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

Title: Users may be unable to access Microsoft Copilot Chat in the browser and may encounter a 400 Request Header error User impact: Users may have been unable to access Copilot Chat in the browser and may have encountered a 400 Request Header error. More info: As a temporary workaround, affected users may have been able to restore access to Microsoft Copilot Chat by clearing their browser cookies and then trying again. Final status: We've completed deployment of our fix to address the browser authentication information misconfiguration and validated after completing an internal monitoring period that our actions successfully remediated impact. If users are still experiencing impact, we advise them to clear their browser cookies and reload Copilot Chat. Scope of impact: Impact was specific to some users attempting to access Microsoft Copilot Chat in the browser. Start time: Friday, July 31, 2026, at 8:00 PM UTC End time: Wednesday, August 5, 2026, at 4:45 AM UTC Root cause: A recent service change affected the way browser authentication information was stored for Microsoft Copilot Chat. This could cause some browser requests to exceed expected limits, which may have prevented users from accessing the service. Next steps: -We're analyzing the offending service change to isolate why the potential for impact wasn't caught prior to the update being deployed to affected users. This analysis will help us to improve our update implementation process moving forward to ensure similar impact scenarios don't occur in the future. This is the final update for the event.

Title: Users may be unable to access Microsoft Copilot Chat in the browser and may encounter a 400 Request Header error User impact: 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 Chat by clearing their browser cookies and then trying again. Current status: Our fix deployment to address the browser authentication information misconfiguration is ongoing, and we anticipate deployment to be completed by our next scheduled update. Scope of impact: Impact is specific to some users attempting to access Microsoft Copilot Chat in the browser. Start time: Friday, July 31, 2026, at 8:00 PM UTC Root cause: A recent service change that affects the way browser authentication information is stored for Microsoft Copilot Chat. 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

Title: Users may be unable to access Microsoft Copilot Chat in the browser and may encounter a 400 Request Header error User impact: 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 Chat by clearing their browser cookies and then trying again. Current status: We've identified the root cause of the issue as a recent service change that affects the way browser authentication information is stored for Microsoft Copilot Chat. This can cause some browser requests to exceed expected limits, which may prevent users from accessing the service. We're currently rolling out a fix and will share an update on deployment timeline in our next scheduled communication. Scope of impact: Impact is specific to some users attempting to access Microsoft Copilot Chat in the browser. Start time: Friday, July 31, 2026, at 8:00 PM UTC Root cause: A recent service change that affects the way browser authentication information is stored for Microsoft Copilot Chat. This can cause some browser requests to exceed expected limits, which may prevent users from accessing the service. Next update by: Tuesday, August 4, 2026, at 11:30 PM UTC

Title: Users may be unable to access Microsoft Copilot Chat in the browser and may encounter a 400 Request Header error User impact: 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 Chat by clearing their browser cookies and then trying again. Current status: We're reviewing service monitoring telemetry to isolate the source of the issue and establish a fix. Scope of impact: Impact is specific to some users attempting to access Microsoft Copilot Chat in the browser.