How to fix Error 4813 on Windows Server 2025 / Windows 11

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Why This Error Happens

Error 4813 occurs when SQL Server’s bulk copy (BCP) utility encounters a mismatch between the expected text/image column length declared in the data file and the actual length of the data stream being read. This typically results from a corrupted or improperly formatted data file, incorrect format file (.fmt) specifications, or truncated data caused by a network interruption during the bulk insert operation. It can also happen when the source data contains embedded delimiters or line breaks that confuse the column-length parser.

Step-by-Step Resolution Guide

Step 1: Validate the Source Data File

  • Open the data file in a text editor (e.g., Notepad++) and confirm there are no unexpected line breaks or null characters within text/ntext fields.
  • Check that the file encoding matches what BCP expects (ANSI vs. UTF-8/Unicode) — mismatched encoding is a common cause of length miscalculation.
  • Run bcp [YourTable] format nul -c -f C:\BCP\format.fmt -S YourServer -T to regenerate a clean format file for comparison.

Step 2: Review and Correct the Format File

  • Inspect C:\BCP\format.fmt to ensure the column lengths and data types match your table schema exactly.
  • Pay special attention to prefix lengths for text/ntext/varchar(max) columns — a prefix length of 0 when the source expects a length-prefixed field will trigger Error 4813.
  • If using a native format file, regenerate it with -n instead of -c to avoid character-mode length ambiguity.

Step 3: Re-run the Bulk Copy with Explicit Parameters

  • Use the -m switch to set a reasonable max error count (e.g., -m 10) so you can capture multiple failure rows instead of stopping at the first one.
  • Run the command with explicit field and row terminators specified:
    bcp YourDB.dbo.YourTable in C:\Data\input.txt -c -t"," -r"\n" -S YourServer -T
  • Check SQL Server error logs at C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG for additional context tied to the failed batch.

Step 4: Check for Network or Driver-Level Truncation

  • If importing over a network share, confirm the file wasn’t altered or truncated during transfer by comparing file size/checksum (Get-FileHash in PowerShell) between source and destination.
  • Update the SQL Server Native Client or ODBC Driver to the latest version compatible with SQL Server 2025, as outdated drivers on Windows Server 2025/Windows 11 can misreport stream lengths.

Alternative or Verification Steps

  • Verify the fix by re-running the bulk insert on a small sample (10–20 rows) before processing the full dataset — a successful completion confirms the format file and data alignment are correct.
  • As an alternative, use OPENROWSET(BULK...) within a T-SQL script instead of the command-line bcp utility, as it often provides more descriptive error messages when a length mismatch occurs.