Users are intermittently unable to access Microsoft 365 Copilot Chat using the Microsoft Edge browser

A post-incident report has been published.

Title: Users are intermittently unable to access Microsoft 365 Copilot Chat using the Microsoft Edge browser User impact: Users were intermittently unable to access Microsoft 365 Copilot Chat using the Microsoft Edge browser. More info: Users were reporting intermittent "Bad Request" errors when accessing m365.cloud.microsoft using the Microsoft Edge browser. Final status: We've confirmed with previously affected users that rolling back the offending change successfully mitigated impact. Scope of impact: Any user attempting to access Microsoft 365 Copilot Chat using the Edge browser could have experienced intermittent impact. Start time: Thursday, April 23, 2026, at 12:00 PM UTC End time: Tuesday, April 28, 2026, at 10:00 PM UTC Root cause: A recent regression in the Office Home sign-in experience was introduced as part of the Proactive Authentication feature rollout, which enables Microsoft Edge to perform silent, preemptive sign-in functionality. As part of this feature, a new ‘EdgeAuth’ cookie was added by the browser, containing an encrypted, signed, and Base64-encoded Azure Active Directory (AAD) access token to support non-interactive authentication flows. Even prior to this change, one of the existing Auth cookies used by Office Home was already near platform size due to multiple factors, including encryption overheads. The addition of the ‘EdgeAuth’ cookie on top of existing other cookies caused the total request header size to exceed Azure Front Door (AFD) limits, resulting in intermittent “Request Header Field Too Long” errors and blocking access to Microsoft 365 Copilot. Next steps: - For a more comprehensive list of next steps and actions, please refer to the Post Incident Report document. We'll provide a preliminary Post-Incident Report within two business days and a final Post-Incident Report within five business days.

Title: Users are intermittently unable to access Microsoft 365 Copilot Chat using the Microsoft Edge browser User impact: Users are intermittently unable to access Microsoft 365 Copilot Chat using the Microsoft Edge browser. More info: Users are reporting intermittent "Bad Request" errors when accessing m365.cloud.microsoft using the Microsoft Edge browser. Current status: We're monitoring internal telemetry which indicates that the impact is now resolved. We're continuing to contact previously affected users to ensure no further action is required. Scope of impact: Any user attempting to access Microsoft 365 Copilot Chat using the Edge browser could experience intermittent impact. Start time: Thursday, April 23, 2026, at 12:00 PM UTC Preliminary root cause: A recent change intended to enhance the Microsoft 365 Copilot app sign-in experience unexpectedly caused the impact. Next update by: Wednesday, April 29, 2026, at 7:00 PM UTC

Title: Users are intermittently unable to access Microsoft 365 Copilot Chat using the Microsoft Edge browser User impact: Users are intermittently unable to access Microsoft 365 Copilot Chat using the Microsoft Edge browser. More info: Users are reporting intermittent "Bad Request" errors when accessing m365.cloud.microsoft using the Microsoft Edge browser. Current status: We've rolled back the change we suspect caused impact. We're contacting previously affected users to confirm the issue is now resolved. Scope of impact: Any user attempting to access Microsoft 365 Copilot Chat using the Edge browser could experience intermittent impact. Next update by: Wednesday, April 29, 2026, at 1:00 AM UTC

Title: Users are intermittently unable to access Microsoft 365 Copilot Chat using the Microsoft Edge browser User impact: Users are intermittently unable to access Microsoft 365 Copilot Chat using the Microsoft Edge browser. More info: Users are reporting intermittent "Bad Request" errors when accessing m365.cloud.microsoft using the Microsoft Edge browser. Current status: Our analysis of client side logs has identified that users are intermittently unable to access the chat feature due to the requests being rejected because browser cookie data is too large and exceeding supported request header limits. We’ve rolled back a recent change that increased cookie creation and are closely monitoring service telemetry to ensure it resolves the impact. Scope of impact: Any user attempting to access Microsoft 365 Copilot Chat using the Edge browser could experience intermittent impact. Next update by: Tuesday, April 28, 2026, at 8:00 PM UTC

Title: Users are intermittently unable to access Microsoft 365 Copilot Chat using the Microsoft Edge browser User impact: Users are intermittently unable to access Microsoft 365 Copilot Chat using the Microsoft Edge browser. More info: Users are reporting intermittent "Bad Request" errors when accessing m365.cloud.microsoft using the Microsoft Edge browser. Current status: We’re analyzing service telemetry and reviewing any recent changes which may have resulted in impact to determine the next steps required to resolve the issue. Scope of impact: Any user attempting to access Microsoft 365 Copilot Chat using the Edge browser could experience intermittent impact. Next update by: Tuesday, April 28, 2026, at 9:30 AM UTC

Title: Users are intermittently unable to access Microsoft 365 Copilot Chat using the Microsoft Edge browser User impact: Users are intermittently unable to access Microsoft 365 Copilot Chat using the Microsoft Edge browser. More info: Users are reporting intermittent "Bad Request" errors when accessing m365.cloud.microsoft using the Microsoft Edge browser. Current status: We're investigating a potential issue with Microsoft 365 Copilot Chat and checking for impact to your organization. We'll provide an update within 30 minutes.