building-mcp-servers

Building MCP Servers

Reference guide for Model Context Protocol server development (January 2026). Covers TypeScript, Python, and C# implementations with Streamable HTTP transport.

Quick Reference

Language	Package	Version	Transport
TypeScript	@modelcontextprotocol/sdk	1.25.1	StreamableHTTPServerTransport
Python	mcp	1.25.0	transport="streamable-http"
C#	ModelContextProtocol.AspNetCore	0.6.0-preview	.WithHttpTransport()

Transport Status

Transport	Status	Use Case
stdio	Supported	Local/CLI (Claude Desktop, Cursor)
Streamable HTTP	Recommended	Remote servers, production
SSE	Deprecated	Legacy only

Streamable HTTP Transport

Single endpoint replaces dual SSE endpoints. Supports stateful sessions or stateless (serverless) mode.

Protocol Flow

Client                              Server
  |------ POST /mcp ---------------->|  (JSON-RPC messages)
  |<----- JSON or SSE response ------|
  |------ GET /mcp ----------------->|  (Optional: server-initiated)
  |<----- SSE stream ----------------|

Required Headers

POST /mcp HTTP/1.1
Content-Type: application/json
Accept: application/json, text/event-stream
Mcp-Session-Id: <session-id>  # After initialization

TypeScript Implementation

Installation

npm install @modelcontextprotocol/sdk zod express
npm install -D @types/express

Basic Server

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/streamableHttp.js";
import { z } from "zod";
import express from "express";

const app = express();
app.use(express.json());

const server = new McpServer({
  name: "my-server",
  version: "1.0.0"
});

// Tool
server.registerTool("add", {
  description: "Add two numbers",
  inputSchema: { a: z.number(), b: z.number() }
}, async ({ a, b }) => ({
  content: [{ type: "text", text: String(a + b) }]
}));

// Resource
server.registerResource(
  "config",
  "config://app",
  { description: "App configuration" },
  async (uri) => ({
    contents: [{ uri: uri.href, text: JSON.stringify({ env: "prod" }) }]
  })
);

// Prompt
server.registerPrompt("review", {
  description: "Code review",
  argsSchema: { code: z.string() }
}, ({ code }) => ({
  messages: [{ role: "user", content: { type: "text", text: `Review:\n${code}` } }]
}));

// Transport
const transport = new StreamableHTTPServerTransport({
  sessionIdGenerator: () => crypto.randomUUID()
});

await server.connect(transport);

app.all("/mcp", async (req, res) => {
  await transport.handleRequest(req, res);
});

app.listen(3000);

Note: Top-level await requires Node.js with ES modules ("type": "module" in package.json or .mjs extension).

Stateless Mode (Serverless)

const transport = new StreamableHTTPServerTransport({
  sessionIdGenerator: undefined  // Disables sessions
});

Python Implementation

Installation

pip install "mcp[cli]"

Basic Server (FastMCP)

from mcp.server.fastmcp import FastMCP, Context
from typing import List

mcp = FastMCP("my-server")

# Tool
@mcp.tool()
def add(a: int, b: int) -> int:
    """Add two numbers."""
    return a + b

# Async tool with progress
@mcp.tool()
async def process_files(files: List[str], ctx: Context) -> str:
    """Process files with progress."""
    for i, f in enumerate(files):
        await ctx.report_progress(i + 1, len(files))
    return f"Processed {len(files)} files"

# Resource
@mcp.resource("config://app")
def get_config() -> dict:
    """App configuration."""
    return {"env": "prod", "version": "1.0.0"}

# Dynamic resource
@mcp.resource("users://{user_id}/profile")
def get_user(user_id: str) -> dict:
    """Get user profile."""
    return {"id": user_id, "name": f"User {user_id}"}

# Prompt
@mcp.prompt()
def code_review(code: str, language: str = "python") -> str:
    """Code review prompt."""
    return f"Review this {language} code:\n\n```{language}\n{code}\n```"

if __name__ == "__main__":
    # Streamable HTTP
    mcp.run(transport="streamable-http", host="0.0.0.0", port=8000, path="/mcp")
    # Or stdio: mcp.run()

FastAPI Integration

from fastapi import FastAPI
from mcp.server.fastmcp import FastMCP

api = FastAPI()
mcp = FastMCP("api-tools")

