mcp-server-builder

Create Model Context Protocol servers to extend Claude's capabilities with custom tools and resources.

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Install skill "mcp-server-builder" with this command: npx skills add patricio0312rev/skills/patricio0312rev-skills-mcp-server-builder

MCP Server Builder

Create Model Context Protocol servers to extend Claude's capabilities with custom tools and resources.

Core Workflow

  • Define purpose: Identify what capabilities to add

  • Choose transport: stdio (local) or HTTP/SSE (remote)

  • Design tools: Define tool schemas and handlers

  • Add resources: Optional file/data access

  • Create prompts: Optional reusable prompts

  • Test locally: Verify with MCP inspector

  • Deploy: Configure for Claude Desktop or API

MCP Architecture Overview

┌─────────────┐ MCP Protocol ┌─────────────┐ │ Claude │◄────────────────────►│ MCP Server │ │ (Host) │ JSON-RPC over stdio │ (Your App) │ └─────────────┘ └─────────────┘ │ ▼ ┌───────────┐ │ Tools │ │ Resources │ │ Prompts │ └───────────┘

Project Setup

TypeScript MCP Server

Create project

mkdir my-mcp-server && cd my-mcp-server npm init -y

Install dependencies

npm install @modelcontextprotocol/sdk zod

Dev dependencies

npm install -D typescript @types/node tsx

// package.json { "name": "my-mcp-server", "version": "1.0.0", "type": "module", "main": "dist/index.js", "bin": { "my-mcp-server": "./dist/index.js" }, "scripts": { "build": "tsc", "dev": "tsx watch src/index.ts", "start": "node dist/index.js" } }

// tsconfig.json { "compilerOptions": { "target": "ES2022", "module": "NodeNext", "moduleResolution": "NodeNext", "outDir": "./dist", "rootDir": "./src", "strict": true, "esModuleInterop": true, "skipLibCheck": true, "declaration": true }, "include": ["src/**/*"] }

Basic Server Structure

// src/index.ts #!/usr/bin/env node

import { Server } from "@modelcontextprotocol/sdk/server/index.js"; import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js"; import { CallToolRequestSchema, ListToolsRequestSchema, ListResourcesRequestSchema, ReadResourceRequestSchema, ListPromptsRequestSchema, GetPromptRequestSchema, } from "@modelcontextprotocol/sdk/types.js";

// Create server instance const server = new Server( { name: "my-mcp-server", version: "1.0.0", }, { capabilities: { tools: {}, resources: {}, prompts: {}, }, } );

// Define tools const TOOLS = [ { name: "get_weather", description: "Get current weather for a location", inputSchema: { type: "object" as const, properties: { location: { type: "string", description: "City name or coordinates", }, units: { type: "string", enum: ["celsius", "fahrenheit"], default: "celsius", }, }, required: ["location"], }, }, { name: "search_database", description: "Search the internal database", inputSchema: { type: "object" as const, properties: { query: { type: "string", description: "Search query", }, limit: { type: "number", default: 10, }, }, required: ["query"], }, }, ];

// List tools handler server.setRequestHandler(ListToolsRequestSchema, async () => { return { tools: TOOLS }; });

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

switch (name) { case "get_weather": { const { location, units = "celsius" } = args as { location: string; units?: string; };

  // Implement your logic here
  const weather = await fetchWeather(location, units);

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

case "search_database": {
  const { query, limit = 10 } = args as { query: string; limit?: number };

  const results = await searchDatabase(query, limit);

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

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

} });

// Start server async function main() { const transport = new StdioServerTransport(); await server.connect(transport); console.error("MCP Server running on stdio"); }

main().catch(console.error);

// Helper functions (implement your logic) async function fetchWeather(location: string, units: string) { // Your implementation return { location, temperature: 22, units, condition: "sunny" }; }

async function searchDatabase(query: string, limit: number) { // Your implementation return { query, results: [], total: 0 }; }

Tools

Tool Schema Definition

// src/tools/index.ts import { z } from "zod";

// Define input schemas with Zod for validation export const GetWeatherSchema = z.object({ location: z.string().describe("City name or coordinates"), units: z.enum(["celsius", "fahrenheit"]).default("celsius"), });

export const SearchSchema = z.object({ query: z.string().min(1).describe("Search query"), filters: z .object({ category: z.string().optional(), dateFrom: z.string().optional(), dateTo: z.string().optional(), }) .optional(), limit: z.number().min(1).max(100).default(10), });

// Convert Zod schema to JSON Schema for MCP export function zodToJsonSchema(schema: z.ZodObject<any>) { // Use zod-to-json-schema package in production return schema; }

