How to fix Error 5836 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: Error 5836 – Lightweight Pooling Not Supported

Why This Error Happens

This error occurs when the lightweight pooling configuration option is enabled via sp_configure, but the running instance of SQL Server 2025 does not support fiber-mode scheduling. Lightweight pooling has been deprecated and is restricted on most modern editions and platforms, meaning the setting is effectively obsolete even though it may still appear as a configurable option. Attempting to apply this value triggers Error 5836 because the SQL Server engine rejects fiber-based scheduling on current Windows Server 2025/Windows 11 architectures.

Step-by-Step Resolution Guide

Step 1: Verify Current Configuration Value

  • Open SQL Server Management Studio (SSMS) and connect to the affected instance.
  • Run EXEC sp_configure 'lightweight pooling'; to check if the value is set to 1.
  • Confirm the run_value column reflects an active (non-zero) setting.

Step 2: Disable Lightweight Pooling

  • Execute the following T-SQL commands:
  • EXEC sp_configure 'show advanced options', 1; RECONFIGURE;
  • EXEC sp_configure 'lightweight pooling', 0;
  • RECONFIGURE;
  • This resets the scheduling mode back to standard thread-based scheduling, which is fully supported.

Step 3: Restart the SQL Server Service

  • Open Services.msc on Windows Server 2025 or Windows 11.
  • Locate SQL Server (MSSQLSERVER) or the named instance service.
  • Right-click and select Restart to apply the configuration change.
  • Alternatively, run net stop MSSQLSERVER && net start MSSQLSERVER from an elevated Command Prompt.

Step 4: Check SQL Server Error Logs

  • Navigate to C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG.
  • Confirm no further references to Error 5836 appear after the restart.
  • Look for the startup message confirming standard thread-mode scheduling is active.

Step 5: Audit Deployment Scripts and Legacy Configurations

  • Search any automated deployment or configuration scripts (PowerShell, DSC, or T-SQL baseline scripts) for references to lightweight pooling.
  • Remove or comment out any legacy configuration commands tied to fiber-mode scheduling to prevent recurrence during future deployments.

Alternative or Verification Steps

  • Run SELECT * FROM sys.configurations WHERE name = 'lightweight pooling'; and confirm both value and value_in_use are set to 0.
  • If the error persists after disabling the setting, check for third-party monitoring tools or legacy maintenance plans that may be resetting the configuration value on a schedule.