Currently in a sharded environment, the replacement document must contain the shard key. If not an error similar to the following can be generated:

"errmsg" : "An upsert on a sharded collection must contain the shard key and have the simple collation. Update request: { q: { _id: 2.0 }, u: { a: 4.0 }, multi: false, upsert: true, collation: { locale: \"simple\" } }, shard key pattern: { _id: \"hashed\" }"

It may be helpful to provide a link to the relevant documentation along with the error message. Users may be confused by the current message since q does contain the shard key and the locale is set as the expected "simple".