Senior Frontend Engineer

About the RoleThousands of engineers sign in daily to inspect traces, update prompts, and run experiments on an open-source LLM engineering platform — think Datadog, but for AI observability. Full LLM inputs and outputs can span tens of thousands of tokens. Your job: make that experience fast and intuitive. You love profiling React renders. You know when to virtualize, memoize, or restructure state. The data is massive — the interface should never feel like it is. This is a high-ownership, senior-level role on a small, remote-first European engineering team with meaningful cross-functional collaboration. What You'll Do• Build performant trace views — Own rendering of large LLM data: identify bottlenecks, optimize data loading, reduce re-renders so even the largest agent traces feel snappy. • Own the React architecture — Shape how components are built, code is structured, and tests are written. Keep the codebase current with React Compiler and emerging patterns. • Drive data visualization — Design and implement virtualized lists, grids, incremental loading, and data-heavy dashboards (cost, latency, evaluation results). • Raise the quality bar — Champion automated testing across unit, integration, and end-to-end layers; conduct thorough code reviews and mentor peers. • Collaborate cross-functionally — Work closely with product and backend engineers to ship features end-to-end across the full LLM development lifecycle. What We're Looking ForRequired: • 5+ years of frontend development experience shipping production web applications • Right to work in the EU without visa sponsorship; ability to be on-site in Berlin approximately one week per month • Strong proficiency in JavaScript and TypeScript • Deep experience with React (or a comparable modern framework) and component-based architecture • Proven track record optimizing data-intensive UIs — virtualization, memoization, code-splitting, and profiling tools are second nature to you • Experience building da