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right now isn't a valid hashtag since it's not a letter or digit. Valid unicode hashtags do work with this endpoint i.e.: https://alpha-api.app.net/stream/0/posts/tag/พล (this example does show that we've got some issues with some international alphabets, but I don't expect will ever be a valid hashtag since it's a symbol)
If I try to perform a hashtag search using non-ASCII characters, I get a 404:
$ curl https://alpha-api.app.net/stream/0/posts/tag/ { "error": { "code": 404, "message": "Not found" } }
If I attempt to %-encode the characters, it's still a 404:
$ curl https://alpha-api.app.net/stream/0/posts/tag/%F0%9F%98%98 { "error": { "code": 404, "message": "Not found" } }