MCP Patterns Skill

Model Context Protocol (MCP) server patterns for building integrations with Claude Code.

Triggers

mcp server, model context protocol, tool handler, mcp resource, mcp tool

Server Structure

Basic MCP Server (Python)

from mcp.server import Server
from mcp.server.stdio import stdio_server

app = Server("my-server")

@app.list_tools()
async def list_tools():
    return [
        {
            "name": "my_tool",
            "description": "Does something useful",
            "inputSchema": {
                "type": "object",
                "properties": {
                    "query": {"type": "string", "description": "Search query"}
                },
                "required": ["query"]
            }
        }
    ]

@app.call_tool()
async def call_tool(name: str, arguments: dict):
    if name == "my_tool":
        result = await do_something(arguments["query"])
        return {"content": [{"type": "text", "text": result}]}
    raise ValueError(f"Unknown tool: {name}")

async def main():
    async with stdio_server() as (read_stream, write_stream):
        await app.run(read_stream, write_stream, app.create_initialization_options())

if __name__ == "__main__":
    import asyncio
    asyncio.run(main())

Project Layout

my-mcp-server/
├── src/
│   └── my_server/
│       ├── __init__.py
│       ├── server.py       # Main server logic
│       ├── tools.py        # Tool handlers
│       └── resources.py    # Resource handlers
├── pyproject.toml
└── README.md

Tool Patterns

Tool with Validation

from pydantic import BaseModel, Field

class SearchInput(BaseModel):
    query: str = Field(..., min_length=1, max_length=500)
    limit: int = Field(default=10, ge=1, le=100)

@app.call_tool()
async def call_tool(name: str, arguments: dict):
    if name == "search":
        # Pydantic validates and parses
        params = SearchInput(**arguments)
        results = await search(params.query, params.limit)
        return {"content": [{"type": "text", "text": json.dumps(results)}]}

Tool with Error Handling

@app.call_tool()
async def call_tool(name: str, arguments: dict):
    try:
        if name == "fetch_data":
            data = await fetch_data(arguments["url"])
            return {"content": [{"type": "text", "text": data}]}
    except httpx.HTTPStatusError as e:
        return {
            "content": [{"type": "text", "text": f"HTTP error: {e.response.status_code}"}],
            "isError": True
        }
    except Exception as e:
        return {
            "content": [{"type": "text", "text": f"Error: {str(e)}"}],
            "isError": True
        }

Multiple Tool Registration

TOOLS = {
    "list_items": {
        "description": "List all items",
        "schema": {"type": "object", "properties": {}},
        "handler": handle_list_items
    },
    "get_item": {
        "description": "Get specific item",
        "schema": {
            "type": "object",
            "properties": {"id": {"type": "string"}},
            "required": ["id"]
        },
        "handler": handle_get_item
    },
    "create_item": {
        "description": "Create new item",
        "schema": {
            "type": "object",
            "properties": {
                "name": {"type": "string"},
                "data": {"type": "object"}
            },
            "required": ["name"]
        },
        "handler": handle_create_item
    }
}

@app.list_tools()
async def list_tools():
    return [
        {"name": name, "description": t["description"], "inputSchema": t["schema"]}
        for name, t in TOOLS.items()
    ]

@app.call_tool()
async def call_tool(name: str, arguments: dict):
    if name not in TOOLS:
        raise ValueError(f"Unknown tool: {name}")
    return await TOOLS[name]["handler"](arguments)

Resource Patterns

Static Resource

@app.list_resources()
async def list_resources():
    return [
        {
            "uri": "config://settings",
            "name": "Application Settings",
            "mimeType": "application/json"
        }
    ]

@app.read_resource()
async def read_resource(uri: str):
    if uri == "config://settings":
        return json.dumps({"theme": "dark", "lang": "en"})
    raise ValueError(f"Unknown resource: {uri}")

Dynamic Resources

@app.list_resources()
async def list_resources():
    # List available resources dynamically
    items = await get_all_items()
    return [
        {
            "uri": f"item://{item.id}",
            "name": item.name,
            "mimeType": "application/json"
        }
        for item in items
    ]

@app.read_resource()
async def read_resource(uri: str):
    if uri.startswith("item://"):
        item_id = uri.replace("item://", "")
        item = await get_item(item_id)
        return json.dumps(item.to_dict())
    raise ValueError(f"Unknown resource: {uri}")

