90 days of telemetry from an open-source MCP server that drives the actual Excel desktop app. The numbers were not where I thought they would be. Half a year ago I asked a simple question: why can AI agents write a React app from scratch but choke on Revenue Model v27 final final.xlsx ? The answer turned out to be boring and important. Agents had spreadsheet libraries . They did not have Excel . So I built Excel MCP Server : an open-source MCP server that drives the real Excel desktop application through COM automation. Not a .xlsx parser. Not Open XML. The actual app, with its calculation engine, VBA, Power Query, pivot caches, and all the quirks that make a workbook a workbook. The "just use openpyxl" problem Every time I talk about this, someone says "why not use a file-based Excel library?" For generating a .xlsx from a server, those libraries are great. Linux containers, no Office install, fast. They are not Excel. They do not run Excel's calculation engine. They do not execute VBA.…