Sebastian Fastner

For Terminaro, a product by Sebastian Software GmbH, I built a web-based appointment booking system for freelancers and companies as a Product Engineer, moving scheduling coordination out of email threads and into a shareable, multilingual booking link. Visitors can view available time slots without registration, book in their own time zone, and cancel appointments through a personal link. I owned the product from the public booking page to the administration area, with a strong focus on GDPR-oriented workflows, a cookie-free booking page, and reliable availability calculation.

React Router 7, TypeScript, and Convex form the technical foundation. The application integrates Google Calendar via OAuth as well as CalDAV-compatible calendars such as iCloud and Fastmail, generates iCal feeds, sends customizable email templates through customer-owned SMTP servers, and protects sensitive credentials through encryption. Better Auth with Pocket ID, Bunny.net Static Hosting with Edge Middleware, security headers, rate limits, health checks, and an extensive Vitest-/Playwright-oriented test base support operations and ongoing development.

For Palamedes, a product by Sebastian Software GmbH, I was responsible as Product Architect for a developer-first product for AI-powered localization designed to move translation work out of the release bottleneck. Instead of optimizing traditional translation tickets, manual exports, and downstream review loops, it focuses on a CLI-first workflow for PO- and MessageFormat-based projects. I brought product strategy, positioning, the open-core split, website, newsletter backend, and API/pipeline infrastructure into a cohesive product approach.

The product core separates an open, local i18n foundation from a commercial translation and control layer. Delta translations, terminology and protected terms, context enrichment, QA reports, targeted retries, and traceable PO metadata form the central product functionality. The website was built with React Router 7, Vanilla Extract, Lingui, and self-hosted Convex; newsletter flows, a preference center, double opt-in, unsubscribe mechanics, Relanto email delivery, Pocket ID admin authentication, and a Convex-based API key and pipeline structure support the product architecture.

Regrello Corp., an AI-focused supply chain management startup acquired by Salesforce in late 2025, needed full internationalization using large language models. I led the initiative and evaluated translation quality from English into six target languages: French, German, Chinese, Spanish, Korean, and Thai. The plugin system translated new or changed app content, workflow titles, and task descriptions within seconds using an event-driven workflow, first into participants' native languages and then into all available languages.

A plugin system for GraphQL resolvers and mutations in the Go backend formed the core of the solution. Translations were integrated transparently into the service APIs and cached in Redis. Temporal workflows with queues, workers, retry mechanisms, throttling, and rate limiting controlled processing. Because plain-text user content could not be stored in the queue for security reasons, I developed a performant encryption system for Temporal. I also built CLI and operations tools for job management, testing new workflows, and retrying failed translations. The backend was restructured and decoupled, making it easier to maintain and understand.

As Senior Frontend Architect, I continued the DWS Robo-Advisor WISE after its 2022 transfer to MorgenFund. I modernized it on Azure, ported the dashboard from React to React Native, and rolled it out to further European and Asian markets.

I introduced continuous deployment for faster, reliable releases and a shared web/mobile architecture. Jest and React Testing Library covered core UI behavior across varying regulatory and localization requirements.

I led the rebuild of 17 Witt Group online shops as a scalable multi-brand platform for international markets. Contentful enabled reusable, SEO-controlled pages and faster launches.

Next.js, React, and Styled Components formed the frontend; Apollo GraphQL unified the backend services. AWS, Kubernetes, and Vault supported secure continuous deployment, while GraphQL query tests protected releases.

As Senior Frontend Architect, I took WISE from DWS prototype to production. Its sales-pitch prototype included an investment-profile wizard; the production platform added risk profiling, DocuSign onboarding, and a dashboard across more than ten European and Asian portals.

After a Vue.js/MobX prototype, I ported the frontend to React and TypeScript, built an enterprise React component library, and owned the Webpack, Babel, and ESLint tooling. Spring Boot connected investment and CRM systems; Keycloak handled OpenID/OAuth. REST services and CI/CD ran on Microsoft Azure.