ln-73-system-design-proposal-builder

# System Design Proposal Builder

**Goal:** Create a proportionate, evidence-backed target system design that turns requirements into explicit boundaries, contracts, data flow, failure behavior, operations, and tradeoffs. Change only the approved design document; do not implement, audit, or approve the delivery.

**Execution contract:** Treat the ordered checkbox workflow below as this skill's Definition of Done. Track every checkbox as `PENDING`, then resolve it to `PROVEN` with concrete evidence, `CLEARED` with evidence that its conditional trigger is absent, or `UNPROVEN`; reading, mentioning, delegating, skipping, or tool failure is not proof.
Before returning, resolve every `PENDING`, count only `PROVEN` and `CLEARED` items as complete, apply this skill's verdict, decision, and approval rules to every `UNPROVEN`, and prepend **Checklist: X/Y complete**<br>**Incomplete: None | section/item — reason; outcome impact; exact next action**; list every `UNPROVEN` item.

## Tool Routing

| Need | Preferred capability | Fallback |
|---|---|---|
| Requirements and constraints | Approved requirements, baseline, decisions, and direct stakeholder input | Mark material gaps and ask the smallest decision question |
| Current implementation and conventions | Repository search, manifests, entrypoints, and architecture artifacts | Treat as greenfield only when the user or repository establishes that fact; otherwise mark current state `UNKNOWN` and return `REVISE` or `BLOCKED` when the gap can change boundaries, compatibility, or migration |
| External capabilities and limits | Current official documentation and specifications | Mark claims `UNVERIFIED`; avoid vendor-dependent commitment |
| Estimates | Reproducible arithmetic from sourced workload assumptions | Use ranges and sensitivity; never present estimates as measurements |
| Document mutation | Minimal patch to the approved target-design artifact | Return `BLOCKED` if scope or path is unsafe |

Use patterns as candidate solutions, not goals. Introduce infrastructure only when a requirement, failure mode, ownership boundary, or measured horizon pays for its lifecycle cost.

## Artifact Rules

- Reuse a clear target-design document; otherwise use `docs/architecture/target-design.md`.
- Read available baseline, current-state, decision, interface, diagram, and migration artifacts by path; none is mandatory.
- Label facts, assumptions, estimates, proposed decisions, and unresolved choices separately.
- Start with requirements and estimates, then domains and contracts, HLD, and only the critical LLD.
- Consider at least two credible alternatives for consequential decisions, including the simplest option.
- Prefer reversible choices and a modular monolith unless evidence justifies independent service boundaries.
- Do not silently change an accepted decision; record the conflict and required governance action.

## Checklist

### 1. Frame the Design

- [ ] Resolve business outcome, actors, critical journeys, scope, non-goals, decision horizon, and intended readers.
- [ ] Read repository instructions and inspect relevant architecture artifacts and current implementation.
- [ ] Extract functional requirements and measurable quality drivers, preserving their source and status.
- [ ] Identify architecture-critical unknowns and ask only questions whose answers change the target shape.
- [ ] Return `BLOCKED` when a required business boundary or safety constraint cannot be responsibly assumed.

### 2. Estimate Before Choosing Components

- [ ] Estimate average and peak request or event rates, concurrency, payload and bandwidth, storage growth, retention, and recovery volume where relevant.
- [ ] Show formulas, ranges, growth horizon, and assumptions; identify the variables that can reverse a choice.
- [ ] Identify likely first bottlenecks and explicit thresholds for deferred scaling mechanisms.
- [ ] Separate availability, latency, durability, consistency, security, cost, and operability requirements from implementation preferences.
- [ ] Reject speculative scale and list complex mechanisms intentionally deferred.

### 3. Define Domains, Data, and Contracts

- [ ] Map business capabilities, domains or modules, ownership, invariants, and allowed dependency direction.
- [ ] Define systems of record, data models at architecture depth, lifecycle, retention, consistency, and transaction boundaries.
- [ ] Define public APIs, events, commands, schemas, errors, idempotency, ordering, versioning, and compatibility expectations.
- [ ] Define trust boundaries, identities, authorization, sensitive data, secrets, abuse controls, and audit needs proportionate to risk.
- [ ] Keep framework and vendor details outside the core model unless they are genuine constraints.

### 4. Build HLD and Critical LLD

- [ ] Describe system context, deployable units, stores, queues, external systems, responsibilities, and labeled data flows.
- [ ] Trace success, overload, dependency failure, partial failure, retry, timeout, degradation, recovery, and cancellation for critical journeys.
- [ ] Deep-dive only the two or three components with the highest correctness, scale, security, or reversibility risk.
- [ ] Define observability, SLI measurement points, health, deployment strategy, rollback, backup, and operator actions.
- [ ] Define ownership, team impact, cost drivers, and operational burden for the proposed topology.

### 5. Decide and Validate

- [ ] Compare credible alternatives against requirements, estimates, failure behavior, complexity, cost, migration, and future triggers.
- [ ] State selected and rejected options with consequences, sensitivity points, and assumptions that would reopen the decision.
- [ ] Identify significant decisions that deserve their own compact decision records without requiring another workflow.
- [ ] Define architecture acceptance evidence: contract checks, load or failure experiments, security validation, recovery proof, and observability signals.
- [ ] O