Authentication Patterns

Environment Variables

import os

API_KEY = os.environ.get("MY_API_KEY")
if not API_KEY:
    raise ValueError("MY_API_KEY environment variable required")

async def make_api_call(endpoint: str):
    async with httpx.AsyncClient() as client:
        response = await client.get(
            f"https://api.example.com/{endpoint}",
            headers={"Authorization": f"Bearer {API_KEY}"}
        )
        response.raise_for_status()
        return response.json()

OAuth Token Refresh

from datetime import datetime, timedelta

class TokenManager:
    def __init__(self):
        self.token = None
        self.expires_at = None

    async def get_token(self) -> str:
        if self.token and self.expires_at > datetime.now():
            return self.token

        # Refresh token
        async with httpx.AsyncClient() as client:
            response = await client.post(
                "https://auth.example.com/token",
                data={"grant_type": "client_credentials", ...}
            )
            data = response.json()
            self.token = data["access_token"]
            self.expires_at = datetime.now() + timedelta(seconds=data["expires_in"] - 60)
            return self.token

token_manager = TokenManager()

State Management

SQLite for Persistence

import aiosqlite

DB_PATH = Path.home() / ".my-mcp-server" / "state.db"

async def init_db():
    DB_PATH.parent.mkdir(parents=True, exist_ok=True)
    async with aiosqlite.connect(DB_PATH) as db:
        await db.execute("""
            CREATE TABLE IF NOT EXISTS cache (
                key TEXT PRIMARY KEY,
                value TEXT,
                expires_at TEXT
            )
        """)
        await db.commit()

async def get_cached(key: str) -> str | None:
    async with aiosqlite.connect(DB_PATH) as db:
        cursor = await db.execute(
            "SELECT value FROM cache WHERE key = ? AND expires_at > datetime('now')",
            (key,)
        )
        row = await cursor.fetchone()
        return row[0] if row else None

async def set_cached(key: str, value: str, ttl_seconds: int = 3600):
    async with aiosqlite.connect(DB_PATH) as db:
        await db.execute(
            "INSERT OR REPLACE INTO cache (key, value, expires_at) VALUES (?, ?, datetime('now', '+' || ? || ' seconds'))",
            (key, value, ttl_seconds)
        )
        await db.commit()

In-Memory Cache

from functools import lru_cache
from cachetools import TTLCache

# Simple TTL cache
cache = TTLCache(maxsize=100, ttl=300)  # 5 minute TTL

async def get_data(key: str):
    if key in cache:
        return cache[key]
    data = await fetch_from_api(key)
    cache[key] = data
    return data

Claude Desktop Configuration

claude_desktop_config.json

{
  "mcpServers": {
    "my-server": {
      "command": "python",
      "args": ["-m", "my_server"],
      "env": {
        "MY_API_KEY": "your-key-here"
      }
    }
  }
}

With uv (Recommended)

{
  "mcpServers": {
    "my-server": {
      "command": "uv",
      "args": ["run", "--directory", "/path/to/my-server", "python", "-m", "my_server"],
      "env": {
        "MY_API_KEY": "your-key-here"
      }
    }
  }
}

Testing Patterns

Manual Test Script

# test_server.py
import asyncio
from my_server.server import app

async def test_tools():
    tools = await app.list_tools()
    print(f"Available tools: {[t['name'] for t in tools]}")

    result = await app.call_tool("my_tool", {"query": "test"})
    print(f"Result: {result}")

if __name__ == "__main__":
    asyncio.run(test_tools())

pytest with Async

import pytest
from my_server.tools import handle_search

@pytest.mark.asyncio
async def test_search_returns_results():
    result = await handle_search({"query": "test", "limit": 5})
    assert "content" in result
    assert len(result["content"]) > 0

@pytest.mark.asyncio
async def test_search_handles_empty():
    result = await handle_search({"query": "xyznonexistent123"})
    assert result["content"][0]["text"] == "No results found"

Common Issues

Issue	Solution
Server not starting	Check command path, ensure dependencies installed
Tool not appearing	Verify list_tools() returns valid schema
Auth failures	Check env vars are set in config, not shell
Timeout errors	Add timeout to httpx calls, use async properly
JSON parse errors	Ensure call_tool returns proper content structure