How to validate extension activation within ClientCallInterceptor?

[pbonito]

I am trying to implement protocol-compliant validation for a UI extension. According to the A2A protocol, the client should confirm that an extension is truly active by checking the server's response headers.

I attempted to use a ClientCallInterceptor for this, but I've encountered a challenge. The intercept() method is designed to modify the request before it is sent. However, once the call is made (especially in send_message_streaming), there doesn't seem to be a corresponding hook to inspect the response headers before the stream is returned to the caller.

Without access to the response headers in the interceptor pipeline, I cannot verify the "handshake" was successful before my extension logic starts processing the payload.

Is there a recommended pattern for using Interceptors to validate extension activation based on response headers? If not, is there an alternative "response-side" middleware hook planned for the SDK to handle protocol handshakes?

Thanks