Founding Engineer

I’m Antonin, co-founder and CTO of River Markets. Oscar (CEO) and I spent 3 years as quants in HFT shops and hedge funds. At nights and weekends we started trading predictions markets for fun and built a ton of trading infrastructure. We are now using this to build the prime brokerage and execution layer for prediction markets: the infrastructure professional and institutional traders need to access, trade, and manage risk across venues like Kalshi and Polymarket at scale. We value ownership, speed, and a high technical bar. We expect to do more with fewer people than a team our size has any right to. We are currently fully connected to 3 exchanges, have 6 proprietary execution algorithms and already have paying users. How we work: AI-native AI is the operating model. Every engineer treats AI as a core part of how they work: writing and reviewing code with AI tooling, automating their own workflows, and building internal systems that lean on LLMs where it makes sense. The bar isn't "can you use Claude": it's "what did you stop doing by hand this week?" We're looking for people who instinctively reach for automation, who multiply their output rather than just grinding harder, and who are excited that the leverage per engineer here is genuinely uncapped. We expect engineers to still understand every line they ship, AI-generated or not. What you’ll do Build and maintain the systems behind our order book, execution stack, exchange connectivity and portfolio management. Integrate with external exchanges and APIs where reliability and latency matter Write high-quality, testable code and raise the bar on everyone else's through review Improve infrastructure, reliability, and observability so we can move fast without breaking our systems Help define our engineering culture, standards, and early hiring, you're setting the defaults the next ten engineers inherit We expect the following: By week 1, you’ll have shipped in production a feature real traders use. By month 1, you’ll own a core system end to end, from architecture to monitoring. By month 3, you’ll have shared our engineering culture and be involved in our next hire What we’re looking for (we don’t expect you to check all the requirements!) Strong engineer (at least 2 YOE -full time- in SWE or similar role) who has designed, shipped, and operated production systems, not just written features, but kept them alive. Fluency in a modern language (Python, Go, Rust, or similar) and comfort across the stack: backend services, data pipelines, and user-facing components Solid grounding in system design, API design, and data modeling for low-latency, scalable applications AI-native by default, you already automate your own work and use AI tooling to ship more, faster Thrives in a small, high-ownership environment with minimal oversight Someone nice (we’ll spend a lot of time together) Bonus experience in fintech, trading, or market infrastructure; a genuine interest in prediction markets; Compensation Equity: 0.5%–1.5% On-site in the San Francisco Bay Area Platinum health insurance plan, we cover 100% of the premium Unlimited PTO (minimum of 3 weeks, we don’t want a burned out team) Food provided in office Dog Friendly office Fresh fruits + snacks from farmer markets and local shops Monthly team building activity 401(k) match fully vested If this sounds like you, email me at antonin+fe[at]rivermarkets.com with a resume and something you built, no cover letter. #J-18808-Ljbffr River Markets (YC P26)