After exploring some technical precedents, I began reverse-engineering Jonathan Yiv's command-line version of Mastermind (starting point #2 above) to figure out how it worked and to simplify it so that when it came time to test it, I wouldn't have to play the entire game to see if it was working.

A lot of the functionality ended up being incompatible with my design philosophy of using this in an SMS setting. Having a computer make guesses is fine for iterating within a command line interface but would likely annoy users on their phones. Also, the colored output in the command line is great, but without a way to change the font color of a text message or a consistent spacing compatible with every phone, I had to omit this feature in the SMS version. Still, I kept the command line interface output in the code to help with testing things locally.