How to fix Error 5849 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 5849 — Online CPU Addition Not Supported

Why This Error Happens

Error 5849 occurs because the installed edition of SQL Server 2025 (typically Standard, Web, or Express) does not include support for dynamic hot-add CPU operations, which are reserved for Enterprise and Datacenter editions. This limitation is enforced at the SQL Server engine level regardless of whether the underlying hardware or Hyper-V/VM configuration supports hot-add CPU. Additionally, even on Enterprise edition, this error can surface if the host OS (Windows Server 2025) or the virtualization layer does not expose the hot-add CPU capability to the guest, causing SQL Server to reject the operation.

Step-by-Step Resolution Guide

Step 1: Verify Your SQL Server Edition

  • Run the following query in SQL Server Management Studio (SSMS) to check your current edition:
  • SELECT SERVERPROPERTY('Edition') AS Edition, SERVERPROPERTY('EngineEdition') AS EngineEdition;
  • Confirm whether the result returns “Standard Edition,” “Web Edition,” or “Express Edition” — these do not support online CPU add.
  • Cross-reference with Microsoft’s official “Editions and supported features” documentation for SQL Server 2025.

Step 2: Upgrade to a Supported Edition (If Required)

  • If your workload requires online CPU addition, initiate an edition upgrade using the SQL Server Installation Center.
  • Navigate to Maintenance > Edition Upgrade in the SQL Server Installation Wizard.
  • Select Enterprise or Datacenter edition and supply a valid Product Key or License file.
  • Restart the SQL Server service (MSSQLSERVER) after the upgrade completes via services.msc or net stop MSSQLSERVER && net start MSSQLSERVER.

Step 3: Confirm Hypervisor and OS Support for Hot-Add CPU

  • Open Hyper-V Manager on the Windows Server 2025 host and check the VM’s settings under Processor > Enable Hot Add.
  • Ensure the VM was created with “Generation 2” support and that hot-add CPU is enabled before the VM is powered on (this setting cannot be changed while the VM is running).
  • If using a third-party hypervisor (VMware, etc.), confirm CPU Hot Plug is enabled in the VM’s hardware configuration settings.

Step 4: Apply Configuration Changes and Restart Services

  • Shut down the VM completely (not just restart) to apply hot-add CPU settings at the hypervisor level.
  • Power the VM back on and allow Windows Server 2025 to fully initialize hardware detection.
  • Restart the SQL Server service to reinitialize CPU affinity and resource detection:
  • net stop MSSQLSERVER && net start MSSQLSERVER

Alternative or Verification Steps

  • After applying changes, attempt the CPU hot-add operation again and check sys.dm_os_sys_info in SQL Server to confirm the increased cpu_count is recognized.
  • If upgrading edition is not feasible, use a scheduled maintenance window to manually add CPUs offline (shutdown VM, modify CPU count, restart VM, and restart SQL Server) instead of relying on hot-add functionality.