Sergej enhanced cross-service authentication in the automatisch/automatisch repository by refining Google OAuth flows for Gmail, Calendar, Drive, Forms, Sheets, Tasks, and YouTube. He enforced account selection and consent prompts to reliably enable refresh tokens, addressing edge-case authentication errors and improving user onboarding. His technical approach involved standardizing OAuth prompt URL encoding, replacing plus signs with spaces to ensure correct parameter interpretation and robust token exchanges. Working primarily with JavaScript, Sergej demonstrated skills in API integration, backend development, and OAuth2 protocol handling. The work delivered targeted, maintainable improvements to authentication reliability and token lifecycle management across multiple Google services.