@mcp.tool()
def query(sql: str) -> dict:
    return {"result": "data"}

api.mount("/mcp", mcp.streamable_http_app())
# Run: uvicorn app:api --port 8000

C# Implementation

Installation

dotnet add package ModelContextProtocol.AspNetCore --prerelease

Basic Server

using System.ComponentModel;
using Microsoft.Extensions.DependencyInjection;
using ModelContextProtocol.Server;

var builder = WebApplication.CreateBuilder(args);

builder.Services
    .AddMcpServer()
    .WithHttpTransport()
    .WithToolsFromAssembly()
    .WithPromptsFromAssembly()
    .WithResourcesFromAssembly();

var app = builder.Build();
app.MapMcp();  // Maps /, /sse, /messages
app.Run();

// Tools
[McpServerToolType]
public static class MyTools
{
    [McpServerTool, Description("Add two numbers")]
    public static int Add(
        [Description("First number")] int a,
        [Description("Second number")] int b) => a + b;

    [McpServerTool, Description("Get weather")]
    public static async Task<string> GetWeather(
        HttpClient http,  // Injected from DI
        [Description("City name")] string city,
        CancellationToken ct)
    {
        var data = await http.GetStringAsync($"https://api.weather.example/{city}", ct);
        return data;
    }
}

// Prompts
[McpServerPromptType]
public static class MyPrompts
{
    [McpServerPrompt, Description("Code review prompt")]
    public static ChatMessage CodeReview(
        [Description("Code to review")] string code) =>
        new(ChatRole.User, $"Review this code:\n\n{code}");
}

// Resources
[McpServerResourceType]
public static class MyResources
{
    [McpServerResource(Name = "config://app"), Description("App config")]
    public static string Config() => """{"env": "production"}""";

    [McpServerResource(UriTemplate = "docs://{topic}")]
    public static string GetDoc([Description("Topic")] string topic) =>
        $"Documentation for {topic}";
}

Stdio Transport (Local)

var builder = Host.CreateApplicationBuilder(args);

builder.Logging.AddConsole(o => o.LogToStandardErrorThreshold = LogLevel.Trace);

builder.Services
    .AddMcpServer()
    .WithStdioServerTransport()
    .WithToolsFromAssembly();

await builder.Build().RunAsync();

Authentication (OAuth 2.1)

MCP servers are OAuth Resource Servers (not Authorization Servers). Key requirements:

• PKCE mandatory (S256 method)
• Resource Indicators (RFC 8707) required
• Bearer tokens in Authorization header
• Audience validation on every request

Discovery Endpoints

GET /.well-known/oauth-protected-resource  # Server metadata
GET /.well-known/oauth-authorization-server  # Auth server metadata

Server Response on 401

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer realm="mcp",
  resource_metadata="https://mcp.example.com/.well-known/oauth-protected-resource"

Token Validation (Python)

import jwt
from jwt import PyJWKClient

class TokenValidator:
    def __init__(self, jwks_uri: str, audience: str):
        self.jwks = PyJWKClient(jwks_uri)
        self.audience = audience

    def validate(self, token: str) -> dict:
        key = self.jwks.get_signing_key_from_jwt(token)
        return jwt.decode(
            token, key.key,
            algorithms=["RS256"],
            audience=self.audience,
            options={"require": ["exp", "aud", "iss"]}
        )

Protected Resource Metadata Response

{
  "resource": "https://mcp.example.com",
  "authorization_servers": ["https://auth.example.com"],
  "scopes_supported": ["read", "write", "admin"],
  "bearer_methods_supported": ["header"]
}

OAuth Middleware Integration (Python/Starlette)

from starlette.applications import Starlette
from starlette.middleware import Middleware
from starlette.middleware.base import BaseHTTPMiddleware
from starlette.responses import JSONResponse
from mcp.server.fastmcp import FastMCP

class BearerAuthMiddleware(BaseHTTPMiddleware):
    def __init__(self, app, validator: TokenValidator):
        super().__init__(app)
        self.validator = validator

    async def dispatch(self, request, call_next):
        auth = request.headers.get("Authorization", "")
        if not auth.startswith("Bearer "):
            return JSONResponse(
                {"error": "unauthorized"},
                status_code=401,
                headers={"WWW-Authenticate": 'Bearer realm="mcp"'}
            )
        try:
            token = auth.replace("Bearer ", "")
            request.state.auth = self.validator.validate(token)
        except Exception:
            return JSONResponse({"error": "invalid_token"}, status_code=401)
        return await call_next(request)

