Building Custom Mcp

Building Custom MCP Servers

Building your own MCP server lets you expose any data source, internal API or business logic to Claude Code. This is where Claude starts to feel like a colleague who has access to all your systems.

---

Setup

npm install @modelcontextprotocol/sdk

---

Minimal MCP Server (Node.js)

// server.mjs — A minimal MCP server with one tool
import { Server } from "@modelcontextprotocol/sdk/server/index.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
import {
  CallToolRequestSchema,
  ListToolsRequestSchema,
} from "@modelcontextprotocol/sdk/types.js";

// Create the server
const server = new Server(
  { name: "my-custom-server", version: "1.0.0" },
  { capabilities: { tools: {} } }
);

// Declare available tools
server.setRequestHandler(ListToolsRequestSchema, async () => ({
  tools: [
    {
      name: "get_current_user",
      description: "Returns information about the currently logged-in user from our internal system.",
      inputSchema: {
        type: "object",
        properties: {
          user_id: {
            type: "string",
            description: "The user's ID"
          }
        },
        required: ["user_id"]
      }
    }
  ]
}));

// Handle tool calls
server.setRequestHandler(CallToolRequestSchema, async (request) => {
  if (request.params.name === "get_current_user") {
    const { user_id } = request.params.arguments;

    // Replace this with a real database call, API call, etc.
    const user = { id: user_id, name: "Jane Smith", role: "Engineering Manager" };

    return {
      content: [{ type: "text", text: JSON.stringify(user, null, 2) }]
    };
  }

  throw new Error(`Unknown tool: ${request.params.name}`);
});

// Start the server
const transport = new StdioServerTransport();
await server.connect(transport);

Register it in ~/.claude/claude.json:

{
  "mcpServers": {
    "my-server": {
      "command": "node",
      "args": ["/path/to/server.mjs"]
    }
  }
}

---

Real-World Example: CRM MCP Server

Expose your CRM contacts to Claude Code:

// crm-server.mjs — Expose contacts, deals and notes to Claude Code
import { Server } from "@modelcontextprotocol/sdk/server/index.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
import { CallToolRequestSchema, ListToolsRequestSchema } from "@modelcontextprotocol/sdk/types.js";
import Database from "better-sqlite3";

const db = new Database(process.env.CRM_DB_PATH || "./crm.db");

const server = new Server(
  { name: "crm-server", version: "1.0.0" },
  { capabilities: { tools: {} } }
);

server.setRequestHandler(ListToolsRequestSchema, async () => ({
  tools: [
    {
      name: "search_contacts",
      description: "Search CRM contacts by name, email, company or tag.",
      inputSchema: {
        type: "object",
        properties: {
          query: { type: "string", description: "Search term" },
          limit: { type: "number", description: "Max results (default 10)" }
        },
        required: ["query"]
      }
    },
    {
      name: "get_contact_history",
      description: "Get the full interaction history for a contact including emails, calls and notes.",
      inputSchema: {
        type: "object",
        properties: {
          contact_id: { type: "string" }
        },
        required: ["contact_id"]
      }
    },
    {
      name: "add_note",
      description: "Add a note to a contact's record.",
      inputSchema: {
        type: "object",
        properties: {
          contact_id: { type: "string" },
          note: { type: "string", description: "The note text" }
        },
        required: ["contact_id", "note"]
      }
    }
  ]
}));

server.setRequestHandler(CallToolRequestSchema, async (request) => {
  const { name, arguments: args } = request.params;

  if (name === "search_contacts") {
    const limit = args.limit || 10;
    const results = db.prepare(
      `SELECT id, name, email, company, tags
       FROM contacts
       WHERE name LIKE ? OR email LIKE ? OR company LIKE ?
       LIMIT ?`
    ).all(`%${args.query}%`, `%${args.query}%`, `%${args.query}%`, limit);

    return { content: [{ type: "text", text: JSON.stringify(results, null, 2) }] };
  }

  if (name === "get_contact_history") {
    const contact = db.prepare("SELECT * FROM contacts WHERE id = ?").get(args.contact_id);
    const history = db.prepare(
      "SELECT * FROM interactions WHERE contact_id = ? ORDER BY created_at DESC"
    ).all(args.contact_id);

    return { content: [{ type: "text", text: JSON.stringify({ contact, history }, null, 2) }] };
  }

  if (name === "add_note") {
    db.prepare(
      "INSERT INTO interactions (contact_id, type, content, created_at) VALUES (?, 'note', ?, datetime('now'))"
    ).run(args.contact_id, args.note);

    return { content: [{ type: "text", text: "Note added successfully." }] };
  }

  throw new Error(`Unknown tool: ${name}`);
});

const transport = new StdioServerTransport();
await server.connect(transport);

Now Claude Code can search your CRM, read contact history and add notes — from within any coding session.

---

Prompting Claude Code to Build an MCP Server

> Build a Node.js MCP server in mcp-servers/contacts-server.mjs that exposes
  my contacts stored in contacts.json.

  Tools to implement:
  1. list_contacts() — returns all contacts
  2. get_contact(id: string) — returns one contact by ID
  3. search_contacts(query: string) — searches by name or email
  4. add_contact(name, email, company, notes) — adds a new contact and saves to contacts.json
  5. update_contact(id, updates) — updates specified fields of a contact

  Also provide the mcpServers config entry I need to add to ~/.claude/claude.json.
  Use the @modelcontextprotocol/sdk package. Include error handling for missing records.

---

Resources and Prompts

Beyond tools, MCP servers can expose resources (readable data) and prompts (reusable templates):

// Expose a resource — Claude can read this directly
server.setRequestHandler(ListResourcesRequestSchema, async () => ({
  resources: [
    {
      uri: "crm://dashboard/summary",
      name: "CRM Dashboard Summary",
      description: "Current pipeline status, total contacts and recent activity",
      mimeType: "application/json"
    }
  ]
}));

// Expose a reusable prompt template
server.setRequestHandler(ListPromptsRequestSchema, async () => ({
  prompts: [
    {
      name: "outreach_email",
      description: "Draft a personalized outreach email for a contact",
      arguments: [
        { name: "contact_id", description: "The contact to write to", required: true }
      ]
    }
  ]
}));

---

Next module: Advanced Patterns