feat(architecture): add architecture.md template and auto-merge on archive

openspec/AGENTS.md

@@ -18,7 +18,7 @@ Instructions for AI coding assistants using OpenSpec for spec-driven development
 Create proposal when you need to:
 - Add features or functionality
 - Make breaking changes (API, schema)
-- Change architecture or patterns  
+- Change architecture or patterns
 - Optimize performance (changes behavior)
 - Update security patterns
 
@@ -125,6 +125,7 @@ openspec validate [change] --strict --no-interactive
 ```
 openspec/
 ├── project.md              # Project conventions
+├── architecture.md         # System architecture (optional, living document)
 ├── specs/                  # Current truth - what IS built
 │   └── [capability]/       # Single focused capability
 │       ├── spec.md         # Requirements and scenarios
@@ -147,7 +148,7 @@ openspec/
 ```
 New request?
 ├─ Bug fix restoring spec behavior? → Fix directly
-├─ Typo/format/comment? → Fix directly  
+├─ Typo/format/comment? → Fix directly
 ├─ New feature/capability? → Create proposal
 ├─ Breaking change? → Create proposal
 ├─ Architecture change? → Create proposal
@@ -211,6 +212,8 @@ Create `design.md` if any of the following apply; otherwise omit it:
 - Security, performance, or migration complexity
 - Ambiguity that benefits from technical decisions before coding
 
+**Note:** Architectural decisions in `design.md` are automatically merged to `architecture.md` when the change is archived. Keep `design.md` focused on the specific change; the global architecture evolves through archived decisions.
+
 Minimal `design.md` skeleton:
 ```markdown
 ## Context
@@ -432,6 +435,63 @@ Only add complexity with:
 3. Review recent archives
 4. Ask for clarification
 
+## Architecture Documentation
+
+### Reading Architecture
+Before creating change proposals that affect system structure, read `openspec/architecture.md` to understand:
+- Current system components and relationships
+- Integration points and data flows
+- Past architectural decisions and their rationale
+
+### When to Update Architecture
+A change has **architectural impact** if it:
+- Adds or removes major components/services
+- Changes component boundaries or responsibilities
+- Adds new integration points or external dependencies
+- Modifies core data flows
+- Introduces new architectural patterns
+
+### Proposing Architectural Changes
+For architecture-impacting changes:
+1. Reference current state from `architecture.md` in `proposal.md`
+2. Document architectural decisions in `design.md`
+3. Include before/after diagrams showing the change
+4. The archive command will automatically merge decisions to `architecture.md`
+
+### Diagram Standards
+Use ASCII diagrams for maximum compatibility:
+
+```
+Component relationships:     Data flow:              Boundaries:
+┌─────────┐                  ──▶ direction           ┌──────────┐
+│ Service │                  ◀── bidirectional       │ Internal │
+└────┬────┘                                          ├──────────┤
+     │                       State transitions:      │ External │
+     ▼                       A ──[event]──▶ B        └──────────┘
+┌─────────┐
+│ Service │
+└─────────┘
+```
+
+## Post-Implementation Checklist
+
+After completing implementation and before archiving, verify:
+
+1. **Architecture currency**: If the change affected system structure, check if `architecture.md` reflects the new state:
+   - Are new components documented?
+   - Are data flows still accurate?
+   - Are integration points up to date?
+
+2. **Decision capture**: Ensure significant decisions are in `design.md` (they'll be auto-merged on archive)
+
+3. **Spec accuracy**: Verify specs match the implemented behavior
+
+**Quick check prompt:**
+```
+"Review my changes and check if openspec/architecture.md needs updating
+ to reflect any structural changes I made"
+```
+
 ## Quick Reference
 
 ### Stage Indicators
@@ -444,6 +504,7 @@ Only add complexity with:
 - `tasks.md` - Implementation steps
 - `design.md` - Technical decisions
 - `spec.md` - Requirements and behavior
+- `architecture.md` - System architecture and component relationships
 
 ### CLI Essentials
 ```bash

openspec/architecture.md

@@ -0,0 +1,84 @@
+# System Architecture
+
+## Overview
+
+<!-- High-level system diagram showing major components and their relationships -->
+<!-- Update this diagram as the system evolves -->
+
+```
+┌─────────────────────────────────────────────────────────────┐
+│                     [Your System Name]                       │
+├─────────────────────────────────────────────────────────────┤
+│                                                              │
+│   ┌──────────┐    ┌──────────┐    ┌──────────┐             │
+│   │ [Client] │───▶│  [API]   │───▶│   [DB]   │             │
+│   └──────────┘    └──────────┘    └──────────┘             │
+│                                                              │
+└─────────────────────────────────────────────────────────────┘
+```
+
+## Components
+
+<!-- Document each major component/service/module -->
+
+### [Component Name]
+- **Responsibility**: [What this component does]
+- **Technology**: [Stack/framework used]
+- **Key files**: [Main entry points, e.g., `src/core/component.ts`]
+
+<!-- Add more components as your system grows -->
+
+## Data Flows
+
+<!-- Document how data moves through your system -->
+
+### [Flow Name]
+```
+[Source] ──▶ [Processing] ──▶ [Destination]
+```
+- **Trigger**: [What initiates this flow]
+- **Data**: [What data moves through]
+
+## Integration Points
+
+<!-- Document external systems and APIs -->
+
+| External System | Purpose | Protocol | Notes |
+|-----------------|---------|----------|-------|
+| [Service]       | [Why]   | [How]    | [Details] |
+
+## Architectural Decisions
+
+<!-- This section is automatically updated when archiving changes with architectural impact -->
+<!-- Each row captures a key decision from design.md files -->
+
+| Date | Decision | Rationale | Status |
+|------|----------|-----------|--------|
+| <!-- Decisions are appended here during archive --> |
+
+## Constraints
+
+<!-- Technical, business, or regulatory constraints affecting architecture -->
+
+- [Constraint 1]: [Description and impact]
+- [Constraint 2]: [Description and impact]
+
+---
+
+## Diagram Reference
+
+<!-- ASCII diagram building blocks for consistency -->
+
+```
+Components:          Relationships:        Boundaries:
+┌─────────┐          ───▶ data flow        ┌──────────┐
+│ Service │          ◀─── reverse flow     │ Internal │
+└─────────┘          ←──▶ bidirectional    ├──────────┤
+                                           │ External │
+Grouping:            States:               └──────────┘
+┌─────────────┐      ○ start state
+│ ┌───┐ ┌───┐ │      ● end state
+│ │ A │ │ B │ │      □ intermediate
+│ └───┘ └───┘ │
+└─────────────┘
+```

schemas/spec-driven/schema.yaml

@@ -105,6 +105,10 @@ artifacts:
       Reference the proposal for motivation and specs for requirements.
 
       Good design docs explain the "why" behind technical decisions.
+
+      Note: Architectural decisions in design.md are automatically merged to
+      openspec/architecture.md when the change is archived. For structural changes,
+      verify architecture.md is updated after implementation.
     requires:
       - proposal