/architecture

name: architecture
description: |
  Greenfield architecture design: map functionality flows, draw components, design APIs,
  classify dependencies, plan observability. For multi-component, API, schema, auth, or
  integration-heavy work. For retrofitting existing code, use codebase-hygiene instead.
allowed-tools: Read Grep Glob LSP Bash
user-invocable: false

Architecture

Design systems from scratch: map flows, then draw components. For retrofitting existing code, use `codebase-hygiene` instead.

Intake Routing

| Request type | Use | | ------------- | ----- | | New system/major feature (greenfield) | This skill | | Existing code with shallow modules | `codebase-hygiene` | | Multi-component integration | This skill | | Single-component refactor | `planning` + `building` |

Functionality-First Design Process

Phase 1: Map Functionality Flows

Map every user flow end-to-end before designing any component:

Flow: [name]
1. [step] → [what the system does] → [what the user sees]
2. [step] → [what the system does] → [what the user sees]
Error paths:
- [error] → [system response] → [user sees]

Every flow must have its error paths mapped. Unmapped error paths become unmapped components.

Phase 2: Map to Architecture

Translate flows into components:

• Each flow step maps to one or more components
• Each error path maps to a component's error handling
• Data crossings between components become interfaces

Phase 3: Design Components

For each component:

• **Interface:** what it receives and returns (the contract)
• **Responsibility:** what it does (one sentence)
• **Dependencies:** what it needs (other components, external services)
• **State:** what it remembers (if anything)
• **Error handling:** what can go wrong and what it does about it

Before finalizing any component boundary, apply the **Deletion Test** and **Two-Adapter Rule** as defined in `cc10x:codebase-design`. A component that fails the deletion test (complexity vanishes if deleted) or fails the two-adapter rule (it is a port with only one adapter — an ordinary caller or test exercising the interface is not an adapter) is not a real boundary yet — fold it into its caller or defer the split until a second concrete need appears.

Architecture Views

System Context (C4 Level 1)

Box diagram: your system + external systems it talks to. One paragraph per external system: what it provides, what you depend on.

Container View (C4 Level 2)

Internal boxes: web app, API, database, queue, worker. Arrows show data flow. One paragraph per container: technology choice, responsibility.

Component View (C4 Level 3)

Inside each container: the modules/classes. Arrows show call relationships. This is what the builder will implement.

LSP-Powered Architecture Analysis

Use LSP to understand existing architecture before designing new:

• **Go to Definition** on key functions to trace the call graph
• **Find References** to understand blast radius of existing interfaces
• **Go to Type Definition** to understand data models
• **Incoming/Outgoing Calls** to map the dependency graph

API Design (Functionality-Aligned)

Design APIs from the flow, not from the data model:

1. **What does the user need to do?** (action, not resource) 2. **What's the minimal interface that enables it?** (fewest endpoints/parameters) 3. **What's the error contract?** (every error case from the flow mapping) 4. **What's the type contract?** (input/output types, not just shapes)

// Good: functionality-aligned
POST /orders/{id}/cancel  →  { status, cancelledAt }

// Bad: data-model-aligned
PUT /orders/{id}  →  { ..., status: "cancelled", ... }

Integration Patterns

For each integration:

| Field | Value | | ------- | ------- | | **System** | [name] | | **Protocol** | [HTTP/gRPC/CLI/message queue] | | **Direction** | [we call them / they call us / both] | | **Contract** | [request/response schema or event schema] | | **Failure mode** | [what happens when it's down] | | **Retry policy** | [retries, backoff, circuit breaker] |

Dependency Classification

| Class | Meaning | Example | | ------- | --------- | --------- | | **Owned** | We control the code and deploy it | Internal service | | **Wrapped** | We depend on it but wrap it in our interface | Third-party SDK behind adapter | | **Consumed** | We depend on it directly, no wrapper | External API called directly | | **Infra** | Platform-level dependency | Database, message queue |

Wrapped dependencies can be swapped. Consumed dependencies cannot. Track which is which — it determines your coupling risk.

Observability Design

For each component:

• **Logging:** what to log (not "everything" — specific events)
• **Metrics:** what to track (business-relevant, not infra noise)
• **Tracing:** what to trace (cross-component flows, not every function call)
• **Alerting:** when to alert (user-visible impact, not internal noise)

Architecture Vocabulary

The deep-module vocabulary (module, interface, depth, seam, adapter, leverage, locality, deletion test, two-adapter rule) is defined in `cc10x:codebase-design`. **Use those terms exactly.**

Three extra terms specific to greenfield architecture (not in codebase-design):

• **Concealed complexity** — work done behind a simple interface. The goal of deep modules.
• **Temporal coupling** — caller must know the order of operations. Design defect — remove or document explicitly.
• **Leaky abstraction** — interface exposes internal details callers must know. Design defect — fix the interface.

Design It Twice

When a module's interface is non-trivial, design it twice:

1. **First design:** the obvious approach. Write it out fully. 2. **Second design:** a different approach (not a refinement of the first).

Compare both. The first design is usually shallow — it mirrors the implementation. The second design reveals what the interface *should* hide. Use the better one, or a hybrid.

**Why:** One-pass int