Can my agent use flows as tools when I'm a licensed M365 Copilot user?

I tried to create an agent in Copilot Studio which drafts responses to emails I receive in Outlook. 

There is no "draft a reply" tool, there is only "Send a reply" or "Draft a message". I don't want an AI agent to immediately send out email replies, I want to review them first, but I also would like to review them in the context of the original message (as opposed to having a bunch of messages in the drafts folder with no visible connection to the original email I received - like the "Draft a message" tool does). 

So I "added a tool" (which would be a "Flow") to the agent which just does 2 HTTP calls to the outlook graph api (one creates the reply, the second adds the generated content to the body).

The flow checker tells me:

More Copilot Credits are needed for this flow to run. Runs from agents by M365 Copilot users and testing don't consume credits.

I am a "M365 Copilot user", so I'd expect this to work, and manually testing the flow works. 

However, when the agent tries to run the flow, it's being blocked with the error:

The environment 'Default-<...>' does not have sufficient Copilot Credits to run workflows.

So, can an agent by a M365 Copilot user run flows? 

Alternatively: is there a way to draft email responses which in Outlook end up visually connected to the original message?