How to fix Error 8020 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 8020 (TDS/RPC CLR Serialization Metadata Error)

Why This Error Happens

This error occurs when a client application sends an RPC call to SQL Server 2025 with a CLR-based parameter (such as a UDT, geometry, geography, or custom CLR type) whose serialized length metadata does not match what SQL Server expects based on the type’s registered assembly definition. This mismatch is commonly caused by an outdated or mismatched client driver (ODBC, OLE DB, or SqlClient) that doesn’t align with the CLR type version deployed on the server, or by a corrupted/incompatible CLR assembly registered in the database. It can also surface after an in-place upgrade to SQL Server 2025 where legacy CLR assemblies were not recompiled against the updated .NET runtime hosted inside SQL Server.

Step-by-Step Resolution Guide

Step 1: Verify and Update Client Drivers

  • Confirm the application is using the latest Microsoft OLE DB Driver (MSOLEDBSQL19+) or Microsoft.Data.SqlClient (v5.x or later) compatible with SQL Server 2025.
  • On Windows 11/Windows Server 2025, check installed driver versions via Control Panel > Programs and Features or run:
  Get-OdbcDriver | Where-Object {$_.Name -like "*SQL Server*"}
  • Uninstall outdated legacy drivers (e.g., SQL Server Native Client 11.0) and reinstall the current Microsoft OLE DB Driver for SQL Server.

Step 2: Re-register and Validate CLR Assemblies

  • Open SQL Server Management Studio (SSMS) and connect to the affected instance.
  • Run the following to identify the problematic assembly tied to the parameter type referenced in the error:
  SELECT * FROM sys.assemblies;
  SELECT * FROM sys.assembly_types;
  • Drop and re-add the CLR assembly using ALTER ASSEMBLY with the WITH PERMISSION_SET clause matching your security requirements:
  ALTER ASSEMBLY YourAssemblyName
  FROM 'C:\Path\To\YourAssembly.dll'
  WITH PERMISSION_SET = SAFE;
  • Ensure the assembly was compiled against the correct .NET target version supported by SQL Server 2025 (.NET 8+ CLR integration).

Step 3: Check TDS Protocol Version Alignment

  • Verify the SQL Server instance is running the latest Cumulative Update (CU) for SQL Server 2025 via:
  SELECT @@VERSION;
  • Confirm Windows Server 2025 / Windows 11 has the latest .NET Framework and .NET runtime updates installed via Windows Update, as TDS/CLR serialization depends on matching runtime libraries.
  • Restart the SQL Server service after updates:
  Restart-Service -Name MSSQLSERVER -Force

Step 4: Rebuild the Parameter Metadata Cache

  • Clear the procedure cache to eliminate stale execution plans referencing outdated CLR parameter metadata:
  DBCC FREEPROCCACHE;
  • Re-execute the failing stored procedure or RPC call to confirm the parameter metadata rebuilds correctly.

Alternative or Verification Steps

  • Run a test RPC call using sqlcmd -S servername -d dbname -Q "EXEC YourProcedure" to confirm the error no longer appears in isolation from the application layer.
  • If the issue persists, capture a network trace using SQL Server Profiler or Extended Events (rpc_completed event) to inspect the raw parameter payload and confirm the CLR type length byte matches the assembly’s current schema.