# Setup
mcp = FastMCP("secure-server")
validator = TokenValidator(
    jwks_uri="https://auth.example.com/.well-known/jwks.json",
    audience="https://mcp.example.com"
)

app = Starlette(
    routes=[Mount("/mcp", app=mcp.streamable_http_app())],
    middleware=[Middleware(BearerAuthMiddleware, validator=validator)]
)

OAuth Middleware Integration (TypeScript/Express)

import jwt from "jsonwebtoken";
import jwksClient from "jwks-rsa";

const client = jwksClient({ jwksUri: "https://auth.example.com/.well-known/jwks.json" });

const authMiddleware = async (req, res, next) => {
  const auth = req.headers.authorization;
  if (!auth?.startsWith("Bearer ")) {
    return res.status(401).json({ error: "unauthorized" });
  }
  try {
    const token = auth.slice(7);
    const decoded = jwt.decode(token, { complete: true });
    const key = await client.getSigningKey(decoded.header.kid);
    req.auth = jwt.verify(token, key.getPublicKey(), {
      audience: "https://mcp.example.com",
      algorithms: ["RS256"]
    });
    next();
  } catch (err) {
    res.status(401).json({ error: "invalid_token" });
  }
};

app.use("/mcp", authMiddleware);

Authenticated Client Request

const transport = new StreamableHTTPClientTransport(
  new URL("https://mcp.example.com/mcp"),
  {
    requestInit: {
      headers: { Authorization: `Bearer ${accessToken}` }
    }
  }
);

Security Anti-Patterns

Anti-Pattern	Fix
Token passthrough to upstream APIs	Use separate tokens for upstream calls
Missing audience validation	Always validate aud claim
Tokens in URLs	Use Authorization header only

Scope Enforcement in Tools

Check scopes before executing sensitive operations:

TypeScript:

const authMiddleware = async (req, res, next) => {
  // ... token validation ...
  req.auth = { sub: decoded.sub, scopes: decoded.scope?.split(" ") || [] };
  next();
};

server.registerTool("delete_user", { /* ... */ }, async ({ userId }, { meta }) => {
  const scopes = meta?.auth?.scopes || [];
  if (!scopes.includes("admin:write")) {
    return { isError: true, content: [{ type: "text", text: "Insufficient scope" }] };
  }
  // ... perform deletion ...
});

Python: Use HTTP middleware to validate, then check in tools:

# With Starlette middleware (see OAuth Middleware Integration above)
# Store validated claims in request.state, then check in tool:

@mcp.tool()
def delete_user(user_id: str) -> str:
    """Delete user - requires admin:write scope."""
    # Scope enforcement happens at HTTP layer via middleware
    # Tool assumes request already passed auth checks
    return f"Deleted user {user_id}"

Note: For advanced middleware with per-tool auth context, use fastmcp package (pip install fastmcp) which provides Middleware class and Context.get_state(). The official mcp package provides FastMCP but with simpler middleware options.

Passing Auth Context to Tool Handlers

TypeScript: Store auth on transport or use a request-scoped context:

// In middleware: attach to request
req.auth = { sub: decoded.sub, email: decoded.email };

// Pass to tool via server context or closure
const sessions = new Map();
app.all("/mcp", async (req, res) => {
  const transport = new StreamableHTTPServerTransport({ /* ... */ });
  sessions.set(transport.sessionId, { auth: req.auth });
  // Tools access via sessions.get(sessionId)
});

Python (with fastmcp package): Use middleware and context state:

# pip install fastmcp
from fastmcp import FastMCP, Context
from fastmcp.server.middleware import Middleware, MiddlewareContext

mcp = FastMCP("my-server")

class AuthMiddleware(Middleware):
    async def on_call_tool(self, context: MiddlewareContext, call_next):
        # Extract and validate token, then store in context
        context.fastmcp_context.set_state("user_id", "user_123")
        context.fastmcp_context.set_state("scopes", ["read", "write"])
        return await call_next()

mcp.add_middleware(AuthMiddleware())

