How to Fix Copilot Chat ‘Failed to Send Message’ Error

You are typing a message in Copilot Chat and click Send. Instead of a reply, you see the error Failed to send message. This problem stops you from using Copilot in Microsoft 365 apps, Teams, or the standalone chat interface. The error usually appears because of a network interruption, a corrupted browser cache, or an expired authentication token. This article explains the root causes and provides step-by-step fixes to restore Copilot Chat.

Why Copilot Chat Shows the Failed to Send Message Error

Copilot Chat relies on a persistent WebSocket connection to stream responses in real time. When you send a message, the client sends a request to the Microsoft 365 backend. If that connection is broken before the server acknowledges the message, the client displays Failed to send message. Common causes include:

• Network timeouts or packet loss during the handshake.
• Expired or revoked OAuth 2.0 access tokens for Microsoft Graph.
• Corrupted local storage in the browser or Microsoft 365 app.
• Third-party browser extensions that block scripts or modify headers.

Steps to Fix the Failed to Send Message Error

Method 1: Clear Browser Cache and Cookies

1. Open your browser settings
   In Chrome, Edge, or Firefox, click the three-dot menu and select Settings.
2. Navigate to Privacy and Security
   Choose Privacy and security then Clear browsing data.
3. Set the time range to All time
   Select All time from the time range dropdown.
4. Check Cookies and Cached images and files
   Ensure both Cookies and other site data and Cached images and files are checked.
5. Click Clear data
   Close all browser tabs and reopen Copilot Chat. Try sending a message again.

Method 2: Re-authenticate Your Microsoft 365 Account

1. Sign out of Copilot Chat
   In the Copilot pane, click your profile picture and select Sign out.
2. Close all browser windows
   This clears any lingering session tokens.
3. Open a new browser window and navigate to copilot.microsoft.com
   Or open Copilot in your Microsoft 365 app (Word, Teams, etc.).
4. Sign in with your work or school account
   Enter your credentials and complete any multi-factor authentication prompts.
5. Send a test message
   Type a short message and press Enter. The error should no longer appear.

Method 3: Disable VPN or Proxy Temporarily

1. Turn off your VPN client
   Right-click the VPN icon in the system tray and select Disconnect or Turn off.
2. Disable system proxy settings on Windows
   Go to Settings > Network and Internet > Proxy. Toggle Use a proxy server to Off.
3. Restart your browser
   Close and reopen the browser. Load Copilot Chat and send a message.
4. Re-enable VPN after testing
   If Copilot works with VPN off, add an exception for copilot.microsoft.com and all subdomains in your VPN or proxy configuration.

Method 4: Disable Conflicting Browser Extensions

1. Open the extensions management page
   In Chrome, type chrome://extensions in the address bar. In Edge, type edge://extensions.
2. Toggle off all extensions
   Switch each extension to Off.
3. Restart the browser
   Close and reopen the browser. Test Copilot Chat.
4. Re-enable extensions one by one
   Turn on each extension and test Copilot after each. When the error returns, you have found the conflicting extension. Remove or update it.

Method 5: Reset the Microsoft 365 App Cache (Desktop Apps)

1. Close all Microsoft 365 apps
   Ensure Word, Excel, Teams, and Outlook are fully closed.
2. Open the Run dialog
   Press Windows key + R.
3. Type the cache path
   Enter %appdata%\Microsoft\Teams for Teams cache or %localappdata%\Microsoft\Office\16.0\OfficeFileCache for other Office apps.
4. Delete the contents of the folder
   Select all files and folders inside and press Delete. Do not delete the folder itself.
5. Restart the app
   Open the Microsoft 365 app and sign in again. Try Copilot Chat.

If Copilot Still Shows the Error After the Main Fixes

Copilot Chat Works on One Device but Not Another

This suggests a device-specific problem. Compare the network settings on both devices. Check if the failing device has strict firewall rules or content filtering enabled. On Windows, open Windows Security > Firewall and network protection and ensure Microsoft Teams and Office are allowed through the firewall.

Error Appears Only in Microsoft Teams

Teams uses a separate WebSocket endpoint for Copilot. In Teams, go to Settings > Apps > Manage apps and find Copilot. Click the three dots and select Repair. If that does not work, clear the Teams cache by deleting the %appdata%\Microsoft\Teams folder contents.

Error Occurs After a Microsoft 365 Update

An update may have reset permissions or changed the authentication flow. Run the Microsoft 365 Support and Recovery Assistant from aka.ms/SaRA. Choose Office > I signed into Office but it doesn’t work and follow the prompts. This tool re-registers Office components and refreshes tokens.

Copilot Chat Error: Browser Version vs Desktop App Behavior

Item	Browser (copilot.microsoft.com)	Desktop App (Teams, Word)
Primary cause	Corrupted cache or cookies	Expired token or app cache
Quick fix	Clear browser data	Sign out and sign back in
Network dependency	Direct HTTPS and WebSocket	Uses app-specific proxy settings
Extension interference	Common	Rare
Reset method	Browser settings reset	Office cache folder deletion

After applying the fixes above, Copilot Chat should send messages without errors. If the problem persists, check the Microsoft 365 service health dashboard at admin.microsoft.com > Health > Service health. A service incident affecting Copilot may require waiting for Microsoft to deploy a fix. As a final step, you can run the Microsoft 365 Support and Recovery Assistant with the Teams scenario selected to automatically detect and repair authentication issues.