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 365 channel.

I've noticed a significant UX discrepancy when it comes to file and image attachments, and I want to confirm my understanding and check whether any workaround or roadmap item exists.

What works:

• ✅ File/image uploads work as expected in the Copilot Studio test pane
• ✅ File/image uploads work in Teams chat when interacting with the agent

What doesn't work:

• ❌ File/image uploads do not reach the agent when interacting via the Microsoft 365 Copilot app (both the desktop app and the web experience at microsoft365.com)

The UX problem:

The M365 Copilot app presents a "+" button in the chat input area with options including "Upload" and "Take screenshot." Users naturally assume these options work. The file even appears as an attachment in the sent message — but the agent never receives it. There's no warning, error, or indication to the user that the attachment was silently dropped.

This creates a misleading experience, particularly for end users who have no visibility into the channel behavior differences.

What I've found so far:

I'm aware this is documented as a known issue for custom engine agents in the Microsoft 365 Copilot Extensibility Known Issues page:

"File attachments — Users can't upload files in agent chats and the agent can't return files for download."

I also found a related GitHub issue (OfficeDev/microsoft-365-agents-toolkit #15325) where a Microsoft team member confirmed this is a "Copilot platform shortage" — not an Agents Toolkit issue — with no published ETA.

My questions for the community and any Microsoft product team members:

1. Is there any currently supported workaround to enable file/image input for a Copilot Studio agent running in the M365 Copilot app (desktop or web)? For example, any manifest configuration, agent settings, or alternate approach?
2. Is this limitation being actively worked on? Is there a roadmap item or Microsoft 365 feature ID that can be tracked for when file attachment support is extended to custom engine agents in the M365 Copilot chat experience?
3. Is the UI behavior (showing upload options that don't work) being addressed separately? Even if full file processing isn't ready, a visible warning or disabled state in the UI would significantly reduce user confusion.

Any insight from others who have hit this — or from Microsoft PMs — is appreciated. Happy to share more configuration details if helpful.

Thanks!

Brian