Microsoft 365 Copilot Custom Engine Agents to Get File Uploads on Web, Targeting January 2027

That date needs some caution. An independent Message Center archive shows the entry was first published on October 16, 2025, with an earlier target of December 2025. It was revised on November 11, 2026. The feature has already slipped about a year. Microsoft also says on the roadmap page that all information is subject to change, so treat January 2027 as a forecast.

The gap this closes​

Microsoft's own documentation describes the current limitation clearly. The Microsoft 365 Copilot extensibility known-issues page says custom engine agents don't support file or image attachments in agent chats. Microsoft also says these issues apply to custom engine agents built using the Microsoft 365 Agents Toolkit and the Microsoft 365 Agents SDK.

Developers have been running into this for a long time. In a GitHub issue filed against the Microsoft 365 Agents Toolkit (#15325), a developer reported that when users send file attachments or paste inline images in a Copilot chat with a Custom Engine Agent, the bot receives attachments=None in the activity payload. The same report says the exact same bot, same manifest, same code — attachments arrive fine in channels. Only the Copilot chat path drops them. The developer also says this happened with supportsFiles: true set on the bot.

A thread in Microsoft's Tech Community describes the same problem. The poster says a Microsoft team member called it a "Copilot platform shortage" — not an Agents Toolkit issue — with no published ETA. The poster also asked directly whether there was a roadmap item or Microsoft 365 feature ID that can be tracked for the fix. Roadmap 503767 appears to be that item.

In short: files and images that users attach in Microsoft 365 Copilot chats are currently not passed to custom engine agents. The bot gets nothing. Roadmap 503767 is Microsoft's planned fix for the web.

What a custom engine agent is​

Microsoft has two main kinds of agents, and they work differently:

• Declarative agents use Copilot's own orchestrator and models. You configure them with instructions, knowledge and actions.
• Custom engine agents let developers control orchestration, AI models and data integrations. Microsoft says they can be built with Copilot Studio, the Microsoft 365 Agents SDK, the Teams SDK or Microsoft Foundry.

Microsoft's extensibility history shows custom engine agents reached general availability in mid-2025. Organizations use them when Copilot's built-in orchestration isn't enough, for example with a custom-trained model for legal analysis or a LangChain or Semantic Kernel pipeline. Many of those scenarios depend on documents: contract review, invoice extraction, or "summarize this spreadsheet against our policy." An agent that can't receive a document is like a mail sorter with no mail slot.

What the roadmap entry does not cover​

The roadmap entry is one sentence long, so it's worth being clear about what it doesn't promise.

• Downloads are a separate issue. Microsoft's known-issues page also says file downloads aren't supported for custom engine agents in Microsoft 365 Copilot. The same agent renders the file correctly in Microsoft Teams and in the Copilot Studio test panel. Roadmap 503767 only covers files going from the user to the agent. It says nothing about agents returning downloadable files.
• Only Web is listed. The entry names the web platform only. Microsoft also documents that custom engine agents aren't supported in Outlook, Word, Excel, PowerPoint, and the Microsoft Edge browser, and this entry doesn't change that.
• No limits are published yet. Microsoft hasn't said which file types are supported, the maximum size, how many files can go in one prompt, how files are delivered to the agent's endpoint, how long they're retained, or what admin controls apply. Don't assume the 512 MB limit from Agent Builder applies here. That limit is for embedding file content as knowledge in declarative agents, which is a different feature.
• Knowledge files are a different feature. Since June 2025, users have been able to upload files from a device or the cloud as knowledge for an agent built in Microsoft 365 Copilot. That gives an agent persistent reference material when it's built. Roadmap 503767 is about giving an agent a specific file during a conversation.

Conflicting advice about Copilot Studio agents​

Some community answers have muddied this. In one Microsoft Q&A thread, a responder says file input for Copilot Studio agents published to Microsoft Teams and Microsoft 365 Copilot is currently supported. They point to Copilot Studio's "Allow file input from users" setting, which lists those channels. In the same thread, they also link to the custom engine agent limitation and note there is no roadmap date for this feature.

What's actually going on is that a setting appearing in the authoring tool doesn't mean the Microsoft 365 Copilot chat client passes the file to your agent. The developer reports and Microsoft's known-issues page show the Copilot chat path is where files get dropped. Until roadmap 503767 ships, test the exact channel your users will use instead of trusting the configuration screen.

What developers and IT admins can do now​

1. Check where your agent runs. If your users need to attach files, Teams currently handles attachments for the same bot, based on the developer reports above. One Tech Community answer recommended 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.
2. Don't assume the attachments field will be filled. Code that reads activity.attachments should handle an empty or missing value in the Copilot chat channel. Tell the user what happened instead of failing silently.
3. Use what works today. Microsoft says pro-code custom engine agents can reach Microsoft 365 content through Microsoft Graph APIs and the Retrieval API. Asking users to put a file in a known SharePoint or OneDrive location and have the agent fetch it is clumsy, but it works.
4. Plan for testing in 2027. When the feature reaches your tenant, re-test the file types you need, size limits, OneDrive selection behavior, and how uploaded content is handled under Purview and your data-loss-prevention policies. Microsoft already says custom engine agent prompts and responses in Copilot Chat and Teams can be reviewed with Content Search or Microsoft Purview. Whether uploaded files get the same treatment still needs to be confirmed.
5. Watch the roadmap entry and the known-issues page. The clearest sign this has shipped will be Microsoft removing the "File attachments" row from the known-issues page.

In short: keep handling missing attachments in your code, send file-heavy work to Teams or a Graph-based workaround for now, and save your detailed testing for when Microsoft publishes the details.

Analysis​

This is a small roadmap entry with a big impact. Microsoft has spent two years pitching Microsoft 365 Copilot as the front door for agents, including ones built on outside orchestration and outside models. But its most flexible agent type can't accept a PDF in the main Copilot chat, even though the same bot accepts files in Teams. That inconsistency has been hurting adoption.

There's a fair counterpoint: adding user-uploaded files to agents whose backends Microsoft doesn't host raises real data-governance questions. If a custom engine agent's endpoint runs outside Microsoft's boundary, where does a user's confidential spreadsheet end up? Microsoft may have been slow here for good reasons. The year-long slip in the target date suggests the work is harder than adding an upload button.

Still, the direction is right. If Microsoft keeps the January 2027 date (and it has already moved once), custom engine agents on the web will finally be able to receive files from users. Until then, plan around the limitation and test in the channels your users actually use.