The obvious place to draw the line is the point where the code you wrote isn't equally available to you and all the other competitors.

This, coupled with a requirement about being clear what pieces are new.

And obviated by the judges having no easy way to learn and isolate which "part" of the app they're testing, and how such compartmentalization goes strongly against human psychology.

I don't pretend it'll be "perfect", but it should let decisions fall (noisily) around what we think of as "fair" (since we're all running that assessment through the same hardware) which is really what we're looking for.

<rules-lawyering-participant>So I get my buddy/silent-partner to develop my cool and unique API beforehand and sneak it unannounced onto a relatively anonymous public Github account before the comp starts, but close enough to the start date that no-one else will have even heard of it yet.

Step 2: ???

Step 3: Profit!

It is fun to consider extreme cases, but I am sure this has never come up before.