It is a real codebase, not a puzzle
You are given a small working project with problems already in it, and asked to take it somewhere better. There is no whiteboard, no algorithm trivia, and nothing that depends on remembering a trick you last used at university.
We built it this way because it is closer to the work. Almost nobody starts a client engagement on a blank page. You inherit something, you read it, you form a view about what is wrong, and you improve it without breaking what already works.
Some of it is fixing, and some of it is building
Part of the task is finding and repairing what is broken. The rest is finishing something that was never written: a function with a clear contract and no implementation behind it.
The second part is the one people underestimate. Repairing shows you can read code. Building the missing piece shows you can take responsibility for a decision nobody has made for you, which is the whole job on an engagement.
What we are reading for
Whether the thing works, obviously. But also whether you noticed the problems that are quiet rather than loud. A failing test announces itself. The faults that cost a client real money usually do not, and finding those is most of what separates a strong submission from a passable one.
We also read for restraint. Rewriting a working module to your own taste is not an improvement, and on a client codebase it is a liability. The strongest submissions change what needs changing and leave the rest alone.
You are not scored on style, and there is no house formatting we expect you to guess.
How to prepare, and what not to bother with
Work in the language you are strongest in. There is no advantage in choosing something you think sounds more impressive; there is a real disadvantage in being slower and less certain in it.
Do not revise algorithms. Do re-read something you wrote a year ago and ask what you would change, because that is the muscle the assessment uses.
Make sure your environment is boring and reliable before you start. A stable connection and an hour where nobody interrupts you is worth more preparation than anything technical.
If something goes wrong during it
Tell us while it is happening, through the support option inside the assessment itself. A dropped connection or a broken page is our problem to fix, not evidence about you, and we would far rather pause the clock than read a submission written under conditions we caused.