When you reply to an email in Outlook with Copilot, you expect a fresh draft based on the latest context. Instead, an old reply from a previous email thread sometimes reappears in the compose window. This happens because Copilot caches its generated suggestions locally and on Microsoft 365 servers to improve performance. The cached suggestion is not automatically cleared when you start a new reply in the same thread. This article explains the caching mechanism that causes old replies to reappear and shows you how to clear the cache or bypass the stale output.
Key Takeaways: Copilot Caching and Stale Replies in Outlook
- Outlook compose window > Copilot icon > Discard draft: Manually removes a cached Copilot suggestion before you send a reply.
- Outlook > File > Options > Mail > Compose messages > Copilot cache: Location where local cache files are stored; clearing this folder forces a fresh generation.
- Ctrl+Z immediately after Copilot inserts a draft: Undoes the cached suggestion and lets you re-prompt Copilot for a new reply.
Why Copilot in Outlook Reuses Old Reply Suggestions
Copilot in Outlook uses a two-layer caching system to reduce latency and server load. The first layer is a local cache stored on your Windows computer inside the Outlook profile folder. The second layer is a server-side cache maintained by Microsoft 365 that stores the last generated suggestion for each email thread you interact with.
When you click Reply in an email thread, Copilot checks the thread ID against its server-side cache. If a cached suggestion exists for that thread, Copilot retrieves it instead of generating a new draft. This behavior is by design: it prevents Copilot from calling the generative AI model repeatedly for the same thread. The problem arises because the cache is not invalidated when you start a fresh reply to a new message within the same thread. The cached suggestion from the previous reply reappears, making it look like Copilot is broken or stuck.
The local cache is stored in the following directory:
%LOCALAPPDATA%\Microsoft\Outlook\CopilotCache
This folder contains JSON files named after thread conversation IDs. Each file holds the last generated draft for that thread. The server-side cache has a time-to-live of 15 minutes. After that, a new prompt triggers a fresh generation. Until the TTL expires, Copilot returns the cached response.
Steps to Clear Copilot Cache and Force a New Reply Draft
Use one of the following methods to remove the cached suggestion and generate a new reply. Method 1 works for a single thread. Method 2 clears all cached drafts globally.
Method 1: Discard the Cached Draft in the Compose Window
- Open the stale reply in the compose window
Click Reply on the email thread that shows the old suggestion. Copilot inserts the cached draft automatically. - Click the Copilot icon in the compose toolbar
The icon looks like a sparkle. A menu opens with options including Discard draft and Regenerate. - Select Discard draft
This deletes the cached suggestion from the server-side cache for this thread. The compose window clears. - Click the Copilot icon again and choose Draft with Copilot
Copilot now generates a new reply based on the current message, not the previous one.
Method 2: Delete the Local Copilot Cache Folder
- Close Outlook completely
Outlook must not be running. Check Task Manager to ensure no Outlook process remains. - Open File Explorer and paste this path into the address bar
%LOCALAPPDATA%\Microsoft\Outlook\CopilotCache
Press Enter. You see one or more JSON files. - Select all files in the folder
Press Ctrl+A to select every file. Right-click and choose Delete. - Restart Outlook
Open Outlook and navigate to the email thread. Click Reply. Copilot generates a fresh draft because the local cache is empty.
Method 3: Use the Keyboard Shortcut to Undo the Cached Draft
- Click Reply in the email thread
Copilot inserts the cached draft into the compose window. - Press Ctrl+Z immediately
This undoes the insertion of the cached draft. The compose window becomes empty. - Click the Copilot icon and select Draft with Copilot
Copilot prompts the generative model again because the server-side cache was not cleared but the local undo resets the UI state. A new draft appears.
If Copilot Still Shows Old Replies After Clearing the Cache
Copilot returns the same old draft even after Discard draft
The Discard draft command only clears the server-side cache for the current thread. If the local cache still holds a file for the same thread, Outlook reloads it on the next Reply click. Delete the local cache folder as described in Method 2 to remove the file completely.
Old reply reappears after a few minutes
This indicates that another Outlook client, such as Outlook on the web or Outlook mobile, generated a new draft and synced it to the server. The server-side cache is updated by any client that uses Copilot on the same mailbox. Close Outlook on all devices, clear the local cache on your primary computer, and then regenerate the draft.
Copilot does not generate any draft after clearing the cache
The Copilot service may be temporarily unavailable or the Microsoft 365 license may not include Copilot. Verify your license in the Microsoft 365 admin center under Billing > Licenses. If the license is active, wait 5 minutes and try again.
Copilot Caching Behavior: Local Cache vs Server Cache
| Item | Local Cache | Server Cache |
|---|---|---|
| Storage location | %LOCALAPPDATA%\Microsoft\Outlook\CopilotCache | Microsoft 365 server |
| File format | JSON files named by thread ID | Binary blob tied to thread ID |
| Time-to-live | No expiration; persists until file is deleted | 15 minutes |
| Cleared by | Manual deletion of folder contents | Discard draft command or TTL expiry |
| Effect on other clients | None; local to the machine | Synced to all Outlook clients on the same mailbox |
Understanding the difference between these two cache layers helps you choose the right fix. If the old reply appears only on one computer, clear the local cache. If it appears on all devices, use Discard draft to clear the server cache.
Now you can identify why Copilot in Outlook returns an old reply and apply the correct cache-clearing method. Start with the Discard draft command inside the compose window. If the problem persists, delete the local cache folder while Outlook is closed. For a quick workaround, press Ctrl+Z after Copilot inserts the draft and then re-prompt it. To prevent stale replies entirely, disable the Copilot auto-suggest feature in Outlook settings under Options > Mail > Copilot and uncheck the box labeled Cache previous suggestions.