Senior Software Engineer (m/f/d)

🚀 Why join

• AI-powered infrastructure company with $10.3M in funding from top-tier investors.
• Exciting product: APIs, attribution models, MCP integration, AI-based recommendation engines, and assistants - not your typical CRUD app.
• Real traction: 30+ new customers every month, 10M+ end users monthly.
• Small team, so you'll genuinely shape how we build things.
• You'll touch everything from frontend widgets to backend infrastructure to data pipelines. The work doesn't get repetitive.
• We actively learn about new tech and AI tools and adopt where we can — we have to, because our customers do.
• Remote-first, but we get together regularly - company-wide and as an engineering team.
• The founding team consists of serial founders and experienced big-tech operators from Twilio, Wise, and Skype.

🌟 What We’re Building

We help SaaS companies turn their users and partners into a sales channel through referrals, sharing, and recommendations - 10M+ end users interact with our system every month.

Our widgets live inside customer products, which means your code runs in environments you don't control. On top of that: APIs that customer developers integrate against, and a portal where customers manage their campaigns and analytics.

Under the hood: attribution across complex user journeys, AI-based recommendation engines, multi-tenant infrastructure serving hundreds of customers, scaling millions of events. Deeper than you'd expect from a referral tool.

Our product is integrated by customer developers - and that world is changing fast. The people building against our APIs and widgets range from corporate engineering teams to solo founders using AI coding agents. Keeping up with how developers work today, and making sure our product fits into all of those workflows, is one of the more interesting challenges here.

Your Mission

• Stack: React · NestJS · Python · AWS
• You work directly with stakeholders - no walls, no ticket factory.
• Build new features and improve existing systems.
• Sometimes we ship fast to learn, sometimes we slow down to fix the foundation - you'll help decide where to lean.
• Real influence on architecture and product direction, not just coding tickets.
• Help us adopt the latest AI improvements to speed up how we build and ship.

About You

Required

• Good communication skills and a collaborative attitude
• Deep backend experience with NestJS or Python (or a comparable framework), and hands-on AWS knowledge from running services in production
• Fluent in TypeScript and JavaScript - it's your daily driver, not a language you've dabbled in
• You treat infrastructure as code - Terraform (or similar) is second nature, and clicking around consoles makes you uncomfortable
• You have strong opinions about observability, backed by real work with OpenTelemetry, distributed tracing, structured logging, and dashboards and alerting that actually help during incidents
• You know when "good enough" is right - and when to invest in quality
• You are a strong problem solver - you can break down complex, ambiguous problems and ship pragmatic solutions, not just debug what's already in front of you
• You contribute ideas and challenge assumptions, not just execute
• You're geniously curious about the product itself - you want to understand what we're building and why, not just pick up tickets and ship code.
• You live at the bleeding edge of agentic coding. Claude Code, Cursor, multi-agent setups, MCP servers, whatever ships next, you're using it and pushing it to its limits. We move fast on adopting what works
• That said, you know how code and systems actually work. You can debug, reason about architecture, and carry your weight in a codebase without an AI assistant holding your hand.
• Strong work ethic - you take ownership, ship what you commit to, and don't need to be chased

Nice-to-haves

• Familiarity with data warehousing / analytics (e.g. Snowflake, ClickHouse, or similar)
• Experience building systems at real scale: high traffic, large user base, high availability,
• Plus solid experience with at least one frontend framework — React, Next.js, or similar.

Benefits

In the end, it’s all about the team and our culture. Understand why we love our job and see what it’s like to work at Cello in our People Playbook. We have some compelling perks and benefits here.