Clarify expected Host behavior when multiple tool calls return UIResources

[JorisAndrade]

Summary

The current specification does not clearly define the expected Host behavior when a model executes multiple tool calls, each returning a UIResource.

Questions

1. Rendering multiple UIResources: If the model runs N tool calls, each with an associated UIResource, should the Host render N iframes? Or is it up to the Host to decide how and when to display these UIs?

2. Priority field behavior: The MCP spec includes a priority annotation (0.0 to 1.0), but it's unclear:

   • What is the expected behavior when priority is not set?
   • What should Hosts do when all UIResources have the same priority (e.g., all 1.0)?
   • How does priority apply to UI rendering decisions specifically?

Understanding the intended behavior would help ensure consistent UX across different Host implementations.

[idosal]

Thanks @JorisAndrade ! Currently, the spec implicitly assumes a new instance per invocation. Hosts may decide to implement variations of their own layout (e.g., a persistent side panel), but each invocation would still create a new instance under the hood. This approach was chosen to simplify the model for the MVP (please see #62).

Now that the spec is stable, we can examine deferred approaches, such as referencing previously rendered instances using viewSessionId. We'd love to hear your thoughts and use cases to shape this part of the spec.

MCP Apps doesn't assign any special meaning or behavior to priority.