Why does my Ruby Gmail API insertion script send emails missing headers?

I recently migrated my Gmail insertion script from Python to Ruby on Rails for consistency, but now the inserted emails come through empty – with no sender or subject.

I built a service that should insert an email with a proper subject, sender, recipient, and content into the inbox. However, no matter what I try, I consistently receive an email with blank header fields.

Below is the extracted code (with dummy values) that outlines the approach:

RUBYCopied
require 'google/apis/gmail_v1'
require 'googleauth'
require 'base64'
require 'mail'

class GmailServiceAccount
  SCOPES = ['https://www.googleapis.com/auth/gmail.insert']

  def initialize(service_account_file)
    @service_account_file = service_account_file
    @token_cache = {}
  end

  def get_service(user_email)
    return @token_cache[user_email] if @token_cache.key?(user_email)

    credentials = Google::Auth::ServiceAccountCredentials.make_creds(
      json_key_io: File.open(@service_account_file),
      scope: SCOPES
    ).dup
    credentials.sub = user_email

    service = Google::Apis::GmailV1::GmailService.new
    service.authorization = credentials
    @token_cache[user_email] = service
    service
  end

  def create_html_message(sender, to, subject, html_content, plain_text = nil)
    plain_text ||= "Please use an HTML capable email client to view this message."

    message = Mail.new do
      from    sender
      to      to
      subject subject
      text_part do
        body plain_text
      end
      html_part do
        content_type 'text/html; charset=UTF-8'
        body html_content
      end
    end

    # Debug: Print the message to ensure it's constructed correctly
    puts "Constructed Message:\n\\#{message}"

    # Ensure the message is properly encoded
    raw = Base64.urlsafe_encode64(message.to_s)
    { raw: raw }
  end

  def insert_html_email(user_email, sender, subject, html_template_path, template_variables = {})
    begin
      html_content = File.read(html_template_path, encoding: 'utf-8')

      template_variables.each do |key, value|
        html_content.gsub!("{{ \\#{key} }}", value.to_s)
      end

      service = get_service(user_email)
      message = create_html_message(sender, user_email, subject, html_content)

      # Debug: Print the raw message to ensure it's not empty
      puts "Encoded Raw Message:\n\\#{message[:raw]}"

      // Include labelIds in the message body
      message_body = {
        raw: message[:raw],
        labelIds: ['INBOX', 'UNREAD']
      }

      result = service.insert_user_message(user_email, message_body)

      { user: user_email, status: 'success', message_id: result.id }
    rescue => e
      { user: user_email, status: 'error', error: e.message }
    end
  end
end

# Command-line execution
if __FILE__ == $0
  service_account_file = 'path/to/your/service-account-key.json'
  user_email = 'recipient@example.com'
  sender = 'sender@example.com'
  subject = 'Test Email'
  html_template_path = 'email_template.html'
  template_variables = { 'name' => 'John Doe' }

  gmail_service = GmailServiceAccount.new(service_account_file)
  result = gmail_service.insert_html_email(user_email, sender, subject, html_template_path, template_variables)

  puts result
end

I have tried running the script standalone in Ruby and even tried suggestions from ChatGPT, but the email still comes through with no header information. How can I modify my approach so that the inserted emails correctly display the sender, subject, and other headers?