async function connectAndDiscover()

in packages/core/src/tools/mcp-client.ts [165:376]

• 163 lines of code
• 33 McCabe index (conditional complexity)

async function connectAndDiscover(
  mcpServerName: string,
  mcpServerConfig: MCPServerConfig,
  toolRegistry: ToolRegistry,
): Promise<void> {
  // Initialize the server status as connecting
  updateMCPServerStatus(mcpServerName, MCPServerStatus.CONNECTING);

  let transport;
  if (mcpServerConfig.httpUrl) {
    const transportOptions: StreamableHTTPClientTransportOptions = {};

    if (mcpServerConfig.headers) {
      transportOptions.requestInit = {
        headers: mcpServerConfig.headers,
      };
    }

    transport = new StreamableHTTPClientTransport(
      new URL(mcpServerConfig.httpUrl),
      transportOptions,
    );
  } else if (mcpServerConfig.url) {
    transport = new SSEClientTransport(new URL(mcpServerConfig.url));
  } else if (mcpServerConfig.command) {
    transport = new StdioClientTransport({
      command: mcpServerConfig.command,
      args: mcpServerConfig.args || [],
      env: {
        ...process.env,
        ...(mcpServerConfig.env || {}),
      } as Record<string, string>,
      cwd: mcpServerConfig.cwd,
      stderr: 'pipe',
    });
  } else {
    console.error(
      `MCP server '${mcpServerName}' has invalid configuration: missing httpUrl (for Streamable HTTP), url (for SSE), and command (for stdio). Skipping.`,
    );
    // Update status to disconnected
    updateMCPServerStatus(mcpServerName, MCPServerStatus.DISCONNECTED);
    return;
  }

  const mcpClient = new Client({
    name: 'gemini-cli-mcp-client',
    version: '0.0.1',
  });

  // patch Client.callTool to use request timeout as genai McpCallTool.callTool does not do it
  // TODO: remove this hack once GenAI SDK does callTool with request options
  if ('callTool' in mcpClient) {
    const origCallTool = mcpClient.callTool.bind(mcpClient);
    mcpClient.callTool = function (params, resultSchema, options) {
      return origCallTool(params, resultSchema, {
        ...options,
        timeout: mcpServerConfig.timeout ?? MCP_DEFAULT_TIMEOUT_MSEC,
      });
    };
  }

  try {
    await mcpClient.connect(transport, {
      timeout: mcpServerConfig.timeout ?? MCP_DEFAULT_TIMEOUT_MSEC,
    });
    // Connection successful
    updateMCPServerStatus(mcpServerName, MCPServerStatus.CONNECTED);
  } catch (error) {
    // Create a safe config object that excludes sensitive information
    const safeConfig = {
      command: mcpServerConfig.command,
      url: mcpServerConfig.url,
      httpUrl: mcpServerConfig.httpUrl,
      cwd: mcpServerConfig.cwd,
      timeout: mcpServerConfig.timeout,
      trust: mcpServerConfig.trust,
      // Exclude args, env, and headers which may contain sensitive data
    };

    let errorString =
      `failed to start or connect to MCP server '${mcpServerName}' ` +
      `${JSON.stringify(safeConfig)}; \n${error}`;
    if (process.env.SANDBOX) {
      errorString += `\nMake sure it is available in the sandbox`;
    }
    console.error(errorString);
    // Update status to disconnected
    updateMCPServerStatus(mcpServerName, MCPServerStatus.DISCONNECTED);
    return;
  }

  mcpClient.onerror = (error) => {
    console.error(`MCP ERROR (${mcpServerName}):`, error.toString());
    // Update status to disconnected on error
    updateMCPServerStatus(mcpServerName, MCPServerStatus.DISCONNECTED);
  };

