Gmail MCP Server

Instructions

Implementation Reference

• src/index.ts:1032-1037 (handler)

  The handler function that executes the create_filter tool logic. It invokes handleTool to authenticate and call the Gmail API's users.settings.filters.create method with the provided parameters, then formats the response.

  async (params) => {
    return handleTool(config, async (gmail: gmail_v1.Gmail) => {
      const { data } = await gmail.users.settings.filters.create({ userId: 'me', requestBody: params })
      return formatResponse(data)
    })
  }

• src/index.ts:1015-1031 (schema)

  Zod schema for input validation of the create_filter tool, defining criteria for matching emails and actions to apply (add/remove labels or forward).

    criteria: z.object({
      from: z.string().optional().describe("The sender's display name or email address"),
      to: z.string().optional().describe("The recipient's display name or email address"),
      subject: z.string().optional().describe("Case-insensitive phrase in the message's subject"),
      query: z.string().optional().describe("A Gmail search query that specifies the filter's criteria"),
      negatedQuery: z.string().optional().describe("A Gmail search query that specifies criteria the message must not match"),
      hasAttachment: z.boolean().optional().describe("Whether the message has any attachment"),
      excludeChats: z.boolean().optional().describe("Whether the response should exclude chats"),
      size: z.number().optional().describe("The size of the entire RFC822 message in bytes"),
      sizeComparison: z.enum(['smaller', 'larger']).optional().describe("How the message size in bytes should be in relation to the size field")
    }).describe("Filter criteria"),
    action: z.object({
      addLabelIds: z.array(z.string()).optional().describe("List of labels to add to messages"),
      removeLabelIds: z.array(z.string()).optional().describe("List of labels to remove from messages"),
      forward: z.string().optional().describe("Email address that the message should be forwarded to")
    }).describe("Actions to perform on messages matching the criteria")
  },

• src/index.ts:1012-1039 (registration)

  The MCP server.tool registration for the create_filter tool, specifying the tool name, description, input schema, and handler function.

  server.tool("create_filter",
    "Creates a filter",
    {
      criteria: z.object({
        from: z.string().optional().describe("The sender's display name or email address"),
        to: z.string().optional().describe("The recipient's display name or email address"),
        subject: z.string().optional().describe("Case-insensitive phrase in the message's subject"),
        query: z.string().optional().describe("A Gmail search query that specifies the filter's criteria"),
        negatedQuery: z.string().optional().describe("A Gmail search query that specifies criteria the message must not match"),
        hasAttachment: z.boolean().optional().describe("Whether the message has any attachment"),
        excludeChats: z.boolean().optional().describe("Whether the response should exclude chats"),
        size: z.number().optional().describe("The size of the entire RFC822 message in bytes"),
        sizeComparison: z.enum(['smaller', 'larger']).optional().describe("How the message size in bytes should be in relation to the size field")
      }).describe("Filter criteria"),
      action: z.object({
        addLabelIds: z.array(z.string()).optional().describe("List of labels to add to messages"),
        removeLabelIds: z.array(z.string()).optional().describe("List of labels to remove from messages"),
        forward: z.string().optional().describe("Email address that the message should be forwarded to")
      }).describe("Actions to perform on messages matching the criteria")
    },
    async (params) => {
      return handleTool(config, async (gmail: gmail_v1.Gmail) => {
        const { data } = await gmail.users.settings.filters.create({ userId: 'me', requestBody: params })
        return formatResponse(data)
      })
    }
  )

• src/index.ts:50-80 (helper)

  Shared helper function handleTool used by create_filter (and other tools) to handle OAuth2 authentication, Gmail client creation, API call execution, and error handling including auth-specific errors.

  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}` });
    }
  }