mcp-builder

pip install fastmcp

from fastmcp import FastMCP
 
# MUST be at module level for FastMCP Cloud
mcp = FastMCP("My Server")
 
@mcp.tool()
async def search_customers(query: str) -> str:
    """Search customers by name or email."""
    # Implementation here
    return f"Found customers matching: {query}"
 
@mcp.resource("customers://{customer_id}")
async def get_customer(customer_id: str) -> str:
    """Get customer details by ID."""
    return f"Customer {customer_id} details"
 
if __name__ == "__main__":
    mcp.run()

my-mcp-server/
├── src/index.ts          # MCP server (for Claude.ai)
├── scripts/
│   ├── search.ts         # CLI version of search tool
│   └── _shared.ts        # Shared auth/config
├── SCRIPTS.md            # Documents available scripts
└── package.json

python server.py

fastmcp dev server.py
# Opens inspector at http://localhost:5173
# Hot reload, detailed logging, tool/resource inspection

python server.py --transport http --port 8000

import asyncio
from fastmcp import Client
 
async def test_server(server_path):
    async with Client(server_path) as client:
        # List everything
        tools = await client.list_tools()
        resources = await client.list_resources()
        prompts = await client.list_prompts()
 
        print(f"Tools: {[t.name for t in tools]}")
        print(f"Resources: {[r.uri for r in resources]}")
        print(f"Prompts: {[p.name for p in prompts]}")
 
        # Call first tool
        if tools:
            result = await client.call_tool(tools[0].name, {})
            print(f"Tool result: {result}")
 
        # Read first resource
        if resources:
            data = await client.read_resource(resources[0].uri)
            print(f"Resource data: {data}")
 
asyncio.run(test_server("server.py"))

git add . && git commit -m "Ready for deployment"
git push -u origin main
# Visit https://fastmcp.cloud, connect repo, add env vars, deploy
# URL: https://your-project.fastmcp.app/mcp

FROM python:3.12-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . .
EXPOSE 8000
CMD ["python", "server.py", "--transport", "http", "--port", "8000"]

# CORRECT
mcp = FastMCP("My Server")
 
@mcp.tool()
def my_tool(): ...
 
# WRONG -- Cloud can't find the server
def create_server():
    mcp = FastMCP("My Server")
    return mcp
 
# FIX for factory pattern -- export at module level
def create_server() -> FastMCP:
    mcp = FastMCP("server")
    return mcp
mcp = create_server()

@mcp.tool()
async def search(
    query: str,           # Required parameter
    limit: int = 10,      # Optional with default
    tags: list[str] = []  # Complex types supported
) -> str:
    """Docstring becomes the tool description."""
    ...

@mcp.tool()
async def get_data(id: str) -> str:
    try:
        result = await fetch_data(id)
        return json.dumps(result)
    except NotFoundError:
        return f"Error: No data found for ID {id}"

import os
from fastmcp import FastMCP
 
mcp = FastMCP("production-server")
API_KEY = os.getenv("API_KEY")
 
@mcp.tool()
async def production_tool(data: str) -> dict:
    if not API_KEY:
        return {"error": "API_KEY not configured"}
    return {"status": "success", "data": data}
 
if __name__ == "__main__":
    mcp.run()

from fastmcp import FastMCP
import os
 
mcp = FastMCP("my-server")
 
# Config
class Config:
    API_KEY = os.getenv("API_KEY", "")
    BASE_URL = os.getenv("BASE_URL", "https://api.example.com")
 
# Helpers
def format_success(data): return {"status": "success", "data": data}
def format_error(msg): return {"status": "error", "message": msg}
 
@mcp.tool()
async def my_tool(query: str) -> dict:
    if not Config.API_KEY:
        return format_error("API_KEY not configured")
    return format_success({"query": query})

_db = None
 
async def get_db():
    global _db
    if _db is None:
        _db = await create_connection(Config.DB_URL)
    return _db

@mcp.resource("health://status")
async def health_check() -> dict:
    return {
        "status": "healthy",
        "version": "1.0.0",
        "checks": {
            "api": "connected",
            "database": "connected"
        }
    }

import httpx
 
_client = None
 
def get_client() -> httpx.AsyncClient:
    global _client
    if _client is None:
        _client = httpx.AsyncClient(
            base_url=Config.BASE_URL,
            headers={"Authorization": f"Bearer {Config.API_KEY}"},
            limits=httpx.Limits(max_connections=20, max_keepalive_connections=5),
            timeout=30.0
        )
    return _client

async def retry_with_backoff(func, max_retries=3, initial_delay=1.0):
    for attempt in range(max_retries):
        try:
            return await func()
        except Exception as e:
            if attempt == max_retries - 1:
                raise
            delay = initial_delay * (2 ** attempt)
            await asyncio.sleep(delay)

from fastmcp import Context
 
@mcp.tool()
async def tool_with_context(param: str, context: Context) -> dict:
    # Context parameter MUST have type hint
    pass

@mcp.tool()
async def long_task(items: list[str], context: Context) -> str:
    for i, item in enumerate(items):
        await context.report_progress(i + 1, len(items), f"Processing {item}")
        await process(item)
    return "Done"

@mcp.tool()
async def summarise(text: str, context: Context) -> str:
    result = await context.request_sampling(
        messages=[{"role": "user", "content": f"Summarise: {text}"}],
        max_tokens=200
    )
    return result

fastmcp dev server.py              # Dev mode with inspector UI
fastmcp run server.py              # Run (stdio)
fastmcp run server.py --transport http --port 8000  # Run (HTTP)
fastmcp inspect server.py          # Inspect without running
fastmcp install server.py          # Install to Claude Desktop
fastmcp deploy server.py --name my-server  # Deploy to Cloud