Bug overview description.
The set API has been giving me trouble, queries that worked for months are suddenly giving errors.

What part(s) of the site page(s) are affected?

These broke in the past few days and I can't find a workaround:

• /set/add_post, probably /set/remove_post as well
• test URL: https://e621.net/set/add_post.json?post_id=65067&set_id=13125
• returns error 500: "Unexpected error (3bb9aa348a70e535b067c8266bd8c137)"
• has this fun error message when you attempt to access the page itself

These broke first (not sure when), but a workaround exists:

• /set/create and /set/update
• test URL: https://e621.net/set/create.json?set[name]=test6514&set[shortname]=test6514&set[public]=false&set[description]=testing
• Returns error 500 regardless of length. These apparently still work when parameters are sent in FormData rather than the URL.

Can you reproduce the bug every time?
Yes

/set/create and /set/update are POST only endpoints and seem to expect the parameters to be sent using POST data instead of in the URL.

/set/add_post and /set/remove_post are also POST only endpoints and are trying to return an error to you, but they can't because there is no render template assigned for them in the manner that you've tried to call them in.

As none of this code has changed in months and months, I don't know what to tell you about them seeming to work before this point.

Oof, I feel dumb. I was sending undefined in the post body when it should have been null. That worked for a long time but now some (not all) pages reject the requests.

Sorry for the false bug report.