Copilot in Outlook Summary Missing for Long Email Thread: Fix

When you open a long email thread in Outlook, the Copilot summary box at the top may not appear. This leaves you scrolling through dozens of messages to find the key points. The issue usually occurs because Copilot cannot process threads that exceed its token limit or that contain unsupported content. This article explains why the summary is missing and provides specific steps to restore it.

Why Copilot in Outlook Stops Generating Summaries for Long Threads

Copilot in Outlook summarizes email threads by analyzing the text content of every message in the thread. The underlying AI model has a token limit, which is the maximum number of words and punctuation marks it can process in one request. When a thread exceeds roughly 15,000 tokens, Copilot cannot return a summary. Tokens accumulate from long email bodies, forwarded messages, inline replies, and attached text files that are converted to text.

A second cause is unsupported content inside a thread. Copilot can only read text, tables, and simple formatting. If a thread contains images with embedded text, PDF attachments, scanned documents, or emails with heavy HTML formatting, Copilot skips the entire thread. The summary box then shows nothing or the message “This conversation cannot be summarized.”

A third cause is a licensing or configuration issue. Copilot for Microsoft 365 requires an active subscription and a Microsoft 365 account that has the Copilot service enabled by an administrator. If the license is missing or the service is disabled, the summary box does not appear at all.

Steps to Fix the Missing Copilot Summary in Outlook

Follow these steps in order. Test after each step by opening a long thread and waiting 3 seconds for the summary to appear.

1. Check the thread length manually
   Select the thread and press Ctrl+A to highlight all messages. Look at the status bar at the bottom of the Outlook window. If the thread contains more than 200 messages or the total text is very long, Copilot may be blocked by the token limit. Try opening a shorter thread to confirm Copilot is working.
2. Remove large attachments from the thread
   Open each email in the thread. Delete any PDF, DOCX, or image files that are attached. Right-click the attachment and select Remove. After removing large attachments, close and reopen the thread.
3. Enable Copilot in the Outlook ribbon
   Go to Outlook > Home > Copilot. If the button is grayed out, click it and select Show Copilot. The summary box should appear at the top of the reading pane. If the button is missing, proceed to the next step.
4. Verify your Copilot license in the Microsoft 365 admin center
   Sign in to admin.microsoft.com. Go to Billing > Your products. Find Copilot for Microsoft 365 and confirm the status shows Active. If the license is expired or not assigned, contact your IT administrator.
5. Disable third-party COM add-ins
   Go to Outlook > File > Options > Add-ins. At the bottom, next to Manage, select COM Add-ins and click Go. Uncheck all add-ins except Microsoft Outlook Social Connector and Microsoft Exchange. Click OK. Restart Outlook and test the summary again.
6. Turn off experimental features in Copilot settings
   Open any email. Click the Copilot icon in the top-right corner of the Outlook window. Select the gear icon to open Settings. Under Experimental, turn off any toggle that says “Enable early preview features.” Close the settings pane and test the thread.
7. Run the Microsoft Support and Recovery Assistant
   Download and install the Microsoft Support and Recovery Assistant from the Microsoft website. Select Outlook > I have a Copilot issue. Follow the on-screen prompts. The tool will check your license, service health, and client configuration.
8. Repair your Outlook profile
   Go to Control Panel > Mail > Show Profiles. Select your profile and click Properties. Click Email Accounts > Repair. Follow the wizard. This fixes corrupted profile data that can block Copilot.

If Copilot Still Has Issues After the Main Fix

Copilot summary appears for short threads but not for long ones

This confirms the token limit is the cause. There is no setting to increase the token limit. Work around it by splitting the thread. Archive messages older than 6 months into a separate folder. Or ask the thread participants to start a new thread for a new topic. Copilot will then summarize the shorter new thread.

Copilot summary shows “This conversation cannot be summarized”

This message appears when the thread contains unsupported content. Look for emails that contain only images, scanned PDFs, or emails that were forwarded from another email system with heavy HTML. Delete those specific messages from the thread. If you cannot delete them, move the entire thread to a folder named “No summary” and ask the sender to resend key information in a plain text email.

Copilot summary box is completely missing even for new emails

This indicates a broader configuration issue. First, check that you are using the new Outlook for Windows or Outlook on the web. The classic Outlook for Windows does not support Copilot summaries. Switch to the new Outlook by toggling the “Try the new Outlook” slider at the top-right of the Outlook window. If the summary still does not appear, run the Microsoft Support and Recovery Assistant as described in step 7 above.

Copilot summary appears but is blank or shows an error

A blank summary often means a network or service issue. Check the Microsoft 365 service health dashboard at admin.microsoft.com > Health > Service health. Look for any advisory under Copilot. If the service is healthy, clear the Outlook cache. Go to Outlook > File > Options > General > Empty Auto-Complete List. Also delete the folder %localappdata%\Microsoft\Outlook\RoamCache and restart Outlook.

Copilot Summary in Outlook vs Manual Search: Key Differences

Item	Copilot Summary	Manual Search
Time to find key points	2-5 seconds	5-20 minutes
Handles long threads	Up to 15,000 tokens	Unlimited
Supports attachments	Text only	Any format
Requires internet	Yes	No
License needed	Copilot for Microsoft 365	None

You can now identify why Copilot summaries are missing and apply the correct fix. Start by checking the thread length and removing large attachments. If the problem persists, verify your license and disable conflicting add-ins. For threads that are too long, split the conversation into smaller threads. As an advanced tip, use Outlook Quick Steps to automatically archive threads older than 30 days so Copilot can summarize active threads without hitting the token limit.