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There is a PR that may help us to support Akka nodes with older versions but the suspicion is that we will have even more issues. This revolves around the ClusterMessageSerializer and that older versions of Akka include fully qualified class names in the messages. This issue is termed Shorter Manifests in Akka docs. Pekko will only work with Akka nodes that emit the manifests in this shorter format (Akka 2.6.5 and above).
If your Akka nodes run a version older than 2.6.5, please upgrade your Akka nodes before attempting to integrate Pekko nodes. Upgrade all the nodes to Akka 2.6.4 first because that version supports the 2 Manifest formats. Once all the nodes are upgraded to Akka 2.6.4, you should then be able to upgrade all nodes to the latest Akka 2.6 release. It is probably a good idea to contact other Akka users and even the Pekko team for advice about such an upgrade.
It is unknown whether the Akka releases made after 2.7.0 (when the license changed to a Business Software License) have any changes that affect Pekko's ability to form clusters with Akka nodes of those versions.
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