The Cognitive Services was purged over a month ago. Resource creation fails with CustomDomainInUse.

Hello Liang LIANG ,

Welcome to Microsoft Q&A .Thank you for reaching out to us.

The CustomDomainInUse error indicates that the requested custom subdomain is currently unavailable for allocation. While this often occurs when a custom subdomain is still associated with an existing or previously deleted resource, the error alone does not confirm the exact cause.

Since the resource was reportedly purged more than a month ago, the next step is to validate whether the original resource state has been fully removed and whether the issue is isolated to the specific custom subdomain. Custom subdomain names are unique to Azure AI Services resources and can generally be reused after the original resource is deleted.

Please check if the following steps help-

1. Verify that the deleted resource no longer exists Please check whether the original resource still appears in the deleted resource inventory by running:

      az cognitiveservices account list-deleted -o table
   

   If the resource is still listed, Azure may still recognize it as a deleted resource and an additional purge may be required. If the resource is not listed, proceed to the next steps.
2. Testing with a new custom subdomain Create a temporary Azure AI Services resource using:
   • A new resource name
   • A completely new custom subdomain
   • The same subscription, region, and resource type
   If the deployment succeeds, the issue is likely specific to the original custom subdomain. If the deployment fails with the same error, the issue may extend beyond the original name and require further investigation.
3. Validating the custom subdomain configuration Please verify that the requested custom subdomain:
   • Contains only letters, numbers, and hyphens
   • Does not end with a hyphen
   • Meets the documented naming requirements
4. Reviewing any networking dependencies If the original resource used networking features, please verify that all associated components were removed successfully, including:
   • Private Endpoints
   • Private Link
   • Private DNS Zones
   • Managed networking configuration
   Incomplete cleanup of Private Endpoint relationships has been documented as a potential cause of purge-related issues and may require additional cleanup
5. Reviewing Activity Logs Please review Activity Logs for:
   • Resource deletion operations
   • Purge operations
   • Failed resource creation attempts
   • Authorization or conflict errors
   This helps confirm whether the purge completed successfully and whether the create request is reaching the service correctly.

The following references might be helpful , please check them out

• Custom subdomains - Foundry Tools | Microsoft Learn
• Recover or purge deleted Microsoft Foundry resources - Foundry Tools | Microsoft Learn
• az cognitiveservices account | Microsoft Learn
• Activity Log in Azure Monitor - Azure Monitor | Microsoft Learn  

Please let us know if the response was helpful

Thank you