Define Architecture

Define durable, easy-to-change architecture defaults for TypeScript full-stack apps; produce an enforceable architecture brief.

• IS: folder structures, module contracts, middleware pipelines, frontend/backend boundaries; architecture briefs; domain-informed deepening of an existing codebase; agent-friendly codebase guardrails.
• IS NOT: scaffolding a new repo (scaffold-nextjs for a Next.js turborepo, scaffold-cli for a TypeScript CLI), multi-tenant domain/isolation/routing (multi-tenant-architecture), or structural review of a local diff (pr-reviewer).

Contents

• Workflow and references
• Setup workflow (new codebase)
• Adoption workflow (existing codebase)
• Validation loop
• Output template
• Related skills
• Gotchas

Workflow and references

Track this checklist:

Architecture progress:
- [ ] Step 1: Pick workflow (new codebase: Setup; existing: Adoption)
- [ ] Step 2: Run it end to end
- [ ] Step 3: Write the brief (Output template)
- [ ] Step 4: Run Validation loop, record results in brief
- [ ] Step 5: Fix failed checks, re-run loop

Load references only when the condition applies:

Setup workflow (new codebase)

1. Constraints first: product scope, team size, compliance/security, expected scale, deploy targets, required integrations, and quality bar.
2. Choose repo shape:
   • apps/ for deployable surfaces (api, web, admin).
   • packages/ for shared libraries (shared, ui, icons, auth, proto).
3. Define backend module contracts, each naming its enforcement (lint boundary rule or type check):
   • handler: transport only.
   • service: business orchestration.
   • dao: database access only.
   • mapper: DB/proto/domain transformations.
   • constants and types: module-local contracts.
4. Define request context and middleware:
   • Carry tenantId, userId, and traceId in an AsyncLocalStorage-backed RequestContext, initialized in every entrypoint (RPC, HTTP, jobs, CLI) and read via getContext(); never thread a ctx parameter through business functions. Implementation in references/api-design.md.
   • Require explicit auth policy per RPC method at registration; a method without one fails registration, never defaults to open.
   • Keep auth, logging, errors, and context in shared middleware, not per-handler code.
5. Define frontend boundaries:
   • Default to Server Components; add "use client" only at leaf components needing interactivity.
   • Server state in TanStack/Connect Query; client state in component state; MobX only for cross-cutting client state that fits neither.
   • Each piece of data has exactly one owner. Never mirror server data into useState or sync two stores with useEffect; both are the red flag that ownership is unclear.
6. Testing and release:
   • Unit tests stay DB-free; integration/E2E run in parallel with dynamically generated IDs so runs never collide on fixtures.
   • Release in small, complete, reversible vertical slices with a rollback plan per change.
   • A slice is complete only when reliability, error paths, observability, and user-facing states are covered; do not defer them to a later polish pass.
7. Agent guardrails: wire dead-code, duplication, boundary, and file-size checks into pre-commit and CI per references/agent-friendly-codebase.md.

Adoption workflow (existing codebase)

Goal: domain-informed deepening, not a rewrite. Load references/deepening-existing.md for the analysis method, opportunity patterns, and output template.

1. Map the domain language and decisions. Read CONTEXT.md, docs/adr/, or local equivalents if present, then read the code for entities, actions, and bounded contexts as the team names them. Note divergence (one concept, three names; or one name, three concepts).
2. Find deepening opportunities. Look for anemic concepts, shallow modules, leaking boundaries, naming divergence, duplicated concepts, primitive obsession, misplaced logic, and tests forced past the public interface. Record each with file paths, never a vague smell.
3. Rank by leverage. Prefer opportunities that pass the deletion test, localize named future changes, have low churn, meet a current requirement, and have a viable testing seam. Rank candidates before designing target interfaces; drop speculative cleanups.
4. Migrate one vertical slice first. Prove the highest-leverage move end to end through one slice before generalizing.
5. Add guardrails. Enforce the new boundary with lint, type, or test checks so it cannot decay, then roll out module by module. For agent-specific guardrails (dead code, duplication, legacy markers, file-size caps) load references/agent-friendly-codebase.md.

Validation loop

Run before finalizing; record results in the brief (Open risks). Each check needs evidence; "looks consistent" is not a pass.

1. Consistency: naming, module boundaries, and middleware rules read the same across every service. Evidence: a contradiction scan with zero findings.
2. Enforceability: every contract names its lint rule, type check, or test. Evidence: an enforcement column or note per contract.
3. Operability: observability, health checks, and rollback path per deployable surface. Evidence: the rollout section names each.
4. Quality gates (only when code changed, e.g. an Adoption slice): run the repo's lint, type-check, and targeted tests (npm run lint, npm run check-types, npm run test --workspace=<pkg> or equivalents). Evidence: passing output.

On failure: fix the brief or conventions, then re-run the loop.

Output template

Use this structure:

# Architecture brief

## Context and constraints
## Repo shape
## Backend module contracts
## Request context and middleware policy
## Frontend boundaries
## Testing strategy
## Quality bar and surface-area budget
## Rollout and rollback plan
## Open risks and follow-ups

Related skills

• scaffold-nextjs or scaffold-cli: scaffold the repo once the brief is agreed.
• multi-tenant-architecture: tenant identification, isolation, and domain strategy.
• pr-reviewer: structural review of a local diff once implemented.
• planning: turn an Adoption opportunity into an implementation plan, then stress-test it.
• agents-md: audit and refactor the AGENTS.md file itself; this skill decides what belongs in tooling instead.
• tidy: diff-scoped cleanup pass; references/agent-friendly-codebase.md covers the repo-wide guardrail setup that keeps those passes small.

Gotchas

• Don't default to microservices for teams under 5: each service adds a deploy pipeline, contract versioning, and on-call surface. Start with a modular monorepo; split when a boundary is proven by team or scale pressure.
• Don't put app-level deps in a monorepo's root package.json: hoisting hides missing declarations, so an app builds locally but breaks deploying alone. Each app owns its deps.
• Don't define module contracts (handler/service/dao) without enforcement: an unenforced contract decays at the first deadline. Add an import-boundary lint rule (e.g. dao may not import handler) the day you write it.
• Don't thread a ctx parameter through business functions instead of AsyncLocalStorage: every signature grows, and adding one field later touches hundreds of call sites.
• Don't place "use client" at page or layout level: it converts the whole subtree to client rendering and forfeits streaming and direct server data access. Push it to leaves.
• Don't propose a big-bang rewrite during Adoption: migrate one vertical slice, verify it, then generalize.
• Don't extract to packages/ early: wait until 3+ apps need the same code; a premature shared package couples release cycles for nothing.
• Don't finalize a brief without a rollback plan per change: an irreversible decision needs a documented fallback before it ships.
• Don't dual-write to a database and a queue/webhook without an outbox (or CDC): one side commits, the other fails, and you silently lose or fabricate a notification. See references/distributed-correctness.md.
• Don't enforce an externally-forceable invariant by construction (unsigned type, hard CHECK): when the outside world forces the state, the system crashes or clamps instead of recording it. Represent it, detect it post-factum, recover explicitly.
• Don't encode in AGENTS.md what a linter can enforce deterministically: a prompt rule decays under context pressure; an exit code in pre-commit and CI does not.
• Don't leave legacy code unmarked next to current patterns: agents grep, find the old endpoint first, and build on it. Mark it with a greppable LEGACY comment or delete it (see references/agent-friendly-codebase.md).