This is an overview of what Citadel Securities looks for in experienced software engineers. We hire engineers who can take an incomplete problem and make progress on it, design systems that hold up as requirements shift, write code other people can maintain and weigh tradeoffs out loud as the picture changes.
Our interview process is built to see all of that: how candidates design a system, how they implement it, and how they revisit their own work as new information comes in.
What We Look For
The core skills we hire for in experienced engineers are system design under ambiguity, code quality, judgment about edge cases and failure modes and clear communication while building.
In practice, that means engineers who keep moving when the spec is half-written, design systems that can evolve without being rewritten, write code that’s clear and testable and talk through their decisions as they go.
Engineering Is Evaluated Over Time
Strong engineering isn’t one good decision. It’s a long string of them, made as the conditions around you keep moving. We’re less interested in whether someone’s first instinct is right and more interested in what they do with it once reality starts pushing back.
Working with Incomplete Information
Real systems are almost never fully specified. Requirements arrive incomplete, constraints aren’t explicit, and the gnarliest edge cases tend to show up late. The engineers who do well here ask sharp questions early, call out what’s missing and then make a defensible assumption and keep going. Waiting for perfect clarity usually isn’t an option. The job is to reduce uncertainty, not eliminate it.
Structuring Systems for Change
A system that works on day one isn’t the bar. Requirements and scale change, and the constraints you designed against quietly shift underneath you. We look for engineers who separate concerns cleanly, surface their assumptions instead of burying them and leave room to extend a system later without tearing it down.
Translating Ideas into Code
We value code that’s readable, testable and faithful to the original design over code that’s clever.
A design only really gets tested when you sit down to implement it. Details you waved past suddenly matter, the interfaces you sketched get exercised for real and the edge cases you knew about (and the ones you didn’t) start showing up. The code we want to read is readable by someone who isn’t the author, lines up with the intent and can be tested and reasoned about without spelunking. Clever is cheap; clear is the harder thing.
Responding When Things Break
Systems don’t behave the way you expected them to. Bugs surface, assumptions you didn’t realize you’d made turn out to be wrong, and performance quietly degrades. What we watch for in those moments is whether the engineer isolates the problem methodically instead of guessing, if they’re willing to adjust the approach (sometimes more than once) and if the work keeps moving while they do it. How someone reacts when the plan breaks tends to tell us more than the plan itself.
Thinking Beyond the Happy Path
A system that works in ideal conditions is, in our mind, half-built. We want engineers who instinctively reach for the edge cases, the failure modes, and the question of how the thing behaves a month or a year in, including under inputs that are malformed, incomplete, or actively adversarial.
Communicating While Building
Engineering isn’t a solo activity. We want engineers who narrate their reasoning as they go, name the tradeoffs they’re making out loud and visibly update when someone brings them new information. That kind of communication is what makes a system easier to build, review and live with later.
Prioritizing Under Constraint
Time runs out, and so does headcount. You can’t build everything in the first pass, and pretending you can is its own kind of failure. We want engineers who can call what has to be solved now, what can wait and what level of “done” is actually appropriate for the piece in front of them. Prioritization isn’t a project-management concern bolted onto engineering. It’s part of the design.
What This Adds Up To
We’re not trying to find out whether someone can produce a perfect answer in a vacuum. We’re trying to see how they approach an unclear problem, how they build a system one decision at a time, how they adjust when the constraints around them move and whether what they leave behind is something the next engineer can build on. The outcome counts. The path to it counts more.
Why This Matters
The systems we build at Citadel Securities run under real constraints. They change constantly, and they live in environments where being correct and being fast both matter, often at the same time. Technical skill is what gets an engineer through the door, but it’s rarely the whole job. The ones who thrive tend to grasp what the system is actually for, which tradeoffs are worth making and how the demands on it will grow over time. Tooling will keep changing; what stays valuable is that sense of which problems are worth solving and why. How an engineer works through a problem in an interview turns out to be a pretty good preview of how they’ll work in that environment, and that’s what we’re really trying to figure out.
