and rework.

Checklist

You MUST create a TodoWrite task for each of these items and complete them in order:

1. Read the approved SRS — from docs/plans/*-srs.md
2. Explore technical context — existing code, frameworks, deployment environment
3. Propose 2-3 approaches — with trade-offs and your recommendation
4. Section-by-section design approval — architecture, data model, API, UI, testing, deployment
5. Write design document — save to docs/plans/YYYY-MM-DD-<topic>-design.md and commit
6. Transition to ATS — REQUIRED SUB-SKILL: Invoke long-task:long-task-ats

The terminal state is invoking long-task-ats. Do NOT invoke any other implementation skill.

Step 1: Read SRS & UCD & Extract Design Inputs

1. Read the approved SRS document from docs/plans/*-srs.md
2. Read the approved UCD style guide from docs/plans/*-ucd.md (if it exists — only present for UI projects)
3. Extract key design drivers:
   • Functional scope — FR count, priority distribution, dependency chains
   • NFR thresholds — performance targets, reliability, scalability that affect architecture
   • Constraints — hard limits that restrict technology/approach choices
   • Interface requirements — external systems, protocols, data formats to integrate with
   • User personas — technical levels that affect API/UI design decisions
   • UCD style tokens (if UCD exists) — color palette, typography, spacing, component catalog → informs frontend architecture and UI/UX section
4. List any SRS Open Questions that must be resolved before design can proceed
   • If unresolved questions affect architecture → ask user via AskUserQuestion before Step 2

Step 2: Explore Technical Context

1. Explore existing code / repos the project will build on
2. Identify technical constraints not in the SRS (e.g., monorepo structure, CI/CD pipeline, existing libraries)
3. Check for a design document template:
   • If the user specified a template path → read and validate it
   • Else → read docs/templates/design-template.md (the default template shipped with this skill)
   • Validation: template must be a .md file containing at least one ## heading

Step 3: Propose Approaches

Present 2-3 implementation approaches with explicit trade-offs:

## Approach A: [Name]
**How it works**: [1-2 sentences]
**Pros**: [bullet list]
**Cons**: [bullet list]
**Best when**: [conditions]
**NFR impact**: [how this approach affects the SRS NFR thresholds]
**Third-party dependencies**: [key libraries/frameworks this approach requires, with versions]

## Approach B: [Name]
...

## Recommendation: Approach [X]
**Reason**: [why this fits best given the SRS constraints and NFRs]

Key: Each approach must be evaluated against the SRS constraints and NFR thresholds. An approach that cannot meet a "Must" NFR is disqualified.

Step 4: Section-by-Section Approval

For non-trivial projects, break the design into sections and get approval per section:

1. Architecture — system components, logical view, tech stack decisions
   • Must include a Logical View (Mermaid graph) showing layers/packages/modules and dependency directions
   • Must include a Component Diagram (Mermaid graph) showing runtime components and interactions
   • Must justify tech stack choices against SRS constraints
   • Must show how NFR thresholds will be met
2. Key feature designs — one chapter per key feature or feature group
   • Each feature chapter MUST include at least:
     • Class diagram (Mermaid classDiagram) — classes/modules, attributes, methods, relationships
     • One behavioral diagram: sequence diagram (Mermaid sequenceDiagram) or flow diagram (Mermaid flowchart)
     • Integration Surface (§4.N.6) — declaring Provides/Requires tables with §6.2 Contract IDs; write "Self-contained" if no cross-feature dependencies
   • For complex features, include ALL four views: class diagram, sequence diagram, flow diagram, and design notes
   • All diagrams MUST use Mermaid format — no ASCII art, no image references
3. Data model — schemas, relationships, storage strategy
   • Must use Mermaid ER diagrams (erDiagram) where applicable
4. API / interface design
   • External interfaces (§6.1) — endpoints, contracts, protocols (trace to SRS IFR-xxx)
   • Internal API contracts (§6.2) — feature-to-feature boundaries; every §3.3 component diagram edge must have a corresponding §6.2 row with Contract ID, request/response schemas, and error codes
5. UI/UX approach (if applicable) — layout strategy, interaction patterns
   • Must address SRS User Personas
   • If UCD document exists: must reference UCD style tokens (colors, typography, spacing) and component catalog
   • Frontend architecture decisions (component library, state management, routing) must be compatible with UCD style tokens
   • Include a mapping: UCD component prompts → concrete implementation components
6. Third-party dependencies — ALL libraries/frameworks with exact version numbers
   • Must verify mutual compatibility between dependencies
   • Must verify compatibility with the project's target runtime version
   • Must note license type for each dependency
   • Must include a dependency graph (Mermaid) for non-trivial dependency chains
7. Testing strategy — high-level test approach decisions only
   • Test philosophy: TDD with quality gates (Red → Green → Refactor → Coverage → Mutation)
   • Tool selections: test framework, coverage tool, mutation tool (with versions — these are design decisions)
   • Coverage thresholds: line >= X%, branch >= Y%, mutation >= Z%
   • Boundary: "Detailed requirement-to-test-category mapping, NFR test methods, and cross-feature integration scenarios are defined in the ATS phase — not here."
8. Deployment / infrastructure (if applicable) — hosting, CI/CD, environments
9. Development plan — milestones, task decomposition, priority ordering
   • Must define milestones with clear exit criteria
   • Must decompose into context-budget-sized features (P0-P3) — each row in §10.2 becomes one feature in feature-list.json; group related right-sized FRs (already validated by SRS G+S heuristics) into vertical slices; include Mapped FRs column for traceability
   • Must show dependency chain (Mermaid graph) identifying the critical path
   • Must include risk assessment with mitigation strategies

