How to fix Error 4858 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 4858 — Bad Value in Format File Attribute

Why This Error Happens

Error 4858 occurs when SQL Server’s bcp utility or BULK INSERT command parses an XML or non-XML format file and encounters an attribute value that doesn’t conform to the expected schema, such as an invalid data type, an out-of-range COLLATION, a malformed TERMINATOR, or an incorrectly specified PREFIX_LENGTH. This typically results from manual editing of the format file, a format file generated against a mismatched table schema, or file corruption during transfer between systems. Since SQL Server 2025 enforces stricter XML schema validation on format files than earlier versions, legacy format files created with older bcp versions may now fail validation that previously passed silently.

Step-by-Step Resolution Guide

Step 1: Identify the Exact Line and Attribute Causing the Failure

  • Open the format file referenced in the error message (commonly a .fmt or .xml file) using a text editor like Notepad++ or VS Code.
  • Locate the exact line number (%d) cited in the error, and inspect the attribute (%ls) and the bad value (%ls) flagged.
  • Common faulty attributes include COLLATION, TERMINATOR, PREFIX_LENGTH, MAX_LENGTH, and XSI:TYPE.

Step 2: Regenerate the Format File from the Current Table Schema

  • Open PowerShell or Command Prompt as Administrator on Windows Server 2025 / Windows 11.
  • Run the following command to regenerate a clean, valid XML format file:
  bcp YourDatabase.dbo.YourTable format nul -c -x -f C:\BCP\YourTable_new.xml -S YourServerName -T
  • Compare the regenerated file against your original to confirm the invalid attribute value and replace it accordingly.

Step 3: Validate Data Type and Collation Consistency

  • Confirm that any COLLATION attribute value matches a collation name actually installed on the SQL Server 2025 instance by running:
  SELECT name FROM sys.fn_helpcollations();
  • Ensure XSI:TYPE values (e.g., SQLCHAR, SQLNVARCHAR, SQLINT) correctly correspond to the destination column’s data type in sys.columns.
  • Save the corrected format file in UTF-8 encoding without BOM corruption, as encoding mismatches can reintroduce invalid attribute values.

Step 4: Re-run the Bulk Operation with the Corrected Format File

  • Execute the BULK INSERT or bcp command again, explicitly pointing to the corrected format file path.
  • Example:
  bcp YourDatabase.dbo.YourTable in C:\BCP\datafile.dat -f C:\BCP\YourTable_new.xml -S YourServerName -T
  • Monitor the command output for successful row count confirmation instead of Error 4858.

Alternative or Verification Steps

  • Verify the fix by running the bulk import against a small sample data file first; a clean completion without Error 4858 confirms the format file is now schema-valid.
  • If the error persists, bypass the format file entirely and use the -c (character format) native bcp switches with explicit field/row terminators on the command line as a temporary workaround while the XML format file is rebuilt from scratch.