MCP Patterns

MCP Server Development Patterns

Package-specific patterns for @youdotcom-oss/mcp. Use these patterns when developing MCP tools and server functionality.

Schema Design

Always use Zod for input/output validation

export const MyToolInputSchema = z.object({
  query: z.string().min(1).describe("Search query"),
  limit: z.number().optional().describe("Max results"),
});

Why this pattern?

• Zod provides runtime validation and TypeScript types
• .describe() adds documentation for MCP tool parameters
• Schema validation catches invalid inputs before API calls
• Type inference from schema ensures type safety

Best practices:

• Use .min(1) for required strings
• Use .optional() for optional parameters
• Always include .describe() for user-facing parameter descriptions
• Export schemas for reuse in tests and documentation

Error Handling

Always use try/catch with typed error handling

try {
  const response = await apiCall();
  return formatResponse(response);
} catch (err: unknown) {
  const errorMessage = err instanceof Error ? err.message : String(err);
  await logger({ level: "error", data: `API call failed: ${errorMessage}` });
  return {
    content: [{ type: "text", text: `Error: ${errorMessage}` }],
    isError: true,
  };
}

Why this pattern?

• MCP tools must return structured responses, never throw
• err: unknown ensures type safety in catch blocks
• Logging errors helps with debugging
• isError: true signals error state to MCP client
• User-friendly error messages in content

Best practices:

• Always catch errors at tool handler level
• Log errors before returning
• Include context in error messages
• Return MCP-compatible error response format

Logging

Use getLogger(mcp) helper, never console.log

import { getLogger } from "../shared/get-logger.ts";

const logger = getLogger(mcp);
await logger({ level: "info", data: `Operation successful: ${result}` });
await logger({ level: "error", data: `Operation failed: ${errorMessage}` });

Why this pattern?

• MCP server logging uses server notification protocol
• console.log doesn't integrate with MCP clients
• getLogger provides proper MCP server notifications
• Logs appear in MCP client debug output

Best practices:

• Use "info" level for successful operations
• Use "error" level for failures
• Include operation context in log messages
• Never use console.log, console.error, or console.warn

Response Format

Return both content and structuredContent

return {
  content: [{ type: "text", text: formattedText }],
  structuredContent: responseData,
};

Why this pattern?

• content provides human-readable text for display
• structuredContent provides machine-readable data
• MCP clients can choose which format to use
• Enables both UI display and programmatic access

Best practices:

• Format text for readability (proper line breaks, formatting)
• Include all relevant data in structured content
• Keep structured content JSON-serializable
• Use consistent formatting across tools

Tool Registration

Use Zod schemas for tool parameter validation. See examples:

• Search tool: src/search/register-search-tool.ts:7-86
• Express tool: src/express/register-express-tool.ts:7-66
• Contents tool: src/contents/register-contents-tool.ts:7-89

Pattern:

mcp.tool(
  'tool-name',
  'Tool description for MCP clients',
  MyToolInputSchema,
  async (params) => {
    // Tool implementation
    try {
      const result = await apiCall(params);
      return {
        content: [{ type: "text", text: formatResult(result) }],
        structuredContent: result,
      };
    } catch (err: unknown) {
      const errorMessage = err instanceof Error ? err.message : String(err);
      await logger({ level: "error", data: errorMessage });
      return {
        content: [{ type: "text", text: `Error: ${errorMessage}` }],
        isError: true,
      };
    }
  }
);

Error Reporting

Include mailto links in error logs using generateErrorReportLink() helper (src/shared/generate-error-report-link.ts:6-37). This creates one-click error reporting with full diagnostic context.

Usage:

import { generateErrorReportLink } from "../shared/generate-error-report-link.ts";

const errorReportLink = generateErrorReportLink({
  subject: "MCP Tool Error",
  toolName: "you-search",
  errorMessage: err.message,
  context: { query: params.query },
});

await logger({
  level: "error",
  data: `Operation failed. Report: ${errorReportLink}`
});

Related Resources

• MCP Specification: https://modelcontextprotocol.io/docs/specification
• Zod Documentation: https://zod.dev
• Package AGENTS.md: packages/mcp/AGENTS.md
• MCP Inspector: Run bun run inspect for interactive testing