hapi/cli/src/codex/happyMcpStdioBridge.ts

/**
 * Happy MCP STDIO Bridge
 *
 * Minimal STDIO MCP server exposing a single tool `change_title`.
 * On invocation it forwards the tool call to an existing Happy HTTP MCP server
 * using the StreamableHTTPClientTransport.
 *
 * Configure the target HTTP MCP URL via env var `HAPPY_HTTP_MCP_URL` or
 * via CLI flag `--url <http://127.0.0.1:PORT>`.
 *
 * Note: This process must not print to stdout as it would break MCP STDIO.
 */

import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
import { StdioServerTransport } from '@modelcontextprotocol/sdk/server/stdio.js';
import { Client } from '@modelcontextprotocol/sdk/client/index.js';
import { StreamableHTTPClientTransport } from '@modelcontextprotocol/sdk/client/streamableHttp.js';
import { z } from 'zod';

function parseArgs(argv: string[]): { url: string | null } {
  let url: string | null = null;
  for (let i = 0; i < argv.length; i++) {
    const a = argv[i];
    if (a === '--url' && i + 1 < argv.length) {
      url = argv[i + 1];
      i++;
    }
  }
  return { url };
}

export async function runHappyMcpStdioBridge(argv: string[]): Promise<void> {
  try {
    // Resolve target HTTP MCP URL
    const { url: urlFromArgs } = parseArgs(argv);
    const baseUrl = urlFromArgs || process.env.HAPPY_HTTP_MCP_URL || '';

    if (!baseUrl) {
      // Write to stderr; never stdout.
      process.stderr.write(
        '[happy-mcp] Missing target URL. Set HAPPY_HTTP_MCP_URL or pass --url <http://127.0.0.1:PORT>\n'
      );
      process.exit(2);
    }

    let httpClient: Client | null = null;

    async function ensureHttpClient(): Promise<Client> {
      if (httpClient) return httpClient;
      const client = new Client(
        { name: 'happy-stdio-bridge', version: '1.0.0' },
        { capabilities: {} }
      );

      const transport = new StreamableHTTPClientTransport(new URL(baseUrl));
      await client.connect(transport);
      httpClient = client;
      return client;
    }

    // Create STDIO MCP server
    const server = new McpServer({
      name: 'Happy MCP Bridge',
      version: '1.0.0',
    });

    // Register the single tool and forward to HTTP MCP
    server.registerTool(
      'change_title',
      {
        description: 'Change the title of the current chat session',
        title: 'Change Chat Title',
        inputSchema: {
          title: z.string().describe('The new title for the chat session'),
        },
      },
      async (args) => {
        try {
          const client = await ensureHttpClient();
          const response = await client.callTool({ name: 'change_title', arguments: args });
          // Pass-through response from HTTP server
          return response as any;
        } catch (error) {
          return {
            content: [
              { type: 'text', text: `Failed to change chat title: ${error instanceof Error ? error.message : String(error)}` },
            ],
            isError: true,
          };
        }
      }
    );

    // Start STDIO transport
    const stdio = new StdioServerTransport();
    await server.connect(stdio);
  } catch (err) {
    try {
      process.stderr.write(`[happy-mcp] Fatal: ${err instanceof Error ? err.message : String(err)}\n`);
    } finally {
      process.exit(1);
    }
  }
}