Microsoft is retiring the =COPILOT() worksheet function in Excel on 14 September 2026. That is 18 days from the date this guide was published, and the retirement notice, Message Center post MC1454373, only reached administrators in mid-August. If anyone in your organisation has been running Excel's Insider or Frontier preview builds over the past year, there is a realistic chance a workbook somewhere has live COPILOT() formulas in it, and the window to deal with them calmly is short.
This is a checklist piece: what the function did, what happens on the date, how to find the workbooks that use it, and where each workflow should move.
What the COPILOT() function was
Introduced in August 2025, COPILOT() let a user put a natural-language prompt directly into a cell, optionally pointing it at ranges of workbook data, and get an AI-generated result back as a spreadsheet value. Summarising feedback comments, classifying rows, generating text and looking up information became things a formula could do. It was never generally available: Microsoft's retirement notice describes it as a preview feature offered through the Excel Insider and Frontier programs, which means the population of affected users is limited but hard to see centrally, because preview enrolment happens at the user and client level.
Why Microsoft is retiring it
Microsoft's stated direction is consolidation: Copilot in Excel continues through the Copilot side pane, which the retirement notice points to as the supported interface for the same categories of task, including text summarisation, data classification, content generation and web-based information lookup. The company's own guidance for the function also drew a firm line around what it should never have been used for. The support page cautions against using COPILOT() for numerical calculations or for any task requiring accuracy or reproducibility, recommends native formulas such as SUM and AVERAGE for that work, warns against use in tasks with legal, regulatory or compliance implications, and notes that results for the same prompt can change over time as the underlying model evolves.
Read plainly, a worksheet function whose output can change between recalculations sits awkwardly inside a tool whose entire value is deterministic recalculation. Moving the AI assistance into the side pane, where a person reviews the output before it lands in a cell, resolves that tension.
What breaks on 14 September
Per MC1454373, the function will be removed from Excel on 14 September 2026 and users will no longer be able to create new COPILOT() formulas after that date. A formula that references a removed function no longer evaluates, so cells still containing COPILOT() after the date should be expected to stop producing results rather than quietly keep their last output. Microsoft says no admin action is required, which is true at the tenant level. It is not true for a team whose weekly report has a column of COPILOT() classifications in it.
The 18-day checklist
- 1Identify who could be affected. The function only ever existed for users on the Excel Insider and Frontier preview programs. Ask team leads who has been running preview builds; in most organisations this is a handful of enthusiasts, often in finance and analytics.
- 2Ask those users directly which workbooks they used it in. A one-line message this week beats a broken report discovered in October.
- 3Search inside each nominated workbook. Open it, press Ctrl+F, search for COPILOT( with the Find options set to look within Formulas. That surfaces every live use in the file.
- 4Freeze the outputs you want to keep. Copy each COPILOT() range and use Paste Special, Values to convert results to static data before the date, and paste the prompt text into an adjacent comment or cell so the logic is not lost with the formula.
- 5Rebuild recurring workflows in a supported surface. One-off summarising, classifying and generating moves to the Copilot side pane in Excel. Repeatable data transformation is a strong fit for Edit with Copilot running Python, which Microsoft listed as released for Windows, Mac and web in its Copilot release notes on 25 August 2026.
- 6Update any internal training or documentation that references the function, which is the one action Microsoft's own notice explicitly recommends.
The honest limits of Copilot in Excel for numbers
The retirement is a good moment to reset expectations about where AI belongs in a spreadsheet, because Microsoft's caution on the function's support page applies just as well to how people use the side pane. Calculations, lookups against workbook data and anything with legal, regulatory or compliance weight belong in native, auditable formulas. Copilot earns its place on the text and structure work around those numbers: summarising commentary, classifying free-text rows, drafting the narrative that goes with the model, and writing or explaining the formulas a person then owns. Teams that hold that line get the benefit without ever putting an AI response in the load-bearing cell.
For finance teams working out what the side pane is actually good at day to day, Frontrow maintains a set of tested prompts at /insights/copilot-prompts-for-excel.