Ln 23 System Design Proposal Builder

---
name: ln-23-system-design-proposal-builder
description: "Designs target system boundaries, contracts and tradeoffs from requirements; does not plan tasks or implement."
---

# 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:** The checklist defines completion. Track each item internally as `PENDING`, `PROVEN` with evidence, `CLEARED` with evidence its condition is absent, or `UNPROVEN` with a gap; reading, delegation, or tool failure is not proof. Reconcile after each section. Before returning, resolve all `PENDING`, count only `PROVEN` and `CLEARED`, and apply verdict and approval rules to every gap.
Preserve intent, scope, and existing authorization. Continue authorized work; ask only for consequential unresolved choices or required external approval. Scale depth to material risk without skipping checks. Preserve dependency and safety order; otherwise choose an appropriate verification method.
Accept equivalent user or repository evidence; no other skill, named artifact, or complete lifecycle is required. Preserve source requirement and decision IDs. Bind reused evidence to relevant source versions, dirty changes, configuration, and environment; invalidate only affected claims.
On continuation, reconcile task, authorization, current state, and unresolved evidence. For long work, return a compact continuation record or update an already authorized artifact; read-only skills do not persist it. Distinguish artifact readiness, verified behavior, and external-action authority.
Prepare authorized work before required approval. If blocked by an instruction, cite its exact source and unresolved boundary; do not invent approval gates from caution.


## 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.
- Compare credible alternatives for consequential decisions, including the simplest feasible option; when constraints permit only one, document why rather than inventing another.
- Prefer reversible choices and the simplest topology fitting the system. For a new application, consider a modular monolith before independent services; do not force that shape onto libraries, plugins, or an established topology.
- Do not silently change an accepted decision; record the conflict and required governance action.

## Checklist

### 1. Frame the Design

- [ ] Resolve business outcome, actors, journeys, scope, non-goals, horizon, readers, language, and the approved canonical destination before editing.
- [ ] 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 components whose correctness, scale, security, or reversibility risk warrants implementation-level detail.
- [ ] 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 appropriate to each material driver: contract checks, load/failure experiments, security validation, recovery proof, or observability signals. Specify prerequisites and pass criteria; do not execute them during design.
- [ ] Outline current-to-target implications and compatibility needs without expanding into a full implementation plan.

### 6. Write and Report

- [ ] Write context, drivers, estimates, domains, contracts, HLD, critical LLD, failure and operations model, security, alternatives, decisions, validation, open questions, and evolution triggers.
- [ ] Preserve existing content outside the approved scope and link shared artifacts only by document path or title.
- [ ] Re-read the proposal for unsupported facts, hidden decisions, mixed abstraction, and unjustified machinery.
- [ ] Demonstrate that each material design boundary is implementable: link source requirements to contracts, data, failure behavior, unresolved choices, and acceptance evidence; distinguish a ready proposal from an accepted decision.
- [ ] Use `READY` only when the design is decision-complete enough for implementation planning; use `REVISE` for material but solvable gaps; use `BLOCKED` when required intent, evidence, authority, or destination is unavailable.

## Self-Check

- [ ] **Reconcile before returning.** Check item-level evidence, requirement coverage, contradictions, scope, verdict, and applicable cleanup. Correct the report or authorized artifacts. Reuse valid evidence; do not automatically rescan the repository or rerun successful commands. Repeat checks only for relevant changes, failures, or unresolved evidence. Disclose remaining gaps.

## Output Contract

Report in the user's language, in this order; retain all five fields and state each fact once. Small results may use one line per field; omit empty tables and do not copy linked artifacts:

1. **Result:** Skill-specific verdict and supported outcome.
2. **Scope:** Reviewed/changed scope, exclusions, baseline, and material assumptions.
3. **Evidence:** Skill-specific fields below; distinguish facts, inferences, and unverified claims. Link artifacts; use tables when useful.
4. **Verification:** Checks/results, unavailable evidence, and applicable cleanup/external state.
5. **Completion:** `Checklist: X/Y complete`; `Incomplete: None` or each `UNPROVEN` item's reason, outcome impact, and exact next action; residual risks and required decisions.

**Skill-specific evidence:** Artifact path; requirements, estimates, boundaries, contracts, HLD/critical LLD, selected decisions, alternatives, evidence, and reopen triggers. Summarize validation/transition needs, compatibility, rollout, rollback, observability, and only unresolved choices that affect implementation planning.