Excel Copilot fails when local workbook filename contains #

Your test shows a filename-parsing problem, but it is outside Copilot in Excel's documented local-file support boundary. Microsoft states that Copilot in Excel is supported only for .xlsx, .xlsb, or .xlsm workbooks saved to OneDrive or Microsoft 365 SharePoint with AutoSave enabled. That explains why the same workbook works on OneDrive, although the “file type” message is misleading. The supported workaround is to save the workbook to OneDrive or SharePoint, confirm AutoSave is on, and retain a modern format. If local storage is mandatory, Microsoft does not document a setting that enables Copilot there; removing # is only an observed workaround and may not remain reliable. Because # works in the cloud copy, report the local-path behavior through Excel's Help > Feedback. Include the Excel version, update channel, exact filename, storage path type, and an error screenshot so the product team can investigate.