Founding Engineer - River Markets (YC P26)

About River Markets


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


As a founding engineer, you'll own core systems end to end, scoping features directly with the founders, designing the architecture, building backend and frontend, integrating with exchange APIs, and watching it run in production. Concretely, you'll:

·       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 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

* Base salary: $130,000–$175,000

* 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)

* Weekly restaurant dinner

* Food provided in office

* Fresh fruits + snacks from farmer markets and local shops

* Monthly team building activity

* 401(k) match fully vested

* We sponsor visas.

If this sounds like you, email me at antonin+fe[at]rivermarkets.com with a resume and something you built, no cover letter.

Interview Process

• Call with Antonin (CTO)
• Call with Oscar (CEO)
• 1 full day onsite