I am trying to code an application around the API. Retrieving posts in JSON works fine, however, since there's no actual link to the "thumbnail" in the JSON, I did guess to obtain the latter, I would have to replace "images" with "thumbnails" and append "thumbnail_" before the hash and convert the extension to "jpeg".

So far this seems correct but it doesn't work all the time. Sometimes the thumbnail link throws 403.

I tried in the browser to understand what I am missing (also searched the forum, but nothing seems to match). The behavior I have in the browser is the following:
- say I load this in the browser simg3.gelbooru.com/thumbn...4a634357dff2b7da721df.jpg I get 403
- if I access the first result page from search, I will eventually see the thumbnail rendered fine there
- reloading the url after the step above, now works

Can anyone hint me around what I am exactly missing to have this work?

Thanks.

If the thumb gives you a 403 it means you didn't send a valid referrer, it has to come from gelbooru or it will be blocked by hotlinking protection.

EDIT: Removed hotlinking protection from thumbs, try now.

Sorry for late response. Thanks for the hint, I actually worked around it. Using server side language to request for the file, encrypting in base64 before displaying in app. You may wish to preserve hotlinking on thumbs just in case.