  if (transport instanceof StdioClientTransport && transport.stderr) {
    transport.stderr.on('data', (data) => {
      const stderrStr = data.toString();
      // Filter out verbose INFO logs from some MCP servers
      if (!stderrStr.includes('] INFO')) {
        console.debug(`MCP STDERR (${mcpServerName}):`, stderrStr);
      }
    });
  }

  try {
    const mcpCallableTool: CallableTool = mcpToTool(mcpClient);
    const discoveredToolFunctions = await mcpCallableTool.tool();

    if (
      !discoveredToolFunctions ||
      !Array.isArray(discoveredToolFunctions.functionDeclarations)
    ) {
      console.error(
        `MCP server '${mcpServerName}' did not return valid tool function declarations. Skipping.`,
      );
      if (
        transport instanceof StdioClientTransport ||
        transport instanceof SSEClientTransport ||
        transport instanceof StreamableHTTPClientTransport
      ) {
        await transport.close();
      }
      // Update status to disconnected
      updateMCPServerStatus(mcpServerName, MCPServerStatus.DISCONNECTED);
      return;
    }

    for (const funcDecl of discoveredToolFunctions.functionDeclarations) {
      if (!funcDecl.name) {
        console.warn(
          `Discovered a function declaration without a name from MCP server '${mcpServerName}'. Skipping.`,
        );
        continue;
      }

      let toolNameForModel = funcDecl.name;

      // Replace invalid characters (based on 400 error message from Gemini API) with underscores
      toolNameForModel = toolNameForModel.replace(/[^a-zA-Z0-9_.-]/g, '_');

      const existingTool = toolRegistry.getTool(toolNameForModel);
      if (existingTool) {
        toolNameForModel = mcpServerName + '__' + toolNameForModel;
      }

      // If longer than 63 characters, replace middle with '___'
      // (Gemini API says max length 64, but actual limit seems to be 63)
      if (toolNameForModel.length > 63) {
        toolNameForModel =
          toolNameForModel.slice(0, 28) + '___' + toolNameForModel.slice(-32);
      }

      sanitizeParameters(funcDecl.parameters);

      // Ensure parameters is a valid JSON schema object, default to empty if not.
      const parameterSchema: Record<string, unknown> =
        funcDecl.parameters && typeof funcDecl.parameters === 'object'
          ? { ...(funcDecl.parameters as FunctionDeclaration) }
          : { type: 'object', properties: {} };

      toolRegistry.registerTool(
        new DiscoveredMCPTool(
          mcpCallableTool,
          mcpServerName,
          toolNameForModel,
          funcDecl.description ?? '',
          parameterSchema,
          funcDecl.name,
          mcpServerConfig.timeout ?? MCP_DEFAULT_TIMEOUT_MSEC,
          mcpServerConfig.trust,
        ),
      );
    }
  } catch (error) {
    console.error(
      `Failed to list or register tools for MCP server '${mcpServerName}': ${error}`,
    );
    // Ensure transport is cleaned up on error too
    if (
      transport instanceof StdioClientTransport ||
      transport instanceof SSEClientTransport ||
      transport instanceof StreamableHTTPClientTransport
    ) {
      await transport.close();
    }
    // Update status to disconnected
    updateMCPServerStatus(mcpServerName, MCPServerStatus.DISCONNECTED);
  }

  // If no tools were registered from this MCP server, the following 'if' block
  // will close the connection. This is done to conserve resources and prevent
  // an orphaned connection to a server that isn't providing any usable
  // functionality. Connections to servers that did provide tools are kept
  // open, as those tools will require the connection to function.
  if (toolRegistry.getToolsByServer(mcpServerName).length === 0) {
    console.log(
      `No tools registered from MCP server '${mcpServerName}'. Closing connection.`,
    );
    if (
      transport instanceof StdioClientTransport ||
      transport instanceof SSEClientTransport ||
      transport instanceof StreamableHTTPClientTransport
    ) {
      await transport.close();
      // Update status to disconnected
      updateMCPServerStatus(mcpServerName, MCPServerStatus.DISCONNECTED);
    }
  }
}