Feature sizing is upstream: FRs are right-sized at the Requirements phase via bidirectional granularity analysis (G1-G6 split + S1-S4 group). §10.2 groups these right-sized FRs into implementation features. Each row should map 1+ related FRs into a vertical slice that productively fills one Worker session (~50% of context window). Feature counts in the scaling table below refer to the final §10.2 row count.

Present each section. Wait for user feedback. Incorporate changes before moving to the next.

For simple projects (< 5 features): Combine all sections into a single approval step, but still include the required diagrams and dependency versions.

Step 4b: Merge Codebase Conventions into Design

Skip this step if docs/rules/ does not exist or contains only a greenfield stub.

If docs/rules/ is populated with convention scan results (from Phase 0-pre codebase scanner):

1. Read all docs/rules/*.md files — coding-style.md, coding-constraints.md, build-and-compilation.md, commit-conventions.md
2. Populate §13 of the design document (Codebase Conventions & Constraints) using the design template's §13 structure:
   • §13.1: Extract "Mandatory Internal Libraries" table from coding-constraints.md
   • §13.2: Extract "Prohibited APIs / Libraries" table from coding-constraints.md
   • §13.3: Extract "Approved 3rd-Party Libraries" table from coding-constraints.md
   • §13.4: Extract "Static Analysis Tools" table from coding-constraints.md (tool name + config path + run command only — do not read config contents)
   • §13.5: Extract key naming and formatting rules from coding-style.md (summary table)
   • §13.6: Extract error handling pattern from coding-constraints.md
   • §13.7: Extract build system and CI/CD summary from build-and-compilation.md
   • §13.8: Extract commit format and branch naming from commit-conventions.md
3. Cross-verify — check for conflicts between scanned conventions and design decisions:
   • §8 (Third-Party Dependencies): new dependencies must not conflict with §13.2 prohibited list
   • §6.2 (Internal API Contracts): libraries used must comply with §13.1 internal library mandates
   • If conflicts exist: mark with "⚠ Design Override: [reason]" and present to user for confirmation
4. Present §13 to user for review (same approval flow as other sections)

Step 5: Write Design Document

Save the approved design to docs/plans/YYYY-MM-DD-<topic>-design.md.

Template usage

Read the template found in Step 2 (user-specified or default docs/templates/design-template.md):

1. Preserve the template's heading structure
2. Replace guidance text under each heading with approved design content
3. Add metadata at top if not already present (Date, Status, SRS Reference, Template path)
4. For uncovered template sections: mark "[Not applicable]"
5. For approved content without matching template section: append as "Additional Notes"

Step 5b: Design Integration Coherence Check

Before transitioning to ATS, mechanically verify cross-feature integration coherence:

1. Contract completeness: For each edge in §3.3 component diagram, verify a corresponding row exists in §6.2 Internal API Contracts. Flag missing rows.
2. Schema consistency: For each §6.2 row, verify that Provider feature's §4.N class diagram includes the Response Schema type, and Consumer feature's §4.N references the Request Schema. Flag mismatches.
3. Dependency completeness: For each feature that appears in a §6.2 "Consumer" column, verify it lists the Provider feature ID in §11.3 dependency chain. Flag missing dependency edges.

