How to fix Error 6956 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 6956 – “Invalid Type Definition for Type ‘%s'”

Why This Error Happens

Error 6956 typically occurs when SQL Server 2025 encounters a corrupted or malformed CLR (Common Language Runtime) user-defined type (UDT) or XML schema collection definition during assembly loading or type registration. This often results from a mismatched assembly version, incomplete deployment of a custom type’s DLL, or database metadata that references a type definition no longer present in the associated assembly. It can also surface after an in-place upgrade to SQL Server 2025 if legacy CLR assemblies are incompatible with the new runtime host.

Step-by-Step Resolution Guide

Step 1: Verify the CLR Assembly and Type Registration

  • Run SELECT * FROM sys.assemblies; to confirm the assembly tied to the affected type is still listed and enabled.
  • Cross-check SELECT * FROM sys.assembly_types; to ensure the type definition matches the assembly’s current version.
  • Look for orphaned entries where the assembly exists but the type metadata is inconsistent.

Step 2: Re-deploy or Re-register the Affected Assembly

  • Drop the problematic type and assembly using DROP TYPE [TypeName]; followed by DROP ASSEMBLY [AssemblyName];.
  • Re-import the correct, updated DLL using CREATE ASSEMBLY [AssemblyName] FROM 'C:\SQLAssemblies\YourAssembly.dll' WITH PERMISSION_SET = SAFE;.
  • Re-create the type definition with CREATE TYPE [TypeName] EXTERNAL NAME [AssemblyName].[ClassName];.

Step 3: Check SQL Server CLR Configuration

  • Confirm CLR integration is enabled by running sp_configure 'clr enabled'; — it should return a value of 1.
  • If disabled, enable it via EXEC sp_configure 'clr enabled', 1; RECONFIGURE;.
  • Restart the SQL Server service through Services.msc or net stop MSSQLSERVER && net start MSSQLSERVER on Windows Server 2025.

Step 4: Validate Compatibility Level and Database Integrity

  • Check the database compatibility level with SELECT name, compatibility_level FROM sys.databases; and ensure it aligns with SQL Server 2025 standards (160 or higher).
  • Run DBCC CHECKDB('YourDatabaseName') WITH NO_INFOMSGS, ALL_ERRORMSGS; to rule out underlying corruption affecting system metadata tables that store type definitions.

Step 5: Rebuild the Type from a Clean Script (If Corruption Persists)

  • Script out all dependent objects (tables, functions, stored procedures) using SQL Server Management Studio’s Generate Scripts wizard.
  • Fully remove the type and assembly, restart the SQL Server instance, then reapply the clean script to rebuild the type from scratch.

Alternative or Verification Steps

  • Verify the fix by querying SELECT * FROM sys.types WHERE name = 'YourTypeName'; and confirming it returns without error, then test the affected stored procedure or function that previously failed.
  • If the issue persists, restore the database from a pre-upgrade backup to a test instance and compare the sys.assembly_types metadata against production to identify the exact point of corruption.