Senior Software Engineer - Full Stack

This Senior Software Engineer - Full Stack role joins a small, remote-first European team at Cello building referral and recommendation systems used by 10M+ end users per month; you’ll work across widgets, APIs, and backend infrastructure to influence product direction, architecture, and how we adopt AI to ship faster. This role matters because your decisions will shape a multi-tenant platform that runs inside customer products and integrates with a wide variety of developer workflows.

What you'll do

• Collaborate directly with stakeholders to define and deliver features without a ticket factory.
• Build new features and improve existing systems across frontend, backend, and data pipelines.
• Decide when to ship fast for learning and when to slow down to strengthen the foundation.
• Influence architecture and product direction beyond executing tickets.
• Help adopt and integrate AI-driven tooling and agentic coding approaches into our engineering workflow.
• Work on frontend widgets that run inside customer products, APIs for integrators, and a customer portal for campaign management.
• Design for attribution across complex user journeys and scale multi-tenant infrastructure handling millions of events.
• Ensure our product fits diverse developer workflows from corporate teams to solo founders using AI agents.

What Cello is looking for

• Strong experience with React (or similar) for frontend work and NestJS/Python (or similar) for backend services.
• Proven AWS experience building and operating cloud infrastructure.
• Familiarity with mobile development (React Native, Flutter, etc.).
• Experience with data warehousing / analytics (e.g., Snowflake, ClickHouse, or similar).
• Excellent communication skills and a collaborative attitude.
• Strong problem-solving skills: break down ambiguous problems and ship pragmatic solutions.
• Ability to judge trade-offs between shipping quickly and investing in quality.
• Ownership mindset: reliable, strong work ethic, and proactive contributor of ideas.
• Deep curiosity about product and developer workflows, and hands-on experience pushing AI coding tools to their limits while still understanding core system behavior.