Present any flagged issues to the user. Resolve before proceeding to ATS.

Step 6: Transition to ATS

Once the design document is saved and committed:

1. Summarize key inputs the ATS skill will need:
   • From SRS: all FR/NFR/IFR requirements with acceptance criteria
   • From Design: testing strategy, tech stack, architecture risk areas
2. REQUIRED SUB-SKILL: Invoke long-task:long-task-ats to generate the Acceptance Test Strategy

Scaling the Design Phase

Section 4 Depth Strategy

For projects with many features, §4.N sections are written at different depths to manage context window constraints:

Thin §4.N format:

### 4.N Feature: <Name> (FR-xxx)
#### 4.N.1 Overview
[1-2 sentences]
#### 4.N.2 Key Types
[List the main classes/types this feature introduces, with one-line purpose each]
#### 4.N.6 Integration Surface
[Provides/Requires tables referencing §6.2]

This is safe because the feature-design SubAgent (Worker Step 4) produces the full class/sequence/flow/algorithm design with access to §6.2 contracts. The thin §4.N serves as an integration specification, not a complete design.

Red Flags

Diagram Requirements

All architectural and design views MUST use Mermaid syntax. This ensures:

• Diagrams are version-controlled alongside the document (no external image files)
• Diagrams are renderable in GitHub, GitLab, and most Markdown viewers
• Diagrams stay in sync with design changes

Required Diagram Types

Diagram Quality Checklist

• Each diagram has a clear title or surrounding heading
• Class diagrams show visibility modifiers (+/-/#) and key methods
• Sequence diagrams show the main success path and at least one error path
• Flow diagrams include decision nodes for all branching logic
• No placeholder diagrams — every diagram reflects actual approved design content
• Every edge in §3.3 component diagram includes Contract ID referencing §6.2

Third-Party Dependency Rules

1. Exact versions required — specify 1.2.3 or a constrained range ^1.2.0 / >=1.2,<2.0; never use latest or omit version
2. Compatibility matrix — verify each dependency is compatible with:
   • The target language/runtime version (e.g., Python >= 3.10, Node >= 18)
   • Other dependencies in the stack (check for known conflicts)
3. License audit — document the license for each dependency; flag any copyleft licenses (GPL, AGPL) that may conflict with project requirements
4. Upgrade path — note any dependencies approaching EOL or with known migration concerns

Development Plan Rules

The development plan section bridges the design document to the Init phase. It MUST include:

1. Milestones — time-boxed phases with clear scope and exit criteria
   • M1 is always "Foundation" (project skeleton, CI, core abstractions)
   • Final milestone is always "Polish & Release" (NFR verification, docs, examples)
2. Task decomposition — features mapped to priorities (P0-P3) with rationale
   • P0: Foundation — required by all other features
   • P1: Core value — the minimum viable feature set
   • P2: Extended — important but not launch-blocking
   • P3: Nice-to-have — defer if timeline is tight
3. Dependency chain — Mermaid graph showing which features block others
4. Paired feature ordering — When the project has both backend and frontend features, organize the task decomposition table so that each backend feature is paired with its corresponding frontend feature. This produces a natural development flow: Backend A → Frontend A → Backend B → Frontend B. The Init phase uses this pairing to order features in feature-list.json.
5. Risk register — technical and schedule risks with mitigation

The Init phase uses this plan to populate feature-list.json with correct priority ordering, paired grouping, and dependency chains.

Integration

Called by: using-long-task (when SRS + UCD exist, no design doc, no feature-list.json) or long-task-ucd (Step 8) Requires: Approved SRS at docs/plans/*-srs.md; optionally approved UCD at docs/plans/*-ucd.md (for UI projects); optionally docs/rules/*.md (codebase conventions from Phase 0-pre scan) Chains to: long-task-ats (after design approval) Produces: docs/plans/YYYY-MM-DD-<topic>-design.md (includes §13 Codebase Conventions if docs/rules/ exists)