The session handler sets the clustermode based on whether there are multiple hosts provided:

        if (isset($CFG->session_redis_host))

Unknown macro: {             // If there is only one host, use the single Redis connection.             // If there are multiple hosts (separated by a comma), use the Redis cluster connection.             $this->host = array_filter(array_map('trim', explode(',', $CFG->session_redis_host)));             $this->clustermode = count($this->host) > 1;         }

https://github.com/moodle/moodle/blob/main/lib/classes/session/redis.php

Some clusters provide a single IP and then use MOVED to direct clients to the correct endpoint.

Please can we allow for clustermode to be explicitly set, even if there only is one IP.

I believe this is also the ask from this comment:

[MDL-59196] Add support for Redis Cluster - Moodle Tracker

From a cursory glance I'm thinking I can specify the same host twice.. if that works I will report back.