For adding hosts with API option with assisted installer, CPU architecture is taken as x86_64 by default. There is no option to specify the CPU architecture of the cluster.

Every time

    1. Try 'Adding hosts with API' day2 operation on a s390x cluster as per the steps given in the documentation-https://docs.redhat.com/en/documentation/assisted_installer_for_openshift_container_platform/2025/html/installing_openshift_container_platform_with_the_assisted_installer/expanding-the-cluster#adding-hosts-with-the-api_expanding-the-cluster     

    2. In step 4.iii, while posting the $INFRA_ENV_REQUEST to /v2/infra-envs API, $INFRA_ENV_ID variable that is being set is taking x86_64 as the CPU architecture by default.

{"cluster_id":"<cluster_id>","cpu_architecture":"x86_64","created_at":"<timestamp>","download_url":"https://api.openshift.com/api/assisted-images/bytoken/<token>/4.21/x86_64/<iso_name>.iso","email_domain":"<domain>","expires_at":"<expiry_timestamp>","href":"/api/assisted-install/v2/infra-envs/xxx","id":"<cluster_id>","kind":"InfraEnv","name":"<infraenv_name>","openshift_version":"4.21","org_id":"<id>","proxy":{},"pull_secret_set":true,"ssh_authorized_key":"<ssh_key>","type":"full-iso","updated_at":"<timestamp>","user_name":"<user>"}

Generated discovery iso is not compatible on s390x

There is a provision to accept the appropriate CPU architecture.