MCP Server Builder

MCP Server Builder

Build Model Context Protocol servers that give LLM clients well-designed tools, resources, and prompts. Uses the official open-source SDKs (both MIT licensed).

When to Use

• "Build an MCP server for X" / "wrap this API as MCP tools."
• "Connect my custom backend to Claude Code / an MCP client."
• Designing tool schemas, choosing a transport, or debugging an MCP server.

Python (FastMCP — recommended for most servers)

pip install "mcp[cli]"

A complete starter server lives in templates/server.py. The core pattern:

from mcp.server.fastmcp import FastMCP
mcp = FastMCP("my-server")

@mcp.tool()
def add(a: float, b: float) -> float:
    """Add two numbers. The docstring IS the model-facing description."""
    return a + b

if __name__ == "__main__":
    mcp.run()   # stdio transport by default

Run it and register with a client:

python templates/server.py
claude mcp add my-server -- python /abs/path/to/server.py

Node / TypeScript

npm install @modelcontextprotocol/sdk zod

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
import { z } from "zod";

const server = new McpServer({ name: "my-server", version: "1.0.0" });
server.tool("add", { a: z.number(), b: z.number() }, async ({ a, b }) => ({
  content: [{ type: "text", text: String(a + b) }],
}));
await server.connect(new StdioServerTransport());

Transports

• stdio — default for local clients (Claude Code, desktop). The client launches your process and talks over stdin/stdout. Never write logs to stdout — use stderr.
• streamable-http — for remote/hosted servers. In Python: mcp.run(transport="streamable-http").

Tool-design best practices

• Name tools by action and write a precise docstring/description — it's what the model uses to decide when to call the tool.
• Keep parameters typed and minimal; use enums/constraints so bad calls fail fast. FastMCP derives the JSON schema from type hints; the TS SDK uses zod.
• Return concise, structured text; avoid dumping huge payloads.
• Make tools idempotent where possible and validate inputs.
• One job per tool — prefer several focused tools over one mega-tool.

Notes & Limits

• Test locally with the MCP Inspector (npx @modelcontextprotocol/inspector).
• For stdio servers, anything on stdout that isn't protocol traffic breaks the client — log to stderr.

Credits

Original skill by Rinu (l3ad3r1) in collaboration with Claude (Anthropic). Built on the official MCP Python SDK and TypeScript SDK (both MIT). The Model Context Protocol is an open standard; all credit for the SDKs and spec belongs to their authors.