1. REST transport [tries to use JSON-RPC transport interface](https://github.com/a2aproject/a2a-python/blob/b306e442b35787dcf88fc28a0fd845c07a3703e3/src/a2a/client/transports/rest.py#L64-L69). 2. JSON-RPC transport [uses the first supported interface](https://github.com/a2aproject/a2a-python/blob/b306e442b35787dcf88fc28a0fd845c07a3703e3/src/a2a/client/transports/jsonrpc.py#L61-L67), while there is no requirement of this [in the spec](https://a2a-protocol.org/latest/specification/#831-supported-interfaces-declaration), first one is the default but it can be for other protocol.