Not a lawyer or in US.

If you reverse engineered rather than copied from Docs or header files I don't believe it should be copyright infringement. Note that the Google Oracle case currently being appealed seems wrong to me as I think copyright on complicated API's are reasonable (many disagree with me) although fair use and antitrust arguments should in many cases allow reimplementation. Anyway in statutory terms I don't see a problem. However...

It is likely that you agreed to Snapchat's terms of service at some point and it is also likely (I haven't read them) that they contain clauses forbidding permission to reverse engineer and/or access the service without using official clients. This opens up possibilities for breach of contract legal action and more worryingly computer crimes prosecution for unauthorized access. This conversion of minor contract breaches into computer hacking crimes is horrible law but it seems to be current reality. Be careful.

I don't know if you are in a better or worse position if you use the software without agreeing to the terms and conditions.