@mcp.tool
async def get_my_profile(ctx: Context) -> dict:
    user_id = ctx.get_state("user_id")  # Set by middleware
    return {"user_id": user_id, "profile": "..."}

Token Refresh Handling

Access tokens are short-lived (typically 1 hour). Strategies:

1. Client-side refresh: Clients refresh tokens before expiration and reconnect
2. Proactive server refresh: Background task refreshes tokens expiring soon
3. On-demand refresh: Return 401, client refreshes and retries

Recommended pattern: Use short-lived access tokens (5-60 min) with refresh tokens. On 401:

// Client retry logic
async function callWithRefresh(tool: string, args: object) {
  try {
    return await client.callTool(tool, args);
  } catch (err) {
    if (err.status === 401) {
      accessToken = await refreshAccessToken(refreshToken);
      transport.updateHeaders({ Authorization: `Bearer ${accessToken}` });
      return await client.callTool(tool, args);
    }
    throw err;
  }
}

Migration: SSE to Streamable HTTP

Server Changes

// OLD (SSE) - Two endpoints
app.get("/sse", async (req, res) => {
  const transport = new SSEServerTransport("/sse/messages", res);
  await server.connect(transport);
});
app.post("/sse/messages", async (req, res) => { /* ... */ });

// NEW (Streamable HTTP) - Single endpoint
app.all("/mcp", async (req, res) => {
  const transport = new StreamableHTTPServerTransport();
  await server.connect(transport);
  await transport.handleRequest(req, res);
});

Client Changes

// OLD
import { SSEClientTransport } from "@modelcontextprotocol/sdk/client/sse.js";
const transport = new SSEClientTransport(new URL("http://localhost:3000/sse"));

// NEW
import { StreamableHTTPClientTransport } from "@modelcontextprotocol/sdk/client/streamableHttp.js";
const transport = new StreamableHTTPClientTransport(new URL("http://localhost:3000/mcp"));

Backward-Compatible Server

// Support both transports during migration
app.all("/mcp", /* new Streamable HTTP handler */);
app.get("/sse", /* legacy SSE handler */);
app.post("/sse/messages", /* legacy message handler */);

Client Configuration

Claude Desktop / Claude Code (stdio)

{
  "mcpServers": {
    "my-server": {
      "command": "node",
      "args": ["/path/to/server.js"],
      "env": { "API_KEY": "secret" }
    }
  }
}

Remote Server (Streamable HTTP)

{
  "mcpServers": {
    "remote": {
      "type": "streamable-http",
      "url": "https://mcp.example.com/mcp"
    }
  }
}

Error Handling

Return tool errors (not protocol errors) for model self-correction:

TypeScript:

server.registerTool("query", { /* ... */ }, async ({ sql }) => {
  if (sql.includes("DROP")) {
    return {
      isError: true,
      content: [{ type: "text", text: "Destructive queries not allowed" }]
    };
  }
  // ...
});

Python: Raise exceptions in tools - they're caught and returned as errors:

@mcp.tool()
def query(sql: str) -> dict:
    if "DROP" in sql.upper():
        raise ValueError("Destructive queries not allowed")
    return {"result": "data"}

Common Mistakes

Mistake	Fix
Using SSE for new servers	Use Streamable HTTP
Writing to stdout in stdio servers	Use stderr for logs
Missing session ID after init	Always include Mcp-Session-Id header
Not validating OAuth audience	Validate token aud matches your server
Forwarding client tokens upstream	Use separate credentials for upstream APIs

Official Resources

SDKs

• TypeScript: https://github.com/modelcontextprotocol/typescript-sdk
• Python: https://github.com/modelcontextprotocol/python-sdk
• C#: https://github.com/modelcontextprotocol/csharp-sdk

Documentation

• Specification: https://modelcontextprotocol.io/specification/2025-11-25
• Transports: https://modelcontextprotocol.io/specification/2025-11-25/basic/transports
• Authorization: https://modelcontextprotocol.io/specification/2025-11-25/basic/authorization

Testing

npx @modelcontextprotocol/inspector node path/to/server.js