mcp-patterns

# 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](https://agenticaifoundation.org/) (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.

> **Pin `mcp<2` until you migrate.** Every Python snippet in this skill targets the v1 SDK
> (`from mcp.server.fastmcp import FastMCP`). The Python SDK released **2.0.0**, `pip install mcp`
> now resolves to it, and upstream's own README says to keep a `<2` upper bound on your requirement
> until you have migrated. `FastMCP` does not appear anywhere in the 2.0 README, so treat these
> snippets as v1-only rather than assuming they still apply. v1.x continues to receive critical bug
> and security fixes on its own branch. Verified 2026-07-31.
> Migration guide: https://py.sdk.modelcontextprotocol.io/migration/

## 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

| Category | Rule | Impact | Key Pattern |
|----------|------|--------|-------------|
| **Server** | `server-setup.md` | HIGH | FastMCP lifespan, Tool/Resource/Prompt primitives |
| **Server** | `server-transport.md` | HIGH | stdio for CLI, Streamable HTTP for production |
| **Auth** | `auth-oauth21.md` | HIGH | PKCE, RFC 8707 resource indicators, token validation |
| **Advanced** | `advanced-composition.md` | MEDIUM | Pipeline, parallel, and branching tool composition |
| **Advanced** | `advanced-resources.md` | MEDIUM | Resource caching with TTL, LRU eviction, lifecycle |
| **Advanced** | `elicitation.md` | MEDIUM | Server-initiated structured input from users |
| **Advanced** | `sampling-tools.md` | MEDIUM | Server-side agent loops with tool calling |
| **Advanced** | `apps-ui.md` | MEDIUM | Interactive UI via MCP Apps + @mcp-ui/* SDK |
| **Client** | `client-patterns.md` | MEDIUM | TypeScript/Python MCP client connection patterns |
| **Security** | `security-injection.md` | HIGH | Description sanitization, encoding normalization |
| **Security** | `security-hardening.md` | HIGH | Zero-trust allowlist, hash verification, rug pull detection |
| **Quality** | `testing-debugging.md` | MEDIUM | MCP Inspector, unit tests, transport debugging |
| **Ecosystem** | `registry-discovery.md` | LOW | Official registry API, server metadata |
| **Ecosystem** | `webmcp-browser.md` | LOW | W3C browser-native agent tools (complementary) |

**Total: 14 rules across 6 categories**

## Key Decisions

| Decision | Recommendation |
|----------|----------------|
| Transport | stdio for CLI/Desktop, Streamable HTTP for production (SSE deprecated) |
| Language | TypeScript for production (better SDK support, type safety) |
| Auth | OAuth 2.1 with PKCE (S256) + RFC 8707 resource indicators |
| Server lifecycle | Always use FastMCP lifespan for resource management |
| Error handling | Return errors as text content (Claude can interpret and retry) |
| Tool composition | Pipeline for sequential, `asyncio.gather` for parallel |
| Resource caching | TTL + LRU eviction with memory cap |
| Tool trust model | Zero-trust: explicit allowlist + hash verification |
| User input | Elicitation for runtime input; never request PII via elicitation |
| Interactive UI | MCP Apps with @mcp-ui/* SDK; sandbox all iframes |
| Token handling | Never pass through client tokens to downstream services |
| Large results | Use `_meta["anthropic/maxResultSizeChars"]` annotation (up to 500K) for results that lose meaning when truncated (CC 2.1.91) |

## 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

| Feature | Spec Version | Status |
|---------|-------------|--------|
| Tools, Resources, Prompts | 2024-11-05 | Stable |
| Streamable HTTP transport | 2025-03-26 | Stable (replaces SSE) |
| OAuth 2.1 + Elicitation (form) | 2025-06-18 | Stable |
| Sampling with tool calling | 2025-11-25