From the logs:
[08/05/25 09:49:31] [SSH] Opening SSH connection to localhost:22. [08/05/25 09:49:32] [SSH] WARNING: The SSH key for this host does not match the key required in the connection configuration. Connections will be denied until the host key matches the configuration key. Key exchange was not finished, connection is closed. SSH Connection failed with IOException: "Key exchange was not finished, connection is closed.", retrying in 15 seconds. There are 10 more retries left. [08/05/25 09:49:47] [SSH] WARNING: The SSH key for this host does not match the key required in the connection configuration. Connections will be denied until the host key matches the configuration key. Key exchange was not finished, connection is closed.
We are currently configuring the agent launcher with the host's RSA key that we extract with ssh-keyscan -t rsa localhost. However Jenkins uses the local SSH client to connect and it looks like it selects ssh-ed25519 during the negotiation phase:
$ ssh -vvv localhost -o PasswordAuthentication=no -o StrictHostKeyChecking=no 2>&1 | grep -i 'Server host key:' debug1: Server host key: ssh-ed25519 SHA256:Y+jV7PPrEBsvPa9K4yXeh2WLWfXPT4c98UJuXx4xUug
releases2003 was recently reimaged