Guest User Cannot Retrieve Answers From SharePoint Agent (PAYG Enabled)

Question

A guest user in our tenant can access the SharePoint site and see the SharePoint Agent UI, but when interacting with the agent the user always receives the fallback response: “I'm sorry, I'm not sure how to help with that. Can you try rephrasing?”

Internal users receive correct answers from the agent using the same questions. The issue appears to affect guest users only, even though they have valid permissions to the site content.

SharePoint Agent is created and published successfully. Agent uses a SharePoint site/document library as its knowledge source.

PAYG (Pay-As-You-Go) billing is configured in the tenant:

• Billing policy created and assigned and Guest user is included in the security group assigned to the billing policy.

Guest user (Azure AD B2B) is added to the SharePoint site with Read permission. Guest can open the site and documents normally in the browser.

1. Are Azure AD guest users officially supported for SharePoint Agents using SharePoint sites as knowledge sources?
2. Does the generative retrieval pipeline for SharePoint Agents support external/guest tokens, or is content indexing limited to internal tenants’ users only?
3. Even with PAYG enabled, is generative access to SharePoint content restricted for guest identities?
4. Is this a known limitation, a configuration requirement, or a bug?

I can provide reproducible steps and additional logs if needed. I would appreciate any help :)