If you build an API with Lambda integration using the API Gateway console, the console automatically requests to add the required permissions for standard Lambda integration. In contrast, when you set up a stage variable to call a Lambda function through your API, you must manually grant your API the permission to invoke the Lambda function. For a stage variable, the API Gateway console notifies you of a sample command that you must run outside the console to set up this permission.

If your Lambda function's resource policy doesn't include the permission for your API to invoke the function, your API returns a 500 status code with the message "Internal server error." To verify that the 500 status code is returned because the permission to invoke the function is missing, check your API Gateway execution logs for a message similar to "Invalid permissions on Lambda function."