Openpyxl silently drops cached formula values when you edit an unrelated cell

A test by an unnamed developer found that openpyxl, a Python library for reading and writing Excel files, silently drops cached formula values when editing an unrelated cell, while Kookerella.FsOpenXmlDsl preserves them. In the test, appending a row below a total cell left the formula intact but emptied the cached value, causing data_only=True readers like pandas.read_excel to return None instead of 450. The bug occurs because openpyxl's Cell class has only one value slot, so loading a workbook in formula mode discards the cached number.

The Silent Bug: How an AI Agent Can Quietly Blank Out Your Excel Formulas Here’s a bug that doesn’t look like a bug. Ask an AI agent to add one line to an existing report. It does. The file opens fine in Excel - Excel recalculates every formula the moment it opens a workbook, so nothing looks wrong. But hand that same file to a second automated system - a PDF export, a dashboard pipeline, another script reading it with pandas - and a number that used to be there is now blank, or zero, or None . Nobody touched that cell. I ran a real, minimal test to find out exactly when and why this happens. The setup A tiny report: three line items and a Total cell holding =SUM B2:B4 , saved with its correct cached value 450 already baked in - exactly what a real .xlsx looks like after someone has opened and saved it in Excel at least once. Item Amount Widgets 100 Gadgets 150 Gizmos 200 Total =SUM B2:B4 cached: 450 The raw XML for that cell confirms it, before anything is touched: <c r="B5" <f SUM B2:B4 </f <v 450</v </c Both <f the formula and <v the last-calculated value are there, as siblings - this is what makes a .xlsx formula cell readable two different ways: recalculate it yourself, or just read <v if you don’t have a formula engine at all. The edit A completely unrelated, deliberately harmless change: append one new row below the total. Nothing about the formula, its range, or the cells it reads is touched. Arm A openpyxl : python import openpyxl wb = openpyxl.load workbook "formula report.xlsx" ws = wb "Report" ws.append "Note", "Reviewed" wb.save "formula arm a edited.xlsx" Arm B Kookerella.FsOpenXmlDsl : js let wb = Workbook.load "formula report.xlsx" let sheet = wb.Sheets | List.find fun s - s.Name = "Report" let editedSheet = { sheet with Cells = sheet.Cells @ / the new row's cells / } Workbook.save "formula arm b edited.xlsx" { wb with Sheets = editedSheet } What’s objectively in each result The same B5 cell, in the same file, after the same category of edit: Original: <c r="B5" <f SUM B2:B4 </f <v 450</v </c Arm A openpyxl : <c r="B5" <f SUM B2:B4 </f <v / </c Arm B Kookerella : <c r="B5" <f SUM B2:B4 </f <v 450</v </c <v is still there in Arm A’s result - it’s just empty. The formula survived. The number didn’t. Why this is worse than it looks Excel itself won’t show this. Open either file in real Excel and every formula recalculates on load - 450 reappears, and you’d never know anything happened. The bug only surfaces when something reads the file without recalculating it. That’s not a rare edge case anymore - it’s exactly what a second automated consumer does: wb = openpyxl.load workbook "formula arm a edited.xlsx", data only=True wb "Report" "B5" .value None data only=True is the mode that reads whatever’s cached instead of the formula text - the same mechanism pandas.read excel uses under the hood. Feed this file into a second script, a dashboard, or any headless pipeline, and the total is silently None where it used to be 450 . No exception, no warning - just a missing number propagating into whatever runs next. Doing the identical edit through Kookerella.FsOpenXmlDsl leaves B5 byte-for-byte intact: data only=True still reads back 450 . Why this happens Not a guess - openpyxl’s own Cell class only has one value slot: python def bind value self, value : ... self. value = value There’s no separate field for “the formula” and “its last cached result” at the same time. When you load a workbook normally data only=False - the mode you need if you want formulas to stay editable rather than frozen as numbers , a formula cell’s .value becomes the formula text . The cached number is never read into memory at all in that mode - data only=True is a separate, mutually exclusive way of loading the same file that substitutes the cached value in place of the formula. So the moment you open a workbook the normal way to edit it, the cached value has nowhere to live - it’s gone before your edit even happens, and saving just writes back what’s left: the formula, with an empty <v . This isn’t an openpyxl bug exactly - it’s a structural consequence of a cell model built around “one value per cell,” which is a reasonable design for a library that never evaluates formulas itself. It just means every save silently costs you the one thing a headless reader downstream actually depends on. Why this matters more now than it used to The old safety net was: someone opens this in real Excel eventually, and Excel fixes it. That assumption gets weaker every time more of a pipeline is agent-to-agent rather than human-in-the-loop - one AI produces a report, a second automated system consumes it, and nobody opens Excel in between. That’s the exact shape of workflow AI agents are increasingly used for, and it’s exactly the shape this bug is invisible inside. Try it Kookerella.FsOpenXmlDsl models a formula cell as Formula expression, cachedValue: float option explicitly, precisely because of this failure mode - see its own README for the reasoning. The MCP server built on it is Kookerella.FsOpenXmlDsl.Mcp /products/fsopenxmldsl-mcp/ : dotnet tool install -g Kookerella.FsOpenXmlDsl.Mcp Don’t want a .NET dependency at all? Download a standalone build for your platform from the latest release https://github.com/Kookerella-Ltd/Kookerella.FsOpenXmlDsl/releases/tag/v0.6.3 instead - the runtime is bundled into the executable, so it’s unzip and run, no install required. { "mcpServers": { "fsopenxmldsl": { "command": "fsopenxmldsl-mcp" } } } It’s usable entirely through JSON via its create workbook from json / generate json tools too - no .NET required on the calling side. See the product page /products/fsopenxmldsl-mcp/ for the full tool list.