How to fix Error 6528 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 6528 – Assembly Not Found in SQL Catalog

Why This Error Happens

Error 6528 occurs when SQL Server attempts to execute a CLR (Common Language Runtime) object—such as a stored procedure, function, or trigger—that references an assembly which no longer exists in the sys.assemblies catalog of the target database. This typically happens because the assembly was dropped manually, the database was restored from a backup that predates the assembly registration, or metadata became desynchronized during a database migration or replication process. In some cases, this also occurs when a user attempts to call a CLR object across database contexts where the assembly was never registered in the current database scope.

Step-by-Step Resolution Guide

Step 1: Identify the Missing Assembly and Affected Object

  • Run the following query to capture the exact error context from SQL Server logs:
  EXEC sp_readerrorlog 0, 1, 'Assembly';
  • Note the exact assembly name and database name reported in the error text (%.*ls placeholders will be populated with real values here).
  • Cross-reference the object (procedure/function/trigger) that triggered the call using:
  SELECT * FROM sys.objects WHERE type IN ('PC', 'FN', 'TR');

Step 2: Verify Assembly Registration in the Catalog

  • Check whether the assembly actually exists in the current database:
  SELECT name, clr_name, permission_set_desc FROM sys.assemblies;
  • If the assembly is missing, confirm you are connected to the correct database context (USE [YourDatabaseName];) since CLR assemblies are database-scoped, not server-scoped.
  • On Windows Server 2025 / Windows 11, also confirm the original .dll source file still exists on disk (commonly under C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Binn\ or your custom deployment path).

Step 3: Re-register the Missing Assembly

  • If the .dll file is available, re-create the assembly manually:
  CREATE ASSEMBLY [YourAssemblyName]
  FROM 'C:\Path\To\YourAssembly.dll'
  WITH PERMISSION_SET = SAFE;
  • If the .dll is unavailable, extract it from a known-good backup or source control repository, then redeploy via SQL Server Data Tools (SSDT) or sqlcmd.
  • Ensure the SQL Server service account has read permissions on the folder containing the .dll file (right-click folder → Properties → Security → add NT SERVICE\MSSQLSERVER).

Step 4: Rebuild Dependent CLR Objects

  • Drop and recreate any stored procedures, functions, or triggers that reference the reinstated assembly to force SQL Server to refresh its internal metadata links:
  DROP PROCEDURE [dbo].[YourCLRProcedure];
  CREATE PROCEDURE [dbo].[YourCLRProcedure]
  AS EXTERNAL NAME [YourAssemblyName].[ClassName].[MethodName];
  • Restart the SQL Server service via services.msc or net stop MSSQLSERVER && net start MSSQLSERVER to clear any cached execution plans referencing the old assembly ID.

Alternative or Verification Steps

  • Confirm the fix by executing the previously failing stored procedure/function directly and checking for successful completion without Error 6528.
  • If the assembly keeps disappearing after restores, check your backup/restore scripts or replication jobs for DROP ASSEMBLY statements, and consider enabling TRUSTWORTHY or using a Certificate-Signed Assembly to prevent unintended removal during security-policy-driven cleanups.