Troubleshooting Guide: SQL Server Error 6563

Why This Error Happens

Error 6563 in Microsoft SQL Server 2025 typically occurs when the CLR (Common Language Runtime) integration attempts to validate or load an assembly method signature that does not match the expected metadata definition stored in the system catalog. This is often triggered by a mismatch between the assembly’s compiled version and the version registered in the database, commonly caused by an incomplete deployment, a corrupted DLL, or an update to the .NET runtime on Windows Server 2025 that is not backward compatible with the referenced assembly. Additionally, this can surface when the executing security context lacks the necessary UNSAFE or EXTERNAL_ACCESS permission level required by the method being invoked.

Step-by-Step Resolution Guide

Step 1: Verify Assembly Permissions and Trust Settings

• Open SQL Server Management Studio (SSMS) and connect to the instance reporting the error.
• Run the following query to check the current permission set of the assembly:

  SELECT name, permission_set_desc FROM sys.assemblies;

• If the permission set is SAFE but the method requires external calls, update it using:

  ALTER ASSEMBLY YourAssemblyName WITH PERMISSION_SET = UNSAFE;

• Ensure the database has TRUSTWORTHY set to ON if the assembly is not signed with a certificate:

  ALTER DATABASE YourDatabaseName SET TRUSTWORTHY ON;

Step 2: Re-register the CLR Assembly

• Drop the existing assembly reference to clear any corrupted metadata:

  DROP ASSEMBLY YourAssemblyName;

• Re-add the assembly from the source DLL file, ensuring the path is accessible by the SQL Server service account:

  CREATE ASSEMBLY YourAssemblyName FROM 'C:\SQLAssemblies\YourAssembly.dll' WITH PERMISSION_SET = SAFE;

• Confirm the file path exists on the Windows Server 2025 host and that the NT SERVICE\MSSQLSERVER account has Read & Execute permissions on that directory.

Step 3: Validate .NET Framework and Runtime Compatibility

• Check the installed .NET runtime version on the server via PowerShell:

  Get-ChildItem "HKLM:\SOFTWARE\Microsoft\NET Framework Setup\NDP" -Recurse

• Ensure the version matches the framework target used to compile the original assembly (e.g., .NET Framework 4.8.1 or the specific CLR version supported by SQL Server 2025).
• If a mismatch is found, recompile the source assembly against the correct target framework and redeploy using Step 2.

Step 4: Restart the SQL Server Service

• Open Services.msc on Windows Server 2025 / Windows 11.
• Locate SQL Server (MSSQLSERVER) or the named instance equivalent.
• Right-click and select Restart to clear the CLR hosting cache and force a fresh load of the assembly metadata.

Alternative or Verification Steps

• To verify the fix, execute the stored procedure or function that previously triggered Error 6563; a successful execution without an exception confirms resolution.
• If the error persists, run DBCC FREESYSTEMCACHE('ALL') to clear the plan cache, as a stale execution plan referencing the old assembly signature may still be cached in memory.