Mention chat agent API

release-notes/v1_85.md

@@ -322,6 +322,10 @@ You cannot publish an extension that uses a proposed API. There may be breaking
 
 This iteration we revived work on test coverage, with initial UI integration and some minor tweaks to the longstanding proposal. While the API is too lengthy to include here, we believe it to be fairly straightforward, and would welcome your input on the proposal in [issue #123713](https://github.com/microsoft/vscode/issues/123713).
 
+### Chat Agents
+
+As mentioned in our recent blog post, [Pursuit of "wicked smartness" in VS Code](https://code.visualstudio.com/blogs/2023/11/13/vscode-copilot-smarter#_extensibility), we have been developing a model for extensions to contribute chat agents to the Copilot chat pane. The chat agent API is proposed, but you can experiment with adding your own chat agent now. Subscribe to [issue #199908](https://github.com/microsoft/vscode/issues/199908) for updates.
+
 ## Engineering
 
 ## Notable fixes