At the moment the query_syntax seems to have a custom syntax. It looks similar to lucene syntax but is not the same. There are a few differences to azure and elastic and I don’t understand whether I am doing something wrong or not. What all three service have in common is that they are built on top of lucene, therefore I though that the query_string operator uses the lucene query syntax as it seems to be done by elastic and azure: https://lucene.apache.org/core/2_9_4/queryparsersyntax.html Documentation about the elastic query string query can be found under: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-query-string-query.html There are a few thing that work differently: 1. The Atlas query_string does not support fuzzy operator like helo~. 2. The Atlas query_string cannot search in multiple fields, because “*” is not supported as a default_path. 3. The Atlas query_string does not seem to support the lucene syntax at all. This makes migration harder and the user cannot use the syntax. I got it working with a custom converter so it is technically possible, but it would be better if Atlas search would just support it: https://github.com/SebastianStehle/MongoSearchTest/blob/main/Mongo.LuceneQuery/LuceneQueryVisitor.cs