MCP Patterns

Patterns for building, composing, and securing Model Context Protocol servers. Based on the 2025-11-25 specification — the latest stable release maintained by the Agentic AI Foundation (Linux Foundation), co-founded by Anthropic, Block, and OpenAI.

Scaffolding a new server? Use Anthropic's mcp-builder skill (claude install anthropics/skills) for project setup and evaluation creation. This skill focuses on patterns, security, and advanced features after initial setup.

Deploying to Cloudflare? See the building-mcp-server-on-cloudflare skill for Workers-specific deployment patterns.

Decision Tree — Which Rule to Read

What are you building?
│
├── New MCP server
│   ├── Setup & primitives ──────► rules/server-setup.md
│   ├── Transport selection ─────► rules/server-transport.md
│   └── Scaffolding ─────────────► mcp-builder skill (anthropics/skills)
│
├── Authentication & authorization
│   └── OAuth 2.1 + OIDC ───────► rules/auth-oauth21.md
│
├── Advanced server features
│   ├── Tool composition ────────► rules/advanced-composition.md
│   ├── Resource caching ────────► rules/advanced-resources.md
│   ├── Elicitation (user input) ► rules/elicitation.md
│   ├── Sampling (agent loops) ──► rules/sampling-tools.md
│   └── Interactive UI ──────────► rules/apps-ui.md
│
├── Client-side consumption
│   └── Connecting to servers ───► rules/client-patterns.md
│
├── Security hardening
│   ├── Prompt injection defense ► rules/security-injection.md
│   └── Zero-trust & verification ► rules/security-hardening.md
│
├── Testing & debugging
│   └── Inspector + unit tests ──► rules/testing-debugging.md
│
├── Discovery & ecosystem
│   └── Registries & catalogs ──► rules/registry-discovery.md
│
└── Browser-native tools
    └── WebMCP (W3C) ───────────► rules/webmcp-browser.md

Quick Reference

Total: 14 rules across 6 categories

Key Decisions

Spec & Governance

• Protocol: Model Context Protocol, spec version 2025-11-25 (latest stable)
• Governance: Agentic AI Foundation (Linux Foundation, Dec 2025)
• Platinum members: AWS, Anthropic, Block, Bloomberg, Cloudflare, Google, Microsoft, OpenAI
• Adoption: 10,000+ servers; Claude, Cursor, Copilot, Gemini, ChatGPT, VS Code
• Spec URL: https://modelcontextprotocol.io/specification/2025-11-25
• 2026 model: Working Groups and Interest Groups are now the primary vehicle for protocol evolution (no more milestone-based releases). Enterprise readiness lands as extensions, not core spec changes.

Feature Maturity

Example

from mcp.server.fastmcp import FastMCP

mcp = FastMCP("my-server")

@mcp.tool()
async def search(query: str) -> str:
    """Search documents. Returns matching results."""
    results = await db.search(query)
    return "\n".join(r.title for r in results[:10])

Common Mistakes

1. No lifecycle management (connection/resource leaks on shutdown)
2. Missing input validation on tool arguments
3. Returning secrets in tool output (API keys, credentials)
4. Unbounded response sizes without _meta annotation — use _meta["anthropic/maxResultSizeChars"] to declare intentionally large results (DB schemas, API specs) so clients/hooks don't truncate them
5. Trusting tool descriptions without sanitization (injection risk)
6. No hash verification on tool invocations (rug pull vulnerability)
7. Storing auth tokens in session IDs (credential leak)
8. Blocking synchronous code in async server (use asyncio.to_thread())
9. Using SSE transport instead of Streamable HTTP (deprecated since March 2025)
10. Passing through client tokens to downstream services (confused deputy)

Ecosystem

Related Skills

• ork:llm-integration — LLM function calling patterns
• ork:security-patterns — General input sanitization and layered security
• ork:api-design — REST/GraphQL API design patterns