Trying Formspree, my AJAX form wasn’t reaching the success function. Using Chrome Developer tools, I just saw:
Searching Formspree’s Help Site for “401” turned up nada. So I switched to Firefox Developer tools, Network tab, and inspected the Response.
Aha! But I actually do want reCAPTCHA spam protection. Luckily I stumbled across this ZEIT guide that says: “To use AJAX on Formspree you must either disable reCAPTCHA or provide your own reCAPTCHA key.”
Much better! I do have my own reCAPTCHA key, and I was able to implement it per Formspree instructions.
This one wasn’t a quick Google to fix so hopefully this helps someone else running into this issue.