Pretty obvious condition would be that it's pre-existing code you wrote. I'm sure that's what the OP intended.

So, someone who is a prolific FOSS contributor is screwed?

If source you previously wrote is available on Github for anyone, would that pass? What if you built an incredibly successful plugin, like lightbox?

Where do you draw the line? Let's say I'm a core developer on Rails, am I not allowed to use Rails then? What happens if I contributed 1 line of code to Rails, does that negate it then?

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.

I think there's a slight distinction between using Rails and entering into a hackathon with the intention to port a recently developed app into a 'mobile' version?

And you didn't write the Ruby(|node|php|python|golang) interpreter either, or gcc/clang that compiled the interpreter, or the kernel that gcc relies on. (or the cpu microcode - it's turtles all the way down).

Personally, I think the Upshot approach was a sleazy hack of the competition rules. On the other hand, with a $1 million prize, you'd have to be an idiot not to expect rules-lawyering to be at least as important in winning as a good idea…