Copilot in Excel edits by default — can we make it plan first on finance workbooks?

You’re right to want a seatbelt here. A finance workbook isn’t a draft email: one confident change to a range or a formula can flow through a forecast, a board pack and a decision before anyone notices. Practitioner threads and accounting forums keep making the same point: the edits look plausible, which is exactly why they need checking.

Here’s what’s going on. Microsoft Support describes three modes in Copilot in Excel: edit, which changes the workbook directly; plan, which produces a step-by-step plan for you to review and confirm before anything changes; and chat, which analyses the data without touching the workbook. Copilot in Excel opens in edit mode by default. Editing reached general availability last winter (roadmap 527835 for the web and 542182 for Windows and Mac, both Launched; Learn’s release notes for 10 March 2026 call it Edit with Copilot in Excel, formerly Agent Mode). Roadmap 560338, Plan Mode for Copilot in Excel, is now Launched for desktop, Mac and web.

Now the part people don’t want to hear. Microsoft’s FAQ says Copilot in Excel is a Microsoft Copilot Chat tool and can’t be turned off independently: the admin controls are the Copilot Chat ones. Learn’s Manage Microsoft Copilot Chat lets you limit Copilot experiences to selected users or groups, or block Copilot Chat for specific users, groups or everyone. I couldn’t find a documented admin setting that forces Plan or Chat mode, or that disables editing for particular files or sites. The mode is the user’s choice in the pane, so this is mostly about habits rather than a switch.

Microsoft is also unusually candid about the limits. Its FAQ says Copilot can misinterpret information or produce inaccurate results and advises against using it for decisions in sensitive areas such as finance; saved Copilot changes are visible to anyone with access to the file, including co-authors in the session; and you can undo them or go back to prior versions. Copilot editing also only works when Calculation Options are set to Automatic.

That last point is where Schofield’s Second Law comes in: data doesn’t really exist unless you have at least two copies of it. In SharePoint and OneDrive, version history is your second copy, and Learn is clear that it’s there to undo unintended changes. Check the finance libraries. The organisation default is 500 major versions with no expiry, but site owners can override it, and Learn warns that an expiry limit can trim every version of a file that hasn’t been edited recently. Changing the organisation default only affects new libraries, so existing finance sites need checking one by one.

Options that actually work:

1. Plan first for anything that matters. Make Plan mode the house rule for budgets, forecasts, reconciliations and board packs, and Chat mode for “tell me what’s in here” questions. Read the plan, run it on a copy, then compare. Best Steady default.
2. Protect the master copy. Keep sign-off workbooks in a library where most people have read access, work on copies, and check version-history limits on those libraries so a bad edit can always be reversed.
3. Scope Copilot by group if you really must. Blocking or limiting Copilot Chat for a group is possible, but it removes far more than Excel editing and pushes people towards unsanctioned tools. Treat it as a short-term measure while you train people.
4. Ban it informally and hope. People will use it anyway, just without a plan step or a review.

12 Panes action / comms: Tell finance: Copilot proposes, a person approves. Use Plan mode, check every changed formula and range, and reconcile totals before you save. Tell site owners: Version history stays on, with no short expiry on finance libraries. Tell the AI administrator: There’s no per-mode switch; access is managed through Copilot Chat controls. Write that down so nobody assumes otherwise. Leadership: Copilot can speed up model-building; it doesn’t replace the review step. This is the Adoption & Rollout pane and the Cost, Control & Value pane working together.

Benefits / risks: Plan mode makes the intended changes visible before they happen, which is the review step finance already understands; edit mode remains the default, so the habit has to be taught; co-authors see saved changes immediately; model choice (including Anthropic models, which an admin must enable as a subprocessor) changes behaviour between sessions; there’s no documented per-mode admin control at the time of writing, so check Learn and Message Centre as this evolves.

I’d suggest a fifteen-minute session with the finance team this month: open a real workbook copy, run the same request in Plan mode and then in edit mode, and restore a prior version together. Once people have seen how easy a rollback is, and how persuasive a wrong formula can look, the plan-first habit tends to stick.

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