> ## Documentation Index
> Fetch the complete documentation index at: https://evoke-f0bfabff.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Agent 06 — Architecture

> Senior Solution Architect. Produces system_design.json and HLD.md covering tech stack, database schema, API contracts, and security design.

<Info>
  **File:** `agents/sdlc/06-architecture.md` · **Model:** Opus · **Tools:** Bash, Read, Write
</Info>

## Purpose

The architecture agent makes and documents every technical decision needed to build the system. Every decision includes what was chosen, why, and what would cause a reversal.

## Core principle

> Every architectural decision is a trade-off. Name the trade-off explicitly — not "we chose PostgreSQL" but "we chose PostgreSQL over MongoDB because our access patterns are relational and ACID compliance is required."

## What it does

1. Reads `jira_tickets.json` and `requirement_spec.json`
2. Selects tech stack with documented rationale
3. Designs database schema
4. Defines API contracts
5. Plans service architecture
6. Includes security design considerations
7. Produces `system_design.json` and `HLD.md`

## Outputs

```
.rstack/runs/<run_id>/specs/
  architecture.md        ← Human-readable HLD
  system_design.json     ← Machine-readable design spec
```

`system_design.json` includes:

```json theme={null}
{
  "tech_stack": {
    "runtime": "Node.js 20",
    "framework": "Fastify",
    "database": "PostgreSQL 16",
    "rationale": "Relational model required; ACID for financial transactions"
  },
  "api_contracts": [...],
  "database_schema": [...],
  "security_design": {
    "auth": "JWT with 24h expiry + refresh tokens in httpOnly cookies",
    "encryption_at_rest": true,
    "encryption_in_transit": "TLS 1.3"
  }
}
```

<Warning>
  Architecture requires explicit `sdlc_approve` before implementation can begin.
</Warning>