Software Engineer Interview Prep

About this loop

Jane Street is a quantitative trading firm whose engineering org is unusual in two ways. First, the firm hires almost entirely for raw aptitude rather than direct experience - the loop is calibrated to identify candidates who can learn fast, reason crisply under pressure, and grow into the firm's specific way of working, with the assumption that everything else (OCaml, the trading domain, the codebase) can be taught. Second, the engineering culture is OCaml-end-to-end at a depth no other firm matches; engineers ship trading systems, research tooling, infrastructure, and even desktop applications in OCaml, and the loop screens explicitly for the kind of structured thinking that fits well with a strongly-typed functional language. The interview process is famous for its mental-math round (compute things like 47 * 53 in your head under time pressure with the interviewer watching closely), probability puzzles (often game-theoretic, multi-step, with a twist that rewards being explicit about your assumptions), and trading game rounds at the onsite (mock trading exercises that probe how you reason about uncertainty and update beliefs in real time). Coding rounds are language-flexible (OCaml fluency is not required for entry; you can use Python, C++, or another language) but the bar on clean, well-structured code is high - Jane Street weights cleanliness, naming, and explicit reasoning over algorithmic tricks. There is essentially no system design round in the conventional FAANG sense - instead, design questions are applied and often connected to actual problems Jane Street systems solve. Behavioral signal exists but is lighter than at most companies; the firm cares more about whether you can think than whether you have a polished STAR story for every situation.

The interview loop

1. 1
   Recruiter screen

   30 minutes. Background, motivation alignment - Jane Street's training-heavy culture is a real fit question, especially for experienced engineers who'd be expected to learn OCaml from scratch. Recruiters screen for aptitude signals (math/puzzle background, prior functional programming exposure) and motivation to work in trading specifically.
2. 2
   Phone screen: mental math

   30-45 minutes. Mental arithmetic under time pressure with the interviewer watching - multi-digit multiplication, division, percentages, sometimes more involved estimation problems. Jane Street is famous for this round. Speed and accuracy both matter; 'I'm not a math person' is not a viable response. Practice for weeks before the loop.
3. 3
   Phone screen: probability and puzzles

   45-60 minutes. Probability puzzles, often game-theoretic or multi-step. Common shapes: expected-value problems, Bayesian updating, optimal stopping, game-theoretic reasoning where you and an opponent both play optimally. The interviewer cares more about how explicitly you state assumptions and how you reason than about whether you arrive at the textbook answer quickly.
4. 4
   Phone screen: coding

   60 minutes. One coding problem in your language of choice - typically Python or C++ for external candidates. Cleanliness and explicit structure matter more than algorithmic cleverness. Jane Street weights code that other engineers could read and modify; clever one-liners score worse than well-structured straightforward code.
5. 5
   Onsite: trading game

   60 minutes. A mock trading exercise - the interviewer plays the market, you make trades based on partial information, and the round probes how you update beliefs as new information arrives. There is no 'right answer' - the round is about reasoning under uncertainty in real time, comfort with being wrong and updating, and ability to explain your strategy as you go.
6. 6
   Onsite: coding deep dive

   60-90 minutes. A more involved coding exercise, sometimes building a small system end-to-end with the interviewer asking follow-up questions about design choices, edge cases, and how you'd extend the code. For external candidates with OCaml experience, this round may use OCaml; for everyone else, language is flexible.
7. 7
   Onsite: behavioral / fit

   45-60 minutes. Lighter than at most companies. Background, motivation for working at Jane Street specifically (the firm cares about this - engineers who treat Jane Street as 'just another job' don't fit), how you learn new things, examples of times you reasoned through hard problems. Generic 'tell me about a time you led a team' answers are less central here than at Amazon-style loops.

What Jane Street actually evaluates

• →Aptitude over experience - the firm trains for everything else, but it cannot train raw reasoning ability
• →Mental math fluency - speed and accuracy under time pressure, no calculator
• →Probability and game-theoretic reasoning - explicit assumptions, comfort with uncertainty, willingness to be wrong
• →Code that other engineers can read - cleanliness, naming, structure over clever one-liners
• →Comfort with functional programming concepts - immutability, types, composition (OCaml is taught on the job, but the patterns help)
• →Genuine motivation for trading specifically - the firm hires people who actually want to work in this domain, not just smart people who'd take any job

Topics tested

System design topics tested in this loop

Curated walkthroughs for the bounded designs that show up in Jane Street's system design rounds. Capacity estimation, architecture, deep-dives, and trade-offs.

Behavioral themes tested in this loop

Sample STAR answers, common prompts, pitfalls, and follow-up strategies for the behavioral themes that decide Jane Street's loop.