RetrievalCallDelta: {
    arguments: RetrievalCallArguments;
    retrieval_type?: RetrievalCallDeltaRetrievalType;
    signature?: string;
    type: "retrieval_call";
}

Used by Vertex Retrieval tools such as Parallel AI, Exa AI, Vertex AI Search,

Type declaration

etc. RetrievalType decides which tool is used