Found out that vim was adding a newline to my session token when I started getting invalid header network errors. Trimming the session token before sending it out might alleviate this problem for others.

If you have previously submitted the correct answer for a particular question (year+day+part), the website appears to respond to new submissions with something along the lines of "You don't seem to be solving the right level. Did you already complete it?" regardless of if the answer is correct. aoc_driver interprets this as response as the server saying the answer is correct.

Idea: Cache the correct answer separately from submission responses (e.g. as an Option<String> for each part).
Then when submitting:

• If this cached value is Some, compare to it instead of submitting to the website.
• If the cached value is None, submit to the website.
  • If the website responds "correct", set the cached answer to Some in addition to handling the submission as it currently does.
  • If the server responds "Did you already complete it?", maybe parse the question page for Your puzzle answer was <code>::before [answer]</code> and save that/those into the cache, then handle same as Some case.
  • If the server responds "incorrect", "rate limit", etc. handle the response same as it currently does.

some external tools, like cargo_aoc will pull input into input/{year_num}/day{day_num}.txt instead of input/{year_num}/{day_num}.txt. Having the aoc_complete macro check for both before deciding there isn't any input might be beneficial.