Gmail MCP Server

Instructions

Implementation Reference

• src/index.ts:702-714 (registration)

  Registration of the 'get_attachment' MCP tool, including description, input schema (messageId and id), and handler function that uses handleTool to call Gmail API.

  server.tool("get_attachment",
    "Get a message attachment",
    {
      messageId: z.string().describe("ID of the message containing the attachment"),
      id: z.string().describe("The ID of the attachment"),
    },
    async (params) => {
      return handleTool(config, async (gmail: gmail_v1.Gmail) => {
        const { data } = await gmail.users.messages.attachments.get({ userId: 'me', messageId: params.messageId, id: params.id })
        return formatResponse(data)
      })
    }
  )

• src/index.ts:708-713 (handler)

  The handler function for 'get_attachment' tool: authenticates via handleTool, retrieves the attachment data from Gmail API using messageId and id, and returns formatted response.

  async (params) => {
    return handleTool(config, async (gmail: gmail_v1.Gmail) => {
      const { data } = await gmail.users.messages.attachments.get({ userId: 'me', messageId: params.messageId, id: params.id })
      return formatResponse(data)
    })
  }

• src/index.ts:704-707 (schema)

  Zod input schema for 'get_attachment' tool defining required string parameters: messageId and id.

  {
    messageId: z.string().describe("ID of the message containing the attachment"),
    id: z.string().describe("The ID of the attachment"),
  },

• src/index.ts:50-80 (helper)

  Shared 'handleTool' helper used by 'get_attachment' (and other tools): creates OAuth2/Gmail clients, validates credentials, executes the provided API callback, handles auth-specific errors gracefully.

  const handleTool = async (queryConfig: Record<string, any> | undefined, apiCall: (gmail: gmail_v1.Gmail) => Promise<any>) => {
    try {
      const oauth2Client = queryConfig ? createOAuth2Client(queryConfig) : defaultOAuth2Client
      if (!oauth2Client) throw new Error('OAuth2 client could not be created, please check your credentials')
  
      const credentialsAreValid = await validateCredentials(oauth2Client)
      if (!credentialsAreValid) throw new Error('OAuth2 credentials are invalid, please re-authenticate')
  
      const gmailClient = queryConfig ? google.gmail({ version: 'v1', auth: oauth2Client }) : defaultGmailClient
      if (!gmailClient) throw new Error('Gmail client could not be created, please check your credentials')
  
      const result = await apiCall(gmailClient)
      return result
    } catch (error: any) {
      // Check for specific authentication errors
      if (
        error.message?.includes("invalid_grant") ||
        error.message?.includes("refresh_token") ||
        error.message?.includes("invalid_client") ||
        error.message?.includes("unauthorized_client") ||
        error.code === 401 ||
        error.code === 403
      ) {
        return formatResponse({
          error: `Authentication failed: ${error.message}. Please re-authenticate by running: npx @shinzolabs/gmail-mcp auth`,
        });
      }
  
      return formatResponse({ error: `Tool execution failed: ${error.message}` });
    }
  }

• src/index.ts:48-48 (helper)

  Shared 'formatResponse' helper: converts API response to MCP content format (JSON string in text block). Used in 'get_attachment' handler.

  const formatResponse = (response: any) => ({ content: [{ type: "text", text: JSON.stringify(response) }] })