Microsoft 365 Alert – Service Degradation – Microsoft 365 Copilot Chat – Users may be unable to access Microsoft Copilot Chat in the browser and may encounter a 400 Request Header error – ONGOING

Announcements Shagufta Naaz

05/08/2026 08:31:00 AM

NHS.net Connect Reference: INC46900485

Microsoft Reference: CP1447620

Issue Status: ONGOING

Issue Description: Users may be unable to access Microsoft Copilot Chat in the browser and may encounter a 400 Request Header error.

More info: As a temporary workaround, affected users may be able to restore access to Microsoft Copilot by clearing their browser cookies and then trying again.

Current Update: 05/08/2026 08:39:00 AM – Microsoft has received reports from a subset of affected users who have reported that they are unable to access Copilot in their browser and are encountering a 400 Request Header error. They have isolated that impact is caused by a recent service change that affects the way browser authentication information is stored for Copilot. This can cause some browser requests to exceed expected limits, which may prevent users from accessing the service. They have initiated deployment of a fix to address the browser authentication information misconfiguration, and they anticipate deployment to be completed by their next scheduled update.

Scope of impact: Impact is specific to some users attempting to access Copilot in the browser.

Root cause: A recent service change that affects the way browser authentication information is stored for Copilot. This can cause some browser requests to exceed expected limits, which may prevent users from accessing the service.

Next update by: Wednesday, August 5, 2026, at 7:00 PM UTC