Build an MCP server
The Model Context Protocol (MCP) is the standard way to expose tools and data to AI assistants like Claude. An MCP server advertises tools the model can call; the official TypeScript SDK runs in Deno through npm. This server exposes a single tool over stdio, the transport desktop AI clients use to launch and talk to local servers.
import { McpServer } from "npm:@modelcontextprotocol/sdk/server/mcp.js";
import { StdioServerTransport } from "npm:@modelcontextprotocol/sdk/server/stdio.js";
import { z } from "npm:zod";A server has a name and version that clients use to identify it.
const server = new McpServer({
name: "dinosaur-facts",
version: "1.0.0",
});Register a tool the model can call. The inputSchema is a Zod schema: the SDK validates arguments against it and advertises the shape to the client, so the model knows what to pass. The handler returns content blocks, here a single piece of text.
const facts: Record<string, string> = {
tyrannosaurus: "Tyrannosaurus rex could bite with 12,800 pounds of force.",
velociraptor: "Velociraptor was about the size of a turkey.",
};
server.registerTool(
"get_dinosaur_fact",
{
description: "Get a fact about a given dinosaur",
inputSchema: { name: z.string().describe("The dinosaur's name") },
},
({ name }: { name: string }) => {
const fact = facts[name.toLowerCase()] ?? `No facts about ${name} yet.`;
return { content: [{ type: "text", text: fact }] };
},
);Connect the server to a transport. StdioServerTransport reads JSON-RPC messages from stdin and writes replies to stdout, which is how a desktop client runs the server as a subprocess. The server now waits for requests; it prints nothing on its own, so keep logging off stdout.
const transport = new StdioServerTransport();
await server.connect(transport);To use this server with Claude Desktop, add it to the client config and point the command at Deno: { "mcpServers": { "dinosaur-facts": { "command": "deno", "args": ["run", "-R", "mcp_server.ts"] } } } Or inspect it interactively with: npx @modelcontextprotocol/inspector