mcp-builder

MCP Builder

Build a working MCP server from a description of the tools you need. Produces a deployable Python server using FastMCP.

Workflow

Step 1: Define What to Expose

Ask what the server needs to provide:

• Tools -- Functions Claude can call (API wrappers, calculations, file operations)
• Resources -- Data Claude can read (database records, config, documents)
• Prompts -- Reusable prompt templates with parameters

A brief like "MCP server for querying our customer database" is enough.

Step 2: Scaffold the Server

pip install fastmcp

Create the server file. The server instance MUST be at module level:

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()

Step 3: Add Companion CLI Scripts (Optional)

For Claude Code terminal use, add scripts alongside the MCP server:

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

CLI scripts provide file I/O, batch processing, and richer output that MCP can't. See assets/SCRIPTS-TEMPLATE.md and assets/script-template.ts for TypeScript templates.

Step 4: Test Locally

Quick test -- run directly:

python server.py

Dev mode with inspector UI (recommended):

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

HTTP mode for remote clients:

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

Automated test script using FastMCP Client:

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"))

Step 5: Pre-Deploy Checklist

Run these checks before deploying. All required checks must pass.

Required (will cause deploy failure):

1. Server file exists
2. Python syntax valid: python3 -m py_compile server.py
3. Module-level server object (not inside a function):

``bash grep -q "^mcp = FastMCP\|^server = FastMCP\|^app = FastMCP" server.py ``

1. requirements.txt exists with PyPI packages only (no git+, -e, .whl, .tar.gz)
2. No hardcoded secrets (check for api_key = "..." patterns excluding os.getenv/os.environ)

Advisory (warnings):

1. fastmcp listed in requirements.txt
2. .gitignore includes .env
3. No circular imports
4. Git repository initialised with remote
5. Server can load: timeout 5 fastmcp inspect server.py

Step 6: Deploy

FastMCP Cloud (simplest):

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

Cloud requirements:

• Module-level server object named mcp, server, or app
• PyPI dependencies only in requirements.txt
• Public GitHub repository
• Environment variables for secrets (no hardcoded values)
• Auto-deploys on push to main, PR preview deployments

Docker (self-hosted):

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"]

Cloudflare Workers (edge): See the cloudflare-worker-builder skill for Workers-based MCP servers.

Critical Patterns

Module-Level Server Instance

FastMCP Cloud requires the server instance at module level:

# 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()

Type Annotations Required

FastMCP uses type annotations to generate tool schemas:

@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."""
    ...

Error Handling

Return errors as strings, don't raise exceptions:

@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}"

Cloud-Ready Server Pattern

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()

Common Errors and Fixes

These are the errors you will hit. Fix them before deploying.

| Error | Cause | Fix | |-------|-------|-----| | RuntimeError: No server object found at module level | Server inside a function | Export mcp = FastMCP(...) at module level | | RuntimeError: no running event loop | Missing async/await | Use async def for async operations | | TypeError: missing required argument 'context' | Context not type-hinted | Add context: Context with type hint | | ValueError: Invalid resource URI | Missing URI scheme | Use data://, file://, info://, api:// | | Resource template parameter mismatch | Name mismatch | user://{user_id} needs def get_user(user_id: str) | | Pydantic validation error | Wrong type hints | Ensure hints match actual data types | | Transport mismatch | Client/server protocol differ | Match both to stdio or both to http | | Import errors with editable package | Package not installed | pip install -e . or add to PYTHONPATH | | DeprecationWarning: mcp.settings | Old API | Use os.getenv() instead | | Port already in use | Stale process | lsof -ti:8000 \| xargs kill -9 | | Schema generation failure | Non-JSON types | Use JSON-compatible types (no NumPy arrays) | | JSON serialization error | datetime/bytes in response | Convert to .isoformat() or string | | Circular import | Factory in __init__.py | Use direct imports, avoid factory pattern | | Python 3.12+ datetime warning | datetime.utcnow() deprecated | Use datetime.now(timezone.utc) | | Import-time execution | Async resource at module level | Use lazy init pattern |

Production Patterns

Self-Contained Server

Keep all utilities in one file to avoid circular imports:

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