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GullettBrian's avatar
GullettBrian
Iron Contributor
Jun 08, 2026

File Uploads Not Passed to Custom Engine Agent in Microsoft 365 Copilot Chat

Hi all, I'm working with a custom agent built in Copilot Studio (full authoring experience — topics, knowledge sources, agent flows) published to both the Microsoft Teams channel and the Microsoft 3...
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AurelienAtibard's avatar
AurelienAtibard
MCT
Jul 07, 2026

Hi Brian,

Your analysis is accurate. This is a confirmed platform limitation: file attachments simply do not work for custom agents in the M365 Copilot app, regardless of how the agent is configured. No manifest setting or agent option will change that today.

The only documented workaround is routing through a Bot Framework Skill that handles attachments natively, which is a significant effort and probably not worth it for most teams.

There is no published roadmap item or feature ID tracking a fix for this. The file upload improvements that shipped recently are about grounding files at authoring time, which is a completely different thing.

You are right to flag the UX issue separately. A file that appears sent but is silently dropped is genuinely misleading. I have not seen any commitment to fix the upload button behavior on its own, but it is worth pushing on through the Microsoft feedback portal or your account team.

In the meantime, the simplest mitigation is a clear message early in the conversation telling users that file uploads are not supported and directing them to Teams if they need that capability.

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