Tool Handler Pattern

// src/tools/handlers.ts import { z } from "zod";

type ToolHandler<T extends z.ZodSchema> = ( args: z.infer<T> ) => Promise<{ content: Array<{ type: string; text: string }> }>;

export function createToolHandler<T extends z.ZodSchema>( schema: T, handler: (args: z.infer<T>) => Promise<any> ): ToolHandler<T> { return async (rawArgs) => { // Validate input const args = schema.parse(rawArgs);

// Execute handler
const result = await handler(args);

// Format response
return {
  content: [
    {
      type: "text",
      text: typeof result === "string" ? result : JSON.stringify(result, null, 2),
    },
  ],
};

}; }

// Usage export const handleGetWeather = createToolHandler( GetWeatherSchema, async ({ location, units }) => { const response = await fetch( https://api.weather.com/v1/current?location=${location}&#x26;units=${units} ); return response.json(); } );

Resources

Static Resources

// src/resources/index.ts const RESOURCES = [ { uri: "config://app-settings", name: "Application Settings", description: "Current application configuration", mimeType: "application/json", }, { uri: "file://docs/readme", name: "Documentation", description: "Project documentation", mimeType: "text/markdown", }, ];

// List resources handler server.setRequestHandler(ListResourcesRequestSchema, async () => { return { resources: RESOURCES }; });

// Read resource handler server.setRequestHandler(ReadResourceRequestSchema, async (request) => { const { uri } = request.params;

switch (uri) { case "config://app-settings": return { contents: [ { uri, mimeType: "application/json", text: JSON.stringify(getAppSettings(), null, 2), }, ], };

case "file://docs/readme":
  const content = await fs.readFile("./README.md", "utf-8");
  return {
    contents: [
      {
        uri,
        mimeType: "text/markdown",
        text: content,
      },
    ],
  };

default:
  throw new Error(`Unknown resource: ${uri}`);

} });

Dynamic Resources

// Resources with templates (e.g., database records) const RESOURCE_TEMPLATES = [ { uriTemplate: "db://users/{userId}", name: "User Profile", description: "Get user by ID", mimeType: "application/json", }, ];

server.setRequestHandler(ListResourceTemplatesRequestSchema, async () => { return { resourceTemplates: RESOURCE_TEMPLATES }; });

server.setRequestHandler(ReadResourceRequestSchema, async (request) => { const { uri } = request.params;

// Parse URI to extract parameters const userMatch = uri.match(/^db://users/(\w+)$/); if (userMatch) { const userId = userMatch[1]; const user = await db.users.findById(userId);

return {
  contents: [
    {
      uri,
      mimeType: "application/json",
      text: JSON.stringify(user, null, 2),
    },
  ],
};

}

throw new Error(Unknown resource: ${uri}); });

Prompts

Reusable Prompts

// src/prompts/index.ts const PROMPTS = [ { name: "code_review", description: "Review code for best practices and issues", arguments: [ { name: "language", description: "Programming language", required: true, }, { name: "focus", description: "What to focus on (security, performance, style)", required: false, }, ], }, { name: "explain_error", description: "Explain an error message and suggest fixes", arguments: [ { name: "error", description: "The error message", required: true, }, { name: "context", description: "Additional context about what you were doing", required: false, }, ], }, ];

// List prompts handler server.setRequestHandler(ListPromptsRequestSchema, async () => { return { prompts: PROMPTS }; });

// Get prompt handler server.setRequestHandler(GetPromptRequestSchema, async (request) => { const { name, arguments: args } = request.params;

switch (name) { case "code_review": return { description: "Code review prompt", messages: [ { role: "user", content: { type: "text", text: Please review the following ${args?.language || "code"} code. ${args?.focus ? Focus particularly on: ${args.focus}` : ""}

Look for:

  • Bugs and potential issues

  • Security vulnerabilities

  • Performance improvements

  • Code style and best practices

  • Suggestions for improvement`, }, }, ], };

