#This is a bad idea.

I like the sentiment of challenges being completely objective.  

As it stands, **there are lots of challenges that would become trivial/impossible if we disallowed non-observable requirements**.

1. *No built-ins*.  I think this is obvious how this makes things trivial in some languages.  If a builtin is shorter than any possible algorithm, then submissions in that language have no choice but to submit that built-in (to stay competitive).
2. *No hard-coding*.  While this is a problematic statement in certain scenarios, **we need it to keep some fastest-code challenges non-trivial**.  Primes become a lot easier to generate if you have the first 10K of them in your source code.  Some sequences become exponentially harder to calculate the higher you go, which means that it *is* feasible to hard-code all answers.

 The following challenges (as well as others) would all become off-topic:

 - http://codegolf.stackexchange.com/questions/51266/extending-oeis-counting-diamond-tilings
 - http://codegolf.stackexchange.com/questions/84592/compute-the-maximum-number-of-runs-possible-for-as-large-a-string-as-possible
 - http://codegolf.stackexchange.com/questions/53018/sum-of-smallest-prime-factors

3. *No retaining state* and *No reflection/modifying other submissions*.  Both of these are *essential* for running all of my KoTHs.  I don't want people storing static variables to remember data from game to game, and I don't want reflection to cheat at the game.  **You might as well kill nearly all of my KoTHs** (the only exceptions are my code-bot challenges).

4. *Submissions must be competitive*.  There is a requirement we place on every challenge by default **and is non-observable**.

5. *Asymptotic complexity restrictions*.  This includes *all* questions from [tag:restricted-complexity] and other challenges where we put a complexity requirement to prevent brute-forcing.

Unless we are able to define all of the above non-observable requirements for every language we use on this site, **we're going to kill *so* many challenges here.**