PowerToys/.github/instructions/typescript-mcp-server.instructions.md

description, applyTo

description	applyTo
Instructions for building Model Context Protocol (MCP) servers using the TypeScript SDK	**/*.ts, **/*.js, **/package.json

TypeScript MCP Server Development

Instructions

• Use the @modelcontextprotocol/sdk npm package: npm install @modelcontextprotocol/sdk
• Import from specific paths: @modelcontextprotocol/sdk/server/mcp.js, @modelcontextprotocol/sdk/server/stdio.js, etc.
• Use McpServer class for high-level server implementation with automatic protocol handling
• Use Server class for low-level control with manual request handlers
• Use zod for input/output schema validation: npm install zod@3
• Always provide title field for tools, resources, and prompts for better UI display
• Use registerTool(), registerResource(), and registerPrompt() methods (recommended over older APIs)
• Define schemas using zod: { inputSchema: { param: z.string() }, outputSchema: { result: z.string() } }
• Return both content (for display) and structuredContent (for structured data) from tools
• For HTTP servers, use StreamableHTTPServerTransport with Express or similar frameworks
• For local integrations, use StdioServerTransport for stdio-based communication
• Create new transport instances per request to prevent request ID collisions (stateless mode)
• Use session management with sessionIdGenerator for stateful servers
• Enable DNS rebinding protection for local servers: enableDnsRebindingProtection: true
• Configure CORS headers and expose Mcp-Session-Id for browser-based clients
• Use ResourceTemplate for dynamic resources with URI parameters: new ResourceTemplate('resource://{param}', { list: undefined })
• Support completions for better UX using completable() wrapper from @modelcontextprotocol/sdk/server/completable.js
• Implement sampling with server.server.createMessage() to request LLM completions from clients
• Use server.server.elicitInput() to request additional user input during tool execution
• Enable notification debouncing for bulk updates: debouncedNotificationMethods: ['notifications/tools/list_changed']
• Dynamic updates: call .enable(), .disable(), .update(), or .remove() on registered items to emit listChanged notifications
• Use getDisplayName() from @modelcontextprotocol/sdk/shared/metadataUtils.js for UI display names
• Test servers with MCP Inspector: npx @modelcontextprotocol/inspector

Best Practices

• Keep tool implementations focused on single responsibilities
• Provide clear, descriptive titles and descriptions for LLM understanding
• Use proper TypeScript types for all parameters and return values
• Implement comprehensive error handling with try-catch blocks
• Return isError: true in tool results for error conditions
• Use async/await for all asynchronous operations
• Close database connections and clean up resources properly
• Validate input parameters before processing
• Use structured logging for debugging without polluting stdout/stderr
• Consider security implications when exposing file system or network access
• Implement proper resource cleanup on transport close events
• Use environment variables for configuration (ports, API keys, etc.)
• Document tool capabilities and limitations clearly
• Test with multiple clients to ensure compatibility

Common Patterns

Basic Server Setup (HTTP)

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

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

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

app.post('/mcp', async (req, res) => {
    const transport = new StreamableHTTPServerTransport({
        sessionIdGenerator: undefined,
        enableJsonResponse: true
    });
    
    res.on('close', () => transport.close());
    
    await server.connect(transport);
    await transport.handleRequest(req, res, req.body);
});

app.listen(3000);

Basic Server Setup (stdio)

import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
import { StdioServerTransport } from '@modelcontextprotocol/sdk/server/stdio.js';

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

// ... register tools, resources, prompts ...

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

Simple Tool

import { z } from 'zod';

server.registerTool(
    'calculate',
    {
        title: 'Calculator',
        description: 'Perform basic calculations',
        inputSchema: { a: z.number(), b: z.number(), op: z.enum(['+', '-', '*', '/']) },
        outputSchema: { result: z.number() }
    },
    async ({ a, b, op }) => {
        const result = op === '+' ? a + b : op === '-' ? a - b : 
                      op === '*' ? a * b : a / b;
        const output = { result };
        return {
            content: [{ type: 'text', text: JSON.stringify(output) }],
            structuredContent: output
        };
    }
);

Dynamic Resource

import { ResourceTemplate } from '@modelcontextprotocol/sdk/server/mcp.js';

server.registerResource(
    'user',
    new ResourceTemplate('users://{userId}', { list: undefined }),
    {
        title: 'User Profile',
        description: 'Fetch user profile data'
    },
    async (uri, { userId }) => ({
        contents: [{
            uri: uri.href,
            text: `User ${userId} data here`
        }]
    })
);

Tool with Sampling

server.registerTool(
    'summarize',
    {
        title: 'Text Summarizer',
        description: 'Summarize text using LLM',
        inputSchema: { text: z.string() },
        outputSchema: { summary: z.string() }
    },
    async ({ text }) => {
        const response = await server.server.createMessage({
            messages: [{
                role: 'user',
                content: { type: 'text', text: `Summarize: ${text}` }
            }],
            maxTokens: 500
        });
        
        const summary = response.content.type === 'text' ? 
            response.content.text : 'Unable to summarize';
        const output = { summary };
        return {
            content: [{ type: 'text', text: JSON.stringify(output) }],
            structuredContent: output
        };
    }
);

Prompt with Completion

import { completable } from '@modelcontextprotocol/sdk/server/completable.js';

server.registerPrompt(
    'review',
    {
        title: 'Code Review',
        description: 'Review code with specific focus',
        argsSchema: {
            language: completable(z.string(), value => 
                ['typescript', 'python', 'javascript', 'java']
                    .filter(l => l.startsWith(value))
            ),
            code: z.string()
        }
    },
    ({ language, code }) => ({
        messages: [{
            role: 'user',
            content: {
                type: 'text',
                text: `Review this ${language} code:\n\n${code}`
            }
        }]
    })
);

Error Handling

server.registerTool(
    'risky-operation',
    {
        title: 'Risky Operation',
        description: 'An operation that might fail',
        inputSchema: { input: z.string() },
        outputSchema: { result: z.string() }
    },
    async ({ input }) => {
        try {
            const result = await performRiskyOperation(input);
            const output = { result };
            return {
                content: [{ type: 'text', text: JSON.stringify(output) }],
                structuredContent: output
            };
        } catch (err: unknown) {
            const error = err as Error;
            return {
                content: [{ type: 'text', text: `Error: ${error.message}` }],
                isError: true
            };
        }
    }
);