    case "explain_error": return { description: "Error explanation prompt", messages: [ { role: "user", content: { type: "text", text: `I encountered this error:

``` ${args?.error} ```

${args?.context ? Context: ${args.context} : ""}

Please explain:

  1. What this error means

  2. Common causes

  3. How to fix it

  4. How to prevent it in the future`, }, }, ], };

    default: throw new Error(Unknown prompt: ${name}); } });

Error Handling

// src/utils/errors.ts import { McpError, ErrorCode } from "@modelcontextprotocol/sdk/types.js";

export class ToolError extends Error { constructor( message: string, public code: ErrorCode = ErrorCode.InternalError ) { super(message); this.name = "ToolError"; }

toMcpError(): McpError { return new McpError(this.code, this.message); } }

// Usage in handlers server.setRequestHandler(CallToolRequestSchema, async (request) => { try { const { name, arguments: args } = request.params; // ... handle tool } catch (error) { if (error instanceof ToolError) { throw error.toMcpError(); } if (error instanceof z.ZodError) { throw new McpError( ErrorCode.InvalidParams, Invalid parameters: ${error.errors.map((e) => e.message).join(", ")} ); } throw new McpError( ErrorCode.InternalError, error instanceof Error ? error.message : "Unknown error" ); } });

Claude Desktop Configuration

// ~/Library/Application Support/Claude/claude_desktop_config.json (macOS) // %APPDATA%\Claude\claude_desktop_config.json (Windows) { "mcpServers": { "my-mcp-server": { "command": "node", "args": ["/path/to/my-mcp-server/dist/index.js"], "env": { "API_KEY": "your-api-key" } }, "npx-server": { "command": "npx", "args": ["-y", "@modelcontextprotocol/server-filesystem", "/path/to/allowed/dir"] } } }

HTTP/SSE Transport

// src/http-server.ts import express from "express"; import { Server } from "@modelcontextprotocol/sdk/server/index.js"; import { SSEServerTransport } from "@modelcontextprotocol/sdk/server/sse.js";

const app = express(); const server = new Server(/* ... */);

// SSE endpoint for MCP app.get("/mcp", async (req, res) => { const transport = new SSEServerTransport("/mcp/messages", res); await server.connect(transport); });

// Message endpoint app.post("/mcp/messages", express.json(), async (req, res) => { // Handle incoming messages });

app.listen(3000, () => { console.log("MCP HTTP server running on port 3000"); });

Testing

MCP Inspector

Install MCP inspector

npx @modelcontextprotocol/inspector

Run your server with inspector

npx @modelcontextprotocol/inspector node dist/index.js

Unit Tests

// tests/tools.test.ts import { describe, it, expect } from "vitest"; import { handleGetWeather } from "../src/tools/handlers";

describe("get_weather tool", () => { it("returns weather data for valid location", async () => { const result = await handleGetWeather({ location: "New York", units: "celsius", });

expect(result.content[0].type).toBe("text");
const data = JSON.parse(result.content[0].text);
expect(data).toHaveProperty("temperature");

});

it("throws error for invalid location", async () => { await expect( handleGetWeather({ location: "", units: "celsius" }) ).rejects.toThrow(); }); });

Complete Example: GitHub MCP Server

// src/index.ts #!/usr/bin/env node

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 { Octokit } from "@octokit/rest";

const octokit = new Octokit({ auth: process.env.GITHUB_TOKEN });

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

const TOOLS = [ { name: "list_issues", description: "List issues in a GitHub repository", inputSchema: { type: "object" as const, properties: { owner: { type: "string", description: "Repository owner" }, repo: { type: "string", description: "Repository name" }, state: { type: "string", enum: ["open", "closed", "all"], default: "open" }, }, required: ["owner", "repo"], }, }, { name: "create_issue", description: "Create a new issue", inputSchema: { type: "object" as const, properties: { owner: { type: "string" }, repo: { type: "string" }, title: { type: "string" }, body: { type: "string" }, labels: { type: "array", items: { type: "string" } }, }, required: ["owner", "repo", "title"], }, }, ];

server.setRequestHandler(ListToolsRequestSchema, async () => ({ tools: TOOLS }));

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

switch (name) { case "list_issues": { const { owner, repo, state = "open" } = args as any; const { data } = await octokit.issues.listForRepo({ owner, repo, state });

  return {
    content: [{
      type: "text",
      text: JSON.stringify(
        data.map((i) => ({ number: i.number, title: i.title, state: i.state })),
        null,
        2
      ),
    }],
  };
}

case "create_issue": {
  const { owner, repo, title, body, labels } = args as any;
  const { data } = await octokit.issues.create({ owner, repo, title, body, labels });

  return {
    content: [{
      type: "text",
      text: `Created issue #${data.number}: ${data.html_url}`,
    }],
  };
}

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

} });

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

Best Practices

  • Validate inputs: Use Zod or similar for schema validation

  • Handle errors gracefully: Return meaningful error messages

  • Use environment variables: Never hardcode secrets

  • Rate limit external calls: Respect API limits

  • Log appropriately: Use stderr for logs (stdout is for MCP)

  • Document tools well: Clear descriptions help Claude use them correctly

  • Keep tools focused: Single responsibility per tool

  • Test with inspector: Verify behavior before deployment

Output Checklist

Every MCP server should include:

  • Server with name and version

  • Capabilities declaration (tools/resources/prompts)

  • Tool schemas with descriptions

  • Input validation with clear error messages

  • Proper error handling and MCP error codes

  • Environment variable configuration

  • Claude Desktop config example

  • MCP inspector testing

  • Unit tests for handlers

  • README with setup instructions

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