[BUG] OS 1.3.7 -> OS 2.19 backfill error: 'KNN87Codec' does not exist

[ragu2003]

What is the bug?

It happens when we start bulk document loader to restore snapshot

bulk-load 2025-09-11 10:30:49,172 ERROR o.o.m.b.w.DocumentsRunner [DocumentBatchReindexer-1] Error during docMigration for index=<index_name>
bulk-load java.lang.IllegalArgumentException: An SPI class of type shadow.lucene9.org.apache.lucene.codecs.Codec with name 'KNN87Codec' does not exist.  You need to add the corresponding JAR file supporting this SPI to your classpath.  The current classpath supports the following names: [Lucene80, Lucene84, Lucene86,
Lucene87, Lucene70, Lucene90, Lucene91, Lucene92, Lucene94, Lucene95, Lucene99, Lucene912, Elasticsearch816[]

What are your migration environments?

Source: OS 1.3.7
Target: OS 2.19
MigrationAssistant: 2.5.0

What is the expected behavior?

Migration assistant to recognize the class and migrate the index.

[peternied]

@ragu2003 Thanks for reporting this issue, can you tell me more information about the index? Would it be possible to publish the index settings, if you prefer not posting to GitHub could you dm me on the OpenSearch Slack so the team can inspect it?