Senior Frontend Engineer

ABOUT LANGFUSE Open Source LLM Engineering Platform that helps teams build useful AI applications via tracing, evaluation, and prompt management (mission https://tracking.us.nylas.com/l/6d586a21a6fc4e1a8aacc7eb75882b72/0/82383757e54352130f65066e1b2fc4708aacab7897561bcb8000fe4c8a9c6a21?cache_buster=1761124921, product https://tracking.us.nylas.com/l/6d586a21a6fc4e1a8aacc7eb75882b72/1/b9fba3a93b6ffcc0f99ecda62767a17cc437fe8fe0b16181d1c43c1391212e3d?cache_buster=1761124921). We are now part of ClickHouse. We're building the "Datadog" of this category; model capabilities continue to improve, but building useful applications is really hard, both in startups and enterprises. Largest open source solution in this category: trusted by 19 of the Fortune 50, >2k customers, >26M monthly SDK downloads, >6M Docker pulls. We joined ClickHouse in January 2026 because LLM observability is fundamentally a data problem and Langfuse already ran on ClickHouse. Together we can move faster on product while staying true to open source and self-hosting, and join forces on GTM and sales to accelerate revenue. Previously backed by Y Combinator, Lightspeed, and General Catalyst. We're a small, engineering-heavy, and experienced team in Berlin and San Francisco. We are also hiring for engineering in EU timezones and expect one week per month in our Berlin office (how we work https://langfuse.com/handbook/how-we-work/principles). In Short: We're looking for a senior frontend engineer to build the UI for the most widely adopted open source LLM engineering platform — where the core challenge is rendering massive, data-intensive views fast and making complex workflows feel simple. WHY FRONTEND ENGINEERING AT LANGFUSE Your work will have an outsized impact. Thousands of users sign into Langfuse daily to inspect traces, update prompts, and run experiments. They obsess over what they see in Langfuse to improve the systems they just shipped to production. Megabytes of JSON data should be available at their fingertips, searchable and presented in a readable way. Your job is to make that experience fast and intuitive. When you ship an improvement to a view, eager users often find your PR and give feedback before you've even released it. They'll tweet about it before you write a changelog post. Everything you build is open source (MIT-licensed) and immediately visible. You will also have direct exposure to how cutting-edge LLM applications are built. The users you serve are some of the best and most ambitious software engineers in the world, and they leverage Langfuse to understand and improve their applications. Deeply understanding their workflows will help you design the best possible UI — and along the way makes you an expert on LLM engineering yourself. WHAT YOU'LL DO - Build performant rendering for large LLM data — Langfuse displays full LLM inputs and outputs, which can be tens of thousands of tokens per observation. You'll own the performance of our trace views, finding bottlenecks in rendering, data loading, and reducing rerenders so that even the largest agent traces feel local-level snappy. You love diving deep into profiling re-renders? This is a match! - Own our React setup - you can shape and own how we build components, structure our code and write tests. Make our React setup thrive for agents and be up-to-date with new features (for example the React Compiler). - Build complex interactive features — recent examples: agent graph visualization https://langfuse.com/changelog/2025-11-05-langfuse-for-agents that infers execution flow from observation timings, a redesigned trace view https://langfuse.com/changelog/2025-03-19-new-trace-view with tree/timeline toggle and search, inline comments https://langfuse.com/changelog/2026-01-07-inline-comments-on-trace-io anchored to specific text selections (like Google Docs), and dashboard widgets for tool usage analytics. These are the kinds of features you'll own end-to-end. - Own UI/UX across the product — you'll work closely with a designer to improve the experience across all of Langfuse. This means thinking about visual consistency and begin to build out a design system. You'll think deeply about how to present complex data in our product. - Ship features end-to-end — while your focus is the frontend, you'll connect to our tRPC APIs and sometimes extend them. We don't have hard boundaries — if a feature needs a backend change, you'll make it. WHAT WE'RE LOOKING FOR - Senior frontend engineer who cares deeply about performance — you love to profile React renders, debugged layout thrashing, and know when to virtualize, memoize, or restructure - Strong in React, TypeScript, and modern CSS — you write clean, maintainable component code - Like to collaborate with a designer to turn complex requirements into intuitive interfaces - Someone who can PM and project-manage themselves — you have strong opinions about what to build and how to ship