How to fix Error 4812 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 4812 — Bulk Copy Row Offset Mismatch

Why This Error Happens

Error 4812 occurs when the BCP (Bulk Copy Program) utility or BULK INSERT operation encounters a data file where the row offsets for text or image (now varchar(max), nvarchar(max), or varbinary(max)) columns do not match what SQL Server expects based on the format file. This typically happens due to a corrupted or malformed data file, a mismatched or outdated format file (.fmt) that doesn’t align with the current table schema, or an incorrect bulk copy mode (character vs. native) being used during the export/import process. It can also surface when a data file was generated by a different version of SQL Server or a third-party tool that structures LOB (Large Object) pointers differently than SQL Server 2025 expects.

Step-by-Step Resolution Guide

Step 1: Validate and Regenerate the Format File

  • Confirm the .fmt file matches the exact column order, data types, and collation of the destination table.
  • Regenerate the format file using the current schema to rule out drift:
  bcp YourDatabase.dbo.YourTable format nul -c -f YourTable_new.fmt -S YourServerName -T
  • Replace the old format file reference in your BCP or BULK INSERT command with this freshly generated file.

Step 2: Re-Export the Source Data Using Matching Bulk Copy Mode

  • If the original export used native mode (-n), ensure the import also uses native mode — mixing character (-c) and native modes is a common cause of offset corruption.
  • Re-run the export from the source system:
  bcp YourDatabase.dbo.YourTable out YourData.dat -n -S SourceServer -T
  • Avoid manually editing .dat files in a text editor, as this can shift byte offsets for LOB columns and trigger Error 4812.

Step 3: Check for File Corruption or Truncation

  • Verify the data file transferred completely (compare file size/checksum between source and destination servers).
  • On Windows Server 2025/Windows 11, use PowerShell to validate integrity:
  Get-FileHash -Path "C:\BulkData\YourData.dat" -Algorithm SHA256
  • If the file was transferred via FTP or a network share, re-copy it using binary mode to prevent silent corruption of LOB data pointers.

Step 4: Rebuild the Bulk Insert Command with Explicit Row Terminators

  • Explicitly define row and field terminators in your BULK INSERT statement rather than relying on defaults:
  BULK INSERT dbo.YourTable
  FROM 'C:\BulkData\YourData.dat'
  WITH (
      FORMATFILE = 'C:\BulkData\YourTable_new.fmt',
      ROWTERMINATOR = '\n',
      FIELDTERMINATOR = '\t',
      TABLOCK
  );
  • This prevents SQL Server from misinterpreting LOB column boundaries due to ambiguous terminators.

Alternative or Verification Steps

  • Run the bulk insert against a small sample (5–10 rows) extracted into a separate test file to confirm the format file and data alignment work before processing the full dataset.
  • If the error persists, switch to OPENROWSET(BULK...) with an explicit SINGLE_BLOB or SINGLE_CLOB option as an alternative import method, bypassing the traditional BCP row-offset parsing entirely.