[BUG] Can't use filter "icu_folding" with "unicode_set_filter"

[MikePieperSer]

What is the bug?

Can't use filter "icu_folding" with "unicode_set_filter".
When try to create an index with this filter settings via java client, the server complains about a missing "[".

When I create an index via JSON directly (Dev Tools), then the client throws an exception, when reading the index settings, complaining an unexpected "[".

How can one reproduce the bug?

        // create an index
        CreateIndexRequest createIndexRequest = new CreateIndexRequest.Builder().index(index)
            .settings(b->b
                .analysis(ab -> ab
                    .filter("de_folding", fb -> fb
                        .definition(tfdb -> tfdb
                            .icuFolding(ifb -> ifb
                                .unicodeSetFilter("^äöüÄÖÜß")
                            )
                        )
                    )
                )
            ).build();
        CreateIndexResponse createIndexResponse = client.indices().create(createIndexRequest);

        final GetIndexResponse getIndexResponse = client.indices().get(b -> b
            .index(index + "*")
            .ignoreUnavailable(true)
            .allowNoIndices(true)
            .expandWildcards(ExpandWildcard.Open));
        System.out.println("Get index reponse: " + getIndexResponse);

What is the expected behavior?

That the index can be created with this filter and can be read back.

What is your host/environment?

AWS Opensearch Serverless / java-client 2.6.0

Do you have any screenshots?

No.

Do you have any additional context?

No.

[dblock]

Looks like a legit bug, want to try and turn it into a (failing) unit test?

[MikePieperSer]

The bug is not in the java-client but in AWS Opensearch Serverless.
When creating an index like this:

PUT icu_sample
{
  "settings": {
    "analysis": {
      "index": {
        "filter": {
          "swedish_folding": {
            "type": "icu_folding",
            "unicode_set_filter": "[^åäöÅÄÖ]"
          }
        }
      }
    }
  }
}

The server creates the index. But then if one tries to get the settings back serverless answers with:

GET icu_sample

{
  "icu_sample": {
        "analysis": {
          "index": {
            "filter": {
              "swedish_folding": {
                "type": "icu_folding",
                "unicode_set_filter": [
                  "^åäöÅÄÖ"
                ]
              }
            }
          }
        }
      }
    }
  }
}

You see "unicode_set_filter" now became an array.
That means, the java-client is absolutely right to complain.

[dblock]

@MikePieperSer Did you open a ticket with AWS?

[MikePieperSer]

Not yet, but it's on my task list.