mcp-patterns

What this skill does

# 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.

## 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 | Stable |
| Elicitation URL mode | 2025-11-25 | Stable |
| MCP Apps (UI extension) | 2026-01-26 | Extension (ext-apps) |
| WebMCP (browser-native) | 2026-02-14 | W3C Community Draft |

## SDK landscape (2026-Q2)

| Package | What it is | When to use |
|---------|-----------|-------------|
| `mcp` (PyPI) **>=1.27** | Official Python SDK — includes the `FastMCP` helper, transport adapters, Inspector | New Python servers. This is the canonical package. |
| `@modelcontextprotocol/sdk` (npm) **>=1.29** | Official TypeScript SDK | New TS servers |
| `fastmcp` (PyPI) | Standalone fork by jlowin — predates `mcp`; API-compatible but diverges on lifespan and middleware | Existing projects pinned to it. New projects should prefer `mcp`. |

> The `fastmcp` fork and the `mcp.server.fastmcp` module are *not the same package*. Imports and `pyproject.toml` entries must agree or stacktraces become cryptic.

### Debugging with Claude Code

Pass `--mcp-debug` to Claude Code when troubleshooting server wiring — it surfaces the raw JSON-RPC frames, handshake failures, and tool-registration events that the default logger swallows:

```bash
claude --mcp-debug "query the local test server"
# or per-session:
export CLAUDE_MCP_DEBUG=1
```

Use alongside the MCP Inspector (`npx @modelcontextprotocol/inspector <cmd>`) — Inspector gives you the client-side frame view, `--mcp-debug` gives you what Claude actually saw.

> **CC 2.1.128 — reconnect tool summarization**: when a server reconnects mid-session, re-announced tools are summarized as `mcp__<server>__* (N tools re-registered)` instead of being enumerated line-by-line. Use the **initial connect** event as the source of truth for tool inventory; treat reconnect summaries as deltas only. See `references/mcp-audit-runbook.md` for grep recipes that work across both formats.

> **CC 2.1.133 — MCP OAuth honors HTTP(S)_PROXY / NO_PROXY / mTLS**: the full MCP OAuth flow (discovery, dynamic client registration, token exchange, token refresh) now respects standard proxy and client-certificate env vars end-to-end. Enterprise deployments behind corporate proxies no longer need OAuth-specific workarounds — the same `HTTPS_PROXY` / `NO_PROXY` / `NODE_EXTRA_CA_CERTS` config that already routes MCP transport now also routes auth. See `configure/references/cc-version-settings.md` (CC 2.1.133 section) for the env-var example. The companion deployment skill `building-mcp-server-on-cloudflare` can drop any prior "proxy-aware OAuth requires manual handling" caveat at this floor.

> **CC 2.1.163 — stdio servers get `CLAUDE_CODE_SESSION_ID`:** on `--resume`, stdio M