How to fix Error 6998 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 6998 – “Invalid Type Definition: Content Model Too Complicated”

Why This Error Happens

This error occurs when SQL Server’s XML parser attempts to validate or compile an XML Schema Collection (XSD) that exceeds the internal complexity limits of the schema processor (MSXML/System.Xml). The root cause is typically an XML schema with an excessive number of enumeration values, deeply nested xs:choice or xs:sequence groups, or a content model that generates too many Deterministic Finite Automaton (DFA) states during compilation. This commonly surfaces when importing large third-party XSD files (such as FpML, HL7, or FIXML schemas) into a CREATE XML SCHEMA COLLECTION statement.

Step-by-Step Resolution Guide

Step 1: Identify the Offending Schema Collection

  • Run the following query to locate recently added or modified XML schema collections:
  • SELECT name, xml_collection_id, create_date FROM sys.xml_schema_collections ORDER BY create_date DESC;
  • Check the SQL Server error log (located at C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG on Windows Server 2025) for the exact %s schema name referenced in Error 6998.
  • Export the schema definition to a local .xsd file for inspection using SQL Server Management Studio (SSMS) 21.

Step 2: Simplify the XML Content Model

  • Open the exported .xsd file in Visual Studio 2022/2025 or XML Notepad.
  • Reduce the number of xs:enumeration facets in any single xs:simpleType restriction; consider splitting large enumeration lists into a reference lookup table instead of inline XML validation.
  • Flatten deeply nested xs:choice blocks — SQL Server’s XML compiler struggles when choice groups exceed roughly 200–300 combined child elements per content model.
  • Replace maxOccurs="unbounded" combined with complex nested choices with simpler xs:sequence structures where business logic allows.

Step 3: Re-register the Modified Schema Collection

  • Drop the existing broken collection (if partially created) using:
  • DROP XML SCHEMA COLLECTION [dbo].[YourSchemaName];
  • Re-import the simplified schema using:
  • CREATE XML SCHEMA COLLECTION [dbo].[YourSchemaName] AS N'<simplified XSD content>';
  • Monitor the command execution; it should complete without throwing Error 6998.

Step 4: Apply Latest SQL Server 2025 Cumulative Update

  • Open PowerShell as Administrator and check your current build number:
  • SELECT @@VERSION;
  • Navigate to the Microsoft Update Catalog and download the latest Cumulative Update (CU) for SQL Server 2025, as early builds have known XML parser DFA-state limits that were patched in later CUs.
  • Install the update and restart the SQL Server (MSSQLSERVER) service via services.msc or net stop MSSQLSERVER && net start MSSQLSERVER.

Alternative or Verification Steps

  • Verify the fix: Re-run the original INSERT, CREATE, or validation query that triggered Error 6998 against a column of type xml bound to the schema collection — it should now process without error.
  • Alternative workaround: If the schema cannot be simplified due to compliance requirements (e.g., regulatory XSDs), store the data as untyped xml or nvarchar(max) instead of binding it to a strict XML SCHEMA COLLECTION, and perform validation at the application layer instead.