Founding Frontend Engineer

Mozart AI is music's first creation network, where the world goes from listening to music, to making it together, to being known for it. 1.5 billion people can play an instrument. We're building the products that turn them into artists, and turn their fans into the next ones. Our customers love us, and include everyone from grandmas using Vibe sessions to Grammy winners using the Studio. Music is having its first explosive moment of creation-driven consumption, and we aspire to shape that world to preserve the social and fun elements of music creation. We've raised over $7M from Balderton Capital (backers of Revolut) and several Silicon Valley legends like Kevin Hartz and Emery Wells, who have backed companies like Uber, Spotify, Reddit, and other massive consumer outcomes. We're a small, talent-dense team building the future of music from our London office. We operate with high ownership, high velocity, and low-ego collaboration — specialist generalists who work hard, learn fast, and ship relentlessly. We care deeply, move fast, and we're here to build something that outlives us. About the Role You'll own the whole frontend, especially the “backend of the frontend”: the architecture, state management, rendering and delivery systems behind a real-time, multiplayer music creation platform in the browser –> WebGL rendering, WASM audio, background jobs, collaborative state, social features. This is not a forms-and-dashboards product. You'll own our hybrid client- and server-side rendering architecture, keep state management clean and stable as the product grows, and build the code delivery pipeline that keeps web and mobile apps always in sync. You will also own and ship major product features end-to-end.This is an intense, in-person role, built for people who want massive impact and grow their careers faster than anywhere else. What You'll Do Own the rendering architecture. Hybrid server- and client-side rendering: data fetching, streaming, caching, code splitting, and the typed client-server boundary, so first paint is fast and every screen after it is instant.Own state management. Design the store architecture that holds a DAW session, live collaborators, in-flight generations, and social state in one predictable model, and choose the right update strategy for each operation.Own the delivery pipeline. CI/CD across web and mobile, feature flags and canary deploys –> users never run stale code against new APIsBuild the social layer. Media-heavy feeds, audio previews, profiles, publishing, and sharing, all smooth on every device size.Make production observable. Readable stack traces, useful errors, and monitoring that separates engineering signal from product analyticsPropose and lead the refactors that matter. Read the codebase, name the structural changes we need, and execute them with AI agents and the verification systems necessaryOwn frontend testing. The right test at every layer, from component stories to visual canvas checks, and the agentic end-to-end suite that verifies changes before merge.Ship product end-to-end. Real features with taste, instrumentation and fast iteration based on real user signal What We're Looking For React depth. You've architected and refactored large React codebases, including design systems, and understand React's internals and trade-offs beyond the APIState management. You've designed production store architectures (e.g., Zustand, Redux, Jotai), kept state predictable at scale, and chosen the right update strategy for each operationSSR in production. You have shipped and operated server-side rendering (TanStack Start, Next.js, or similar) and understand hydration, streaming, caching, and the framework trade-offs.Server-push architecture. You have built systems where the server reliably drives the client: background jobs finishing, collaborators acting, the database changing.Browser mastery. You know Chrome DevTools inside out, know the client-storage layers and their quirks, and build UIs that survive disconnects and resume in-flight work cleanly.Delivery infrastructure. Feature flags, canary deploys, CI/CD across platforms, and production-grade source maps.Refactor leadership. You read large codebases fast, identify the structural changes that matter, and lead migrations to completion, with AI agents as a daily tool.Testing judgment. You know a pure function, a store, a canvas renderer, and a user flow each deserve a different kind of test, you’ve implemented all at scale, including agentic E2E tests that run in CIAgent-native. You think in agentic workflows and build software that is operable and verifiable by agents, not just humansTruth seeker with taste. You talk to users, watch sessions, and read analytics, and you have strong opinions about motion, sound, and feel that you can defend on their merits.High ownership, low ego. You ship fast, own outcomes after release, and do whatever needs doing5+ years of professional experience Nice-To-Haves WebGL and canvas rendering (PixiJS, Three.js, shaders)CRDT and multiplayer-state experience (Yjs, presence, conflict resolution)Media-heavy or audio-streaming frontendsReact Native (Expo), Swift, or KotlinExperience shipping consumer products at an early-stage startupYou write, produce, or play music