Went through the Jane Street SWE loop in Q1 2026 for a senior IC role, NYC. Wanted to document the system design round since I couldn't find much that was specific.
First thing: Jane Street does NOT do the standard 45-minute "design Twitter" exercise. Their system design is more of an open-ended technical conversation that lasts about 60-75 minutes. The interviewer presented a real-ish problem, something like building a pricing engine that needs to handle bursts of market data at low latency. They want to see how you think about tradeoffs, not whether you can draw a canonical architecture diagram.
What they actually care about: Can you reason about latency vs. throughput tradeoffs concretely. Not just "oh we'd use a queue" but why, at what point, and what breaks. Data model choices under load. I got asked to walk through what happens to my schema when volume 10x-es. Failure modes. Where does the system fall over first? How would you detect it?
I came from infra/platform so I kept pulling toward kubernetes orchestration and distributed systems patterns. The interviewer pushed back gently whenever I jumped to tooling before reasoning from first principles. That was the big signal: they want the reasoning chain, not the tool name.
No whiteboard in the remote version. I was asked to share my screen and use whatever I wanted to sketch. I just used Excalidraw.
Round lasted 75 minutes including a Q+A where they let me ask about their actual infrastructure. That part was genuinely interesting. Their internal systems are written in OCaml more than most people realize.
I think I did well on this round but got dinged somewhere else in the loop. Happy to answer specifics if you're prepping for this level.