Power Platform Community Forum Thread Details

It sounds more like an issue with the Copilot Studio authoring UI than with the instruction itself. Microsoft’s current documentation confirms that you should be able to type / in the Instructions editor and select resources such as agents, tools, variables, topics, or Power Fx expressions.

I’d try these checks:

Refresh Copilot Studio and reopen the parent agent.

Try the same instruction in an InPrivate/Incognito window.
Test another browser, preferably Edge or Chrome.
Temporarily disable browser extensions/ad blockers.
Make sure the child agent/resource you want to reference is available in the same environment and properly connected to the parent agent.
If both the Add button and / autocomplete are unresponsive, open F12 → Console and check for JavaScript errors while clicking Add or typing /.
If this started recently and happens in multiple browsers, it may be a Copilot Studio authoring-side issue. Capture the browser console error and a screenshot/video and raise it with Microsoft Support.

Also, if by PFX you mean Power Fx, you can reference Power Fx expressions from the instruction editor using / when the feature is available for that agent.

If the issue is specifically that the Add button does nothing, I'd focus on the UI/browser/tenant issue first rather than changing the agent instructions.