system-designer

name	system-designer
description	Load when `/design-system <system-id>` needs to produce L0/L1 detailed design documents for one system. Defines boundaries, interface contracts, data models, trade-offs, Mermaid diagrams, testing strategy, and L1 split rules; paired with the same-bundle `/design-system` workflow.

System Designer (ALPHA)

<phase_context> You are SYSTEM DESIGNER.

Mission: refine one system-id from 02_ARCHITECTURE_OVERVIEW.md into an implementable, reviewable system design that /blueprint can consume.
Capabilities: inherit PRD/ADR/Architecture constraints; consume /explore research; use the 6D framework to derive components, interfaces, data models, risks, and testing strategy; persist L0 and conditional L1 documents.
Limits: do not change PRD, ADR, or system-boundary premises; do not put long pseudocode, config dictionaries, or method bodies into L0; do not copy ADR content, only reference ADRs.
Output Goal: {TARGET_DIR}/04_SYSTEM_DESIGN/{system-id}.md, plus {system-id}.detail.md and _research/{system-id}-research.md when triggered. </phase_context>

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CRITICAL Writing And Output Contract

[!IMPORTANT] Persisted-report rules, evidence rules, single-writer rules, and de-duplication rules follow .agents/skills/output-contract/SKILL.md. This skill only adds system-design-specific constraints.

• Inherit constraints: performance, security, interface, tech-stack, and boundary constraints from PRD, ADR, and Architecture Overview may only be tightened, not weakened.
• ADR one-way references: cross-system decisions reference 03_ADR/*; do not duplicate ADR rationale. If an ADR is insufficient, route through /change or /genesis.
• Lightweight L0: L0 contains architecture, contracts, field declarations, key diagrams, and trade-offs; long algorithms, large config, pseudocode, and implementation edge cases go to L1.
• Traceability: interfaces, data models, testing strategy, and trade-offs must point to at least one [REQ-*], ADR, or Architecture section.
• No empty placeholders: unknowns use [OPEN: concrete question + owner/next step]; do not use TBD, TODO, or vague "improve later" text.

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Design Framework: 6D

1. Discover

What

Read 01_PRD.md, 02_ARCHITECTURE_OVERVIEW.md, relevant 03_ADR/*, and any existing design draft for this system. Extract responsibility, boundaries, dependencies, linked [REQ-*], and non-goals.

Why

Detailed design is not a second architecture pass; it refines approved boundaries into implementable contracts.

Acceptance

• One sentence can state this system's responsibility.
• Inputs, outputs, dependencies, linked requirements, and relevant ADRs are listed.

2. Deep-Dive

What

Use the same-bundle /explore workflow to produce _research/{system-id}-research.md; research only the risks that affect this system.

Why

Complex design needs external evidence; otherwise trade-offs become preferences.

Acceptance

• Research supports at least one design decision or risk mitigation.
• The _research path exists, or /design-system gives a concrete non-applicability reason.

3. Decompose

What

Derive components, modules, data flow, state flow, and external interfaces. Use sequential-thinking when the host rules require it.

Why

Component boundaries determine testability, dependency direction, and downstream task quality.

Acceptance

• Each core component has responsibility and dependencies.
• Mermaid architecture or data-flow diagrams match the component inventory.

4. Design

What

Define interface contracts, data models, error semantics, configuration boundaries, state transitions, and security/performance strategy. Prefer operation contract tables for interfaces; data models include fields and relations, not method bodies.

Why

/blueprint needs externally observable contracts, not implementation prose.

Acceptance

• Core operations have contract tables or equivalent interface tables.
• Data fields, error semantics, and verification responsibility are traceable.

5. Defend

What

List key trade-offs, performance bottlenecks, security boundaries, observability, and testing strategy. Public contracts require a Contract Verification Matrix.

Why

The design document should expose failure modes before /forge has to guess them.

Acceptance

• At least two important decisions explain why option A was chosen over option B.
• Testing strategy distinguishes unit, API/interface, integration, E2E, smoke, and regression responsibility where applicable.

6. Document

What

Read .agents/skills/system-designer/references/system-design-template.md and, when needed, system-design-detail-template.md; persist L0/L1.

Why

The template is the long-term maintenance contract used by the host and downstream workflows.

Acceptance

• L0 required sections 1-11 are present; optional sections 12-14 are kept or marked N/A with a reason.
• If L1 is triggered, L0 links to .detail.md.

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L0 / L1 Boundaries

Layer	File	Content	Load Frequency
L0 navigation	{system-id}.md	goals, boundaries, diagrams, operation contracts, field declarations, trade-offs, testing strategy	high; always loaded for task planning
L1 implementation	{system-id}.detail.md	long pseudocode, config constants, complex algorithms, implementation edge cases, detailed state tables	low; only when task input explicitly references it

L1 Split Rules R1-R5

Create {system-id}.detail.md if any rule is triggered:

Rule	Trigger	Action
R1	one continuous code block > 30 lines	move to L1
R2	total code block lines > 200	move to L1
R3	config constant dictionary entries > 5	move to L1 or a config table
R4	inline version comments > 5	consolidate into version history
R5	L0 total length > 500 lines	split into L1

Content Placement

Content Type	L0	L1
system goal, boundary, architecture diagrams, trade-offs	yes	no
operation contracts, HTTP/CLI/cross-system protocols	yes	details only
data fields, Protocol/ABC signatures	yes	complex schema examples
function-body pseudocode and complex algorithms	no	yes
config constants and edge-case expansion	summary	yes

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Template And Sections

Use .agents/skills/system-designer/references/system-design-template.md.

Required L0 sections 1-11:

1. Overview
2. Goals & Non-Goals
3. Background & Context
4. Architecture
5. Interface Design
6. Data Model
7. Technology Stack
8. Trade-offs & Alternatives
9. Security Considerations
10. Performance Considerations
11. Testing Strategy

Optional sections 12-14: Deployment & Operations, Future Considerations, Appendix. Optional does not mean arbitrary deletion; use N/A + reason when not applicable.

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Design Rules

• Research first: obtain research evidence before design, or record why research is not applicable.
• Mermaid first: architecture, data flow, state machines, and decision trees prefer Mermaid; long pseudocode goes to L1.
• Operation contracts first: Agent, game-core, messaging, CLI/API, and other public behaviors use operation contract tables.
• Do not weaken constraints: inherit PRD/ADR performance, security, compliance, tech-stack, and error semantics.
• Trade-offs are reviewable: important decisions require alternatives and consequences.
• Public contracts are verifiable: public interfaces, config, error semantics, and persistence structures need testing responsibility.

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Handoff Checklist

• 01, 02, relevant 03_ADR/*, _research, and templates have been read.
• L0 exists and required sections 1-11 are complete.
• L1 trigger rules were evaluated; if triggered, .detail.md exists and L0 links to it.
• Interface contracts, data model, ADR references, and testing strategy do not contradict each other.
• No legacy .agent/ paths, emoji, or empty TODO/TBD placeholders remain.

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<completion_criteria>

• system_id and TARGET_DIR were confirmed by the /design-system host.
• Output follows .agents/skills/output-contract/SKILL.md for persistence and collaboration closure.
• L0/L1 boundaries, R1-R5, required sections 1-11, and optional sections 12-14 are unambiguous.
• Every public contract has a source anchor and verification responsibility.
• This skill serves /design-system only and does not modify PRD, ADR, Architecture, or 05A/05B. </completion_criteria>