Lead Full-Stack Engineer

We're looking for an ambitious, hands-on Lead Full-Stack Developer who builds with AI coding agents every day and knows how to make that fast and reliable.

Our platform builds and quality-checks reinforcement-learning tasks for frontier AI labs: authoring, review, automated QA in cloud sandboxes, and customer acceptance. It's built quickly by a small team working heavily with AI agents. This is an opportunity to own how we build software: ship features, spot what to improve, deliver our roadmap, and establish engineering practices from scratch. You can then grow into leading the engineering team.

What You'll Do

• Ship, constantly: Take feature requests from our researchers, reviewers and operators, and deliver them to production within days, across a FastAPI backend and a Next.js frontend.
• Vibe code with discipline: Build with AI coding agents as your main tool, and set the standard for doing it well: clear context, tight scope, tests that mean something, and diffs that get read.
• Establish engineering best practices from 0 to 1: Introduce code review, linting, testing and CI gates that make the team safer without slowing it down.
• Find what to improve: Spot weak points in the code and product, such as tangled modules, missing tests or slow pages, and fix them incrementally while features keep shipping.
• Deliver the plan: Turn our existing roadmap into sequenced, shippable work, and push back with evidence when something should change.
• Guard correctness where it matters: Make sure the rules behind reviews, acceptance and payouts are enforced in code and covered by tests.
• Shape the future team: Review others' work, mentor, share knowledge, and potentially lead the engineering team as it grows.

What We're Looking For

• Proven engineering experience: A track record of shipping production software as a hands-on senior or lead engineer.
• AI-native workflow: You build with Claude Code, Cursor, Codex or similar every day, and you have opinions on how to keep agent-written code correct.
• Full-stack generalist: Strong Python (FastAPI, SQLAlchemy, asyncio), TypeScript and React (Next.js), and PostgreSQL. Comfortable with any part of the stack on any given day.
• Builder mentality: You enjoy creating systems and processes, solving ambiguous problems, and taking ideas all the way to production.
• Strong ownership and autonomy: You take responsibility for the things nobody else sees, because you see them. You drive them to measurable results without constant supervision.
• Pragmatic quality: You know the difference between practices that protect a team and ceremony that slows it down.
• Ambition and drive: A highly motivated, resourceful and results-oriented person who wants to grow quickly, stays hands-on, and is ready to lead a team forward.

We value demonstrated achievements, intellectual curiosity and the ability to execute over years of experience or impressive job titles.

Nice to Have

• Experience establishing engineering practices in a startup or high-growth company.
• Experience combining hands-on development with tech-lead or team-lead responsibilities.
• Experience improving a codebase written largely by AI agents.
• Experience building agent tooling: project context files, custom review prompts, MCP servers.
• Experience with marketplaces, payouts or review workflows, where correctness carries money.
• Exposure to LLM evaluation, reinforcement learning or AI research tooling.

Why Join ARIMLABS?

• Build something from scratch: Take ownership of how we build software and set the standards the team grows with.
• Exceptional growth opportunities: Join a fast-scaling AI research company at an early stage, grow alongside the business, and potentially lead your own team.
• High autonomy: Freedom to experiment, make decisions and ship without unnecessary bureaucracy.
• Work with exceptional people: Collaborate with highly skilled researchers, engineers and domain experts working on challenging problems at the frontier of artificial intelligence.
• Exposure to frontier AI: Work with frontier models every day, both in your own workflow and in the product you build.
• Entrepreneurial environment: Work in a company that values initiative, speed, ownership and ambitious thinking.
• Career flexibility: Shape your long-term career toward engineering leadership or deep technical ownership as the organization expands.
   

Compensation & Working Conditions

• Engagement: B2B contract.
• Time off: 24 days of paid time off annually.
• Autonomy: Significant freedom to organize your work and establish effective processes.