How to fix Error 6995 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 6995 – Invalid XML Schema Type Definition

Why This Error Happens

This error occurs because SQL Server’s XML Schema Collection (XSC) implementation explicitly disallows the use of the built-in ID and IDREF XML Schema types (or any custom types derived from them) when defining element types or extending complex types. This restriction exists because ID and IDREF are identity-constraint types reserved for internal XML uniqueness validation, and permitting them as general-purpose element types would conflict with SQL Server’s internal XML indexing and validation engine. The error is almost always triggered by a malformed or improperly migrated .xsd file being registered via CREATE XML SCHEMA COLLECTION or ALTER XML SCHEMA COLLECTION.

Step-by-Step Resolution Guide

Step 1: Identify the Offending Schema Collection

  • Run the following query to locate the specific schema collection causing the failure: SELECT name, schema_id FROM sys.xml_schema_collections WHERE name = '%s';
  • Check the SQL Server Error Log (located at C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG on Windows Server 2025) to capture the exact timestamp and batch that triggered Error 6995.
  • Open the source .xsd file in a text editor or Visual Studio to inspect the type declarations.

Step 2: Remove or Refactor Invalid Type References

  • Search the XSD file for any <xs:element> or <xs:complexType> definitions that use type="xs:ID" or type="xs:IDREF".
  • Replace these references with a safe substitute type, such as xs:string or xs:token, which preserves data format without violating SQL Server’s identity-constraint restrictions.
  • If the schema uses a custom simple type derived by restriction from ID/IDREF (e.g., <xs:simpleType name="CustomID"><xs:restriction base="xs:ID">), change the base attribute to xs:string instead.
  • Save the corrected .xsd file to a working directory, such as C:\SQLSchemas\Corrected\.

Step 3: Re-register the Corrected Schema Collection

  • Drop the existing broken schema collection if it was partially created: DROP XML SCHEMA COLLECTION [SchemaName];
  • Re-import the corrected schema using T-SQL: CREATE XML SCHEMA COLLECTION [SchemaName] AS 'C:\SQLSchemas\Corrected\schema.xsd';
  • Confirm the command completes without raising Error 6995 in SQL Server Management Studio (SSMS) or Azure Data Studio.

Step 4: Validate Dependent Objects

  • Check any tables or stored procedures referencing the XML Schema Collection using sp_depends or the sys.sql_expression_dependencies catalog view.
  • Recompile affected stored procedures with EXEC sp_recompile 'dbo.YourProcedureName'; to ensure they bind correctly to the updated schema.

Alternative or Verification Steps

  • Verify the fix by inserting a sample XML document that conforms to the corrected schema into a typed XML column and confirming no validation errors are thrown.
  • If modifying the .xsd source is not feasible (e.g., third-party schema), consider storing the XML as untyped (XML data type without a schema collection reference) and enforcing ID/IDREF-like uniqueness constraints at the application layer instead.