Keito said:
Yep, pretty much. Select file, get it's MD5, request JSON url (https://e621.net/post/show.json?md5=%filename%) using name without extension, if result is empty, request it again using MD5 i've got before, if it's empty again just skip the file. It's not really spamming since it takes 2 seconds or so for each file, but technically it is. :)
I doubt it's hard for server to catch up with this tool working, i guess it's like 3-4 people browsing e621 at the same time...

Oh, well, if it takes two seconds, then it doesn't matter too much. Question is why it takes so long though, jou only take the filename, right? even calculating the hash anew takes like 4 ms per image here.
2 secs per file would mean it would take multiple days for my (kinda massive, granted, been collecting for years) local library

with multiple tags I mean that something like that used to work before, now a year past:
show.json?md5=%hash%,%hash2%,%hash3%
(I don't remember how it was exactly, I just know I could check a hundred or so hashes with one server call)

Yes, it's actually mentioned in the OP post. I'll just quote it :)

Ah, my bad :) didn't read the readme, sorry

For your TODO, I fear they're not really possible or rather, make not much sense. All the date e621 has is the upload date and, some times, if it is tagged, a tag for the year. I've seen years old pics uploaded and given a recent date. As for alternative sources, weasyl API has no way to get by hash. FN API is not out yet. FA has none. So you'd be out of luck there. Best bet would be to call a reverse image search, but TinEye wants $200 for even their lowest tier (500 searches) and google has no access to boot and likes to use captchas for sources it deems questionable. InadescentAPI, another one, wants 75 pounds for their lowest tier and claims to use, beside others, google, so my scam alert is tingling. I don't think you can really do that. At least not if the file comes from e621 and hasn't the ID of, say, FA in it ;)

well thanks for the answers :)

and thanks savageorange
personally I'm a fan of APNG.. full color instead of just the 156/frame of gifs, true transparency instead of "transparent or not" and its files are smaller than gifs. Too bad the support for it is abysmal