Diffing two 834 files without crashing Excel

If you work with enrollment files, you know the request: "What changed between last month's 834 and this month's?" The files are 80 MB, 300 MB, sometimes more. Notepad++ freezes, Excel truncates or crashes, and the answer is due this afternoon. This article walks through a method that works with any tool, then shows how I do it with a small free CLI I wrote. All examples use synthetic data. Why the obvious approaches fail Opening it in Excel. X12 is not a table. An 834 is a stream of segments separated by a terminator, often with no line breaks at all. Excel's import wizard can split on ~, but one member spans many segments, so you get a ragged mess. Past about a million rows it cuts the file off, and the warning is easy to miss. A plain text diff. If both files are one long line, the diff says everything is different. If the carrier reorders members between runs, every move looks like a delete plus an add. Line-based diff has no notion of "member." Notepad++ with a comp