mcp-builder

type	skill
lifecycle	stable
inheritance	inheritable
name	mcp-builder
description	Build MCP servers for LLM tool integration — Python (FastMCP), Node/TypeScript (MCP SDK), or C#/.NET (Microsoft MCP SDK)
tier	standard
applyTo	**/mcp*,**/*mcp-server*,**/*model-context-protocol*
currency	2026-04-22T00:00:00.000Z

MCP Server Development Guide

Build high-quality MCP servers that enable LLMs to interact with external services

Staleness Watch: See EXTERNAL-API-REGISTRY.md for source URLs and recheck cadence

The quality of an MCP server is measured by how well it enables LLMs to accomplish real-world tasks.

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When to Build vs Use Existing

Microsoft MCP Servers

Before building custom, check if Microsoft already provides one:

Server	Type	Description
Azure MCP	Local	48+ Azure services (Storage, KeyVault, Cosmos, SQL, etc.)
Foundry MCP	Remote	https://mcp.ai.azure.com - Models, deployments, evals, agents
Fabric MCP	Local	Microsoft Fabric APIs, OneLake, item definitions
Playwright MCP	Local	Browser automation and testing
GitHub MCP	Remote	https://api.githubcopilot.com/mcp

Decision Matrix

Scenario	Recommendation
Azure service integration	Use Azure MCP Server (48 services covered)
AI Foundry agents/evals	Use Foundry MCP remote server
Custom internal APIs	Build custom server (this guide)
Third-party SaaS integration	Build custom server (this guide)

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Server Types

Type	Transport	Use Case	Example
Local	stdio	Desktop apps, single-user, local dev	Azure MCP Server via NPM/Docker
Remote	Streamable HTTP	Cloud services, multi-tenant, Agent Service	https://mcp.ai.azure.com (Foundry)

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Phase 1: Research and Planning

1.1 Understand Modern MCP Design

API Coverage vs. Workflow Tools: Balance comprehensive API endpoint coverage with specialized workflow tools. When uncertain, prioritize comprehensive API coverage.

Tool Naming and Discoverability: Clear, descriptive tool names help agents find the right tools quickly. Use consistent prefixes (e.g., github_create_issue, github_list_repos) and action-oriented naming.

Context Management: Design tools that return focused, relevant data. Agents benefit from concise tool descriptions and the ability to filter/paginate results.

Actionable Error Messages: Error messages should guide agents toward solutions with specific suggestions and next steps.

1.2 Study MCP Protocol Documentation

Start with the sitemap: https://modelcontextprotocol.io/sitemap.xml

Key pages to review:

• Specification overview and architecture
• Transport mechanisms (streamable HTTP, stdio)
• Tool, resource, and prompt definitions

1.3 Select Language and Transport

Language Selection:

Language	Best For	SDK
TypeScript (recommended)	General MCP servers, broad compatibility	@modelcontextprotocol/sdk
Python	Data/ML pipelines, FastAPI integration	mcp (FastMCP)
C#/.NET	Azure/Microsoft ecosystem, enterprise	Microsoft.Mcp.Core

Transport Selection:

Transport	Use Case	Characteristics
Streamable HTTP	Remote servers, multi-tenant, Agent Service	Stateless, scalable, requires auth
stdio	Local servers, desktop apps	Simple, single-user, no network

---

Phase 2: Implementation

TypeScript Server (Recommended)

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-mcp-server",
  version: "1.0.0",
});

// Register a tool with Zod schema
server.tool(
  "get_weather",
  "Get current weather for a city",
  { city: z.string().describe("City name") },
  async ({ city }) => ({
    content: [{ type: "text", text: JSON.stringify({ city, temp: "72°F" }) }],
  })
);

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

Python Server (FastMCP)

from mcp.server.fastmcp import FastMCP
from pydantic import BaseModel, Field

mcp = FastMCP("my-mcp-server")

class WeatherParams(BaseModel):
    city: str = Field(description="City name")

@mcp.tool(description="Get current weather for a city")
async def get_weather(params: WeatherParams) -> dict:
    return {"city": params.city, "temp": "72°F"}

if __name__ == "__main__":
    mcp.run()

C#/.NET Server

using Microsoft.Mcp.Core;

var server = new McpServerBuilder()
    .WithName("my-mcp-server")
    .WithVersion("1.0.0")
    .AddTool("get_weather", "Get current weather", async (string city) =>
        new { city, temp = "72°F" })
    .Build();

await server.RunAsync();

---

Tool Design Best Practices

Input Schema

• Use Zod (TypeScript) or Pydantic (Python) for validation
• Include constraints and clear descriptions
• Add examples in field descriptions

Output Schema

• Define outputSchema where possible for structured data
• Use structuredContent in tool responses (TypeScript SDK feature)
• Helps clients understand and process tool outputs

Annotations

Annotation	Purpose
readOnlyHint: true	Tool doesn't modify state
destructiveHint: true	Tool makes irreversible changes
idempotentHint: true	Safe to retry
openWorldHint: true	Tool accesses external systems

Implementation Patterns

// Good: Async, error handling, pagination
server.tool(
  "list_items",
  "List items with pagination",
  {
    page: z.number().default(1),
    limit: z.number().max(100).default(20),
  },
  async ({ page, limit }) => {
    try {
      const items = await api.listItems({ page, limit });
      return {
        content: [{
          type: "text",
          text: JSON.stringify({ items, page, hasMore: items.length === limit }),
        }],
      };
    } catch (error) {
      return {
        content: [{
          type: "text",
          text: `Error: ${error.message}. Try: check API key, verify endpoint, reduce page size.`,
        }],
        isError: true,
      };
    }
  }
);

---

Phase 3: Testing

Build and Verify

TypeScript:

npm run build
npx @modelcontextprotocol/inspector ./dist/server.js

Python:

python -m py_compile your_server.py
npx @modelcontextprotocol/inspector -- python your_server.py

MCP Inspector

Interactive debugging for any MCP server:

npx @modelcontextprotocol/inspector /path/to/your/mcp-server

Code Quality Checklist

• No duplicated code (DRY principle)
• Consistent error handling with actionable messages
• Full type coverage
• Clear tool descriptions
• Pagination support where applicable
• Proper async/await for I/O operations

---

Phase 4: Create Evaluations

Evaluation Purpose

Test whether LLMs can effectively use your MCP server to answer realistic, complex questions.

Create 10 Evaluation Questions

1. Tool Inspection: List available tools and understand capabilities
2. Content Exploration: Use READ-ONLY operations to explore data
3. Question Generation: Create 10 complex, realistic questions
4. Answer Verification: Solve each question yourself to verify answers

Evaluation Requirements

Each question must be:

• Independent: Not dependent on other questions
• Read-only: Only non-destructive operations required
• Complex: Requiring multiple tool calls and deep exploration
• Realistic: Based on real use cases humans would care about
• Verifiable: Single, clear answer that can be verified by string comparison
• Stable: Answer won't change over time

Output Format

<evaluation>
  <qa_pair>
    <question>Find discussions about AI model launches with animal codenames. One model needed a specific safety designation that uses the format ASL-X. What number X was being determined?</question>
    <answer>3</answer>
  </qa_pair>
  <!-- More qa_pairs... -->
</evaluation>

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Common Issues

Issue	Cause	Solution
Tools not appearing	Server not responding to tools/list	Verify tool registration, check server startup
Stdout pollution	Debug output on stdout	Move debug output to stderr
Connection refused	Port conflict or server crash	Check port availability, review logs
Timeout	Slow API calls	Add timeout handling, implement pagination
Schema validation	Invalid input schema	Use Zod/Pydantic with proper constraints

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Related Skills

This skill complements:

• mcp-development — Core MCP protocol patterns and architecture
• azure-architecture-patterns — When building Azure-integrated MCP servers
• testing-strategies — For comprehensive MCP server testing

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References

• MCP Protocol Specification
• TypeScript SDK
• Python SDK
• MCP Servers Directory
• MCP Inspector