Microsoft 365 Alert – Service Degradation – Microsoft Copilot (Microsoft 365) – Users’ Copilot for Sales add-in may not appear on Outlook mobile and Outlook for Mac clients – RESOLVED

Announcements Arpan Das

01/04/2026 09:10:00 AM

NHSmail Reference: INC46787832

Microsoft Reference: CP1266855

Issue Status: RESOLVED

Issue Description: Users’ prompts in GPT 5.4 Think Deeper mode in Microsoft 365 Copilot may have returned a failure response.

More info: This event only impacted GPT 5.4 Think Deeper, and other models could’ve been utilized to circumvent impact. Additionally, this issue affected users in all regions except the European Union.

Final Update: 01/04/2026 08:50:00 AM– Microsoft has confirmed that their rollback of the responsible change has successfully mitigated this impact.

Scope of impact: This event may have impacted users in any region outside the European Union using GPT 5.4 Think Deeper Mode in Copilot, who may have gotten prompt response failures.

Root cause: A recent OpenAI code change introduced a regression to the request flow utilized by the GPT 5.4 Think Deeper mode, resulting in this impact.

Next Steps: Microsoft are revewing the responsible code change to better understand what led to the error that resulted in impact and prevent similar future occurrences.