MCP Patterns

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

server-setup.md

server-transport.md

auth-oauth21.md

advanced-composition.md

advanced-resources.md

elicitation.md

sampling-tools.md

apps-ui.md

client-patterns.md

security-injection.md

security-hardening.md

testing-debugging.md

registry-discovery.md

webmcp-browser.md

Key Decisions

asyncio.gather

Spec & Governance

• Protocol: Model Context Protocol, spec version 2025-11-25
• 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

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 (Claude has context limits)
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

mcp-builder

building-mcp-server-on-cloudflare

@mcp-ui/*

Related Skills

• llm-integration

  — LLM function calling patterns

• security-patterns

  — General input sanitization and layered security

• api-design

  — REST/GraphQL API design patterns