Founding Frontend Engineer

Mozart AI; Greater London; On-site; GBP 90,000 - 130,000; Full time

Job summary

Mozart AI in London seeks a senior frontend engineer to own the rendering architecture and state management across the client and server. You will drive the delivery pipeline, implement robust observability, and lead important refactors using AI-assisted verification.

This in-person role offers rapid career growth in a fast-paced startup environment. You will shape end-to-end features with careful testing, measurable user impact, and a focus on delivery quality.

Qualifications

• 5+ years of professional software development experience.
• Experience with large React codebases and design systems.
• Proven ability to lead migrations and refactors.

Responsibilities

• Own our client- and server-side rendering architecture and data flow.
• Own the state management architecture across the app.
• Own the delivery pipeline with CI/CD and canary deploys.
• Make production observable with strong error handling and monitoring.
• Lead refactors that matter using AI agents and verification systems.
• Own frontend testing across layers, including E2E testing in CI.
• Ship end-to-end features with fast iteration based on user signals.

Skills

React depth

State management

SSR in production

Server-push architecture

Delivery infrastructure

Refactor leadership

Testing judgment

Agent-native

Truth seeker

Product taste

High ownership, low ego

5+ years experience

Tools

TanStack Start

Next.js

Redux

Zustand

Jotai

CI/CD

Source maps

Job description

We are building a world where everyone can feel the joy of creating music, together, in real time. More than 300k users love us, 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 while preserving the social and fun elements of music creation. We've raised over $7m from Balderton Capital (backers of Revolut) and Silicon Valley legends like Kevin Hartz (ex-Eventbrite CEO), Emery Wells (ex Frame.io CEO), backers of Uber, Spotify, Reddit, and other massive consumer outcomes.

It takes one hell of a team to bring the joy of music creation to everyone in the world, and we’re building it from our London office. We operate with high ownership, high velocity, and low-ego. We are all specialist generalists who work hard, learn fast, and ship relentlessly. We care deeply, and we're here to build something that outlives us.

About the Role

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 our client- and server-side rendering architecture. Data fetching, streaming, caching, chunking, and the typed client–server boundary
• Own the state management architecture.
• Own the delivery pipeline. CI/CD across web and mobile, feature flags and canary deploys –> users never run stale code against new APIs
• Make production observable. Readable stack traces, useful errors, and monitoring that separates engineering signal from product analytics
• Propose 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 necessary
• Own frontend testing. The right kind of test at every layer –> from component stories to visual canvas checks –> and lead our agentic end-to-end testing for the frontend
• 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 API
• State 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 operation
• SSR in production. You've shipped and operated SSR (e.g., TanStack Start, Next.js) and understand hydration, streaming, caching, and framework trade-offs
• Server-push architecture. You’ve build systems where the server reliably drives the client → background jobs finishing, collaborators acting, the database changing
• 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 CI
• Agent-native. You think in agentic workflows and build software that is operable and verifiable by agents, not just humans
• Truth seeker. You talk to users, watch sessions, and read analytics –> chasing what users need, not what you assume
• Product taste. You have strong opinions about motion, sound, and feel, and can defend design decisions on their merits
• High ownership, low ego. You ship fast, own outcomes after release, and do whatever needs doing
• 5+ years of professional experience

Nice-To-Haves

• WebGL and canvas rendering (e.g., PixiJS, Three.js, shaders)
• React Native (Expo), Swift, or Kotlin
• Experience shipping consumer products –> you know how building for consumers differs from B2B
• CRDT and multiplayer-state experience (e.g., Yjs, presence, conflict resolution)
• Media-heavy or audio-streaming frontends
• You make music – listening, exploring, producing. A big plus.
• Experience working at an early stage startup or fast-paced environment. A big plus as well.