If you’re getting HTTP 503 errors when using something that accesses the API it’s because WaniKani isn’t able to handle the request before timing out.
Attempting to request a large amount of data from the API will timeout after ~30s and return a 503 error. This makes the https://www.wanikani.com/api/v1.2/user/<key>/vocabulary/ link useless and intermittently breaks anything that uses it.
This is still a problem.
I just responded to an email reporting this exact issue. I'll just copy and paste my response here...
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It is something I am aware of and is one of the higher priority issues I would like to address. Unfortunately, fixing this issue would probably require an application restructuring, which is something that won’t happen immediately.
The one immediate work-around I can suggest for you is to do multiple API calls at small group intervals. For example, in groups of 10:
https://www.wanikani.com/api/user/0f8073554e19479b2289a040f2f3bbee/vocabulary/1,2,3,4,5,6,7,8,9
It is not ideal (5 requests have to be made for 50 levels at 10 intervals), but it’ll get the results you want without the time outs. I would experiment with the intervals and see what produces the best results (20? 25?)
Sorry about the inconvenience and thanks for the understanding.