Many artist ids are not returning any recommendations from the API. I'm assuming this is a bug.
Just a couple of examples, using the developer console, omitting the authentication bearer part of the header:
curl -X "GET" "https://api.spotify.com/v1/recommendations?market=US&seed_artists=5hAhrnb0Ch4ODwWu4tsbpi" -H "Accept: application/json" -H "Content-Type: application/json"
curl -X "GET" "https://api.spotify.com/v1/recommendations?market=US&seed_artists=2YZyLoL8N0Wb9xBt1NhZWg" -H "Accept: application/json" -H "Content-Type: application/json"
Odd, I've just tried again, copied your first line and added my own authorisation to the end, and it returns values. So possibly something to do with your account?
$ curl -X "GET" "https://api.spotify.com/v1/recommendations?market=US&seed_artists=5hAhrnb0Ch4ODwWu4tsbpi" -H "Accept: application/json" -H "Content-Type: application/json" -H "Authorization: Bearer xxx"
{
"tracks" : [ {
"album" : {
"album_type" : "ALBUM",
"artists" : [ {
"external_urls" : {
"spotify" : "https://open.spotify.com/artist/2iUVQjheBnvOt8vaBrxXJz"
},
"href" : "https://api.spotify.com/v1/artists/2iUVQjheBnvOt8vaBrxXJz",
"id" : "2iUVQjheBnvOt8vaBrxXJz",
"name" : "Cass McCombs",
"type" : "artist",
"uri" : "spotify:artist:2iUVQjheBnvOt8vaBrxXJz"
} ],
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Env: prod