Claude 3.5 Sonnet System Prompt Architecture: XML Tagging, MCP Tool-Calling & Prompt Caching

1. The Canonical XML Tag Hierarchy

Anthropic trained Claude to parse nested XML structures as first-class control flows. Instead of unstructured Markdown bullet points, a production Claude system prompt should be structured into four distinct functional zones:

2. Leveraging Prompt Caching for 90% Cost Reduction

Claude 3.5 Sonnet supports prompt caching with a 5-minute Time-To-Live (TTL). When building agents that inspect an entire codebase or large documentation suite, you can place a cache breakpoint at the end of the static knowledge base:

// Anthropic TypeScript SDK Example
const response = await anthropic.messages.create({
  model: 'claude-3-5-sonnet-20241022',
  max_tokens: 4096,
  system: [
    {
      type: 'text',
      text: 'Here is our full 50,000-token PostgreSQL database schema...',
      cache_control: { type: 'ephemeral' } // 👈 Cache breakpoint!
    }
  ],
  messages: [{ role: 'user', content: 'Generate cohort retention SQL.' }]
});

Every user query hitting this agent within 5 minutes reads the 50,000 cached tokens at $0.375 per million tokens instead of the standard $3.75 per million—cutting total spend by 90%.

3. Model Context Protocol (MCP) Standard Integration

Anthropic's open-source Model Context Protocol (MCP) eliminates brittle custom API integrations. By exposing MCP tools to Claude, you can connect your agent directly to local files, databases, GitHub PRs, and Slack channels.

When writing prompts for MCP-enabled Claude agents, always instruct the model to execute an explicit Discovery Step prior to tool invocation:

4. Ready-to-Use Production Template: Fullstack Senior Architect

<role>
You are a Principal Software Architect with 15+ years of experience in distributed systems, TypeScript, and modern Cloudflare/AWS cloud architecture.
</role>

<constraints>
1. Never emit placeholder comments like '// implement here' or '// rest of code stays same'. Always provide complete, deployable blocks.
2. Enforce strict TypeScript: no 'any', no implicit coercion, use unknown + Zod validation for external data.
3. Apply modern error handling with early returns (guard clauses) to minimize indentation depth.
</constraints>

<instructions>
When asked to refactor or design a feature:
- Step 1: Analyze the edge cases and dependencies inside <architecture_plan> tags.
- Step 2: Emit the verified, production-ready code inside standard fenced code blocks.
- Step 3: Conclude with a 2-sentence explanation of trade-offs made.
</instructions>