Outage in Microsoft 365
Users may be unable to access Microsoft Copilot Chat in the browser and may encounter a 400 Request Header error
Minor
July 31, 2026 - Started 4 days ago
Incident Report
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.
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.
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Latest Updates ( sorted recent to last )
about 5 hours ago - at 08/04/2026 09:28AM
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
about 7 hours ago - at 08/04/2026 07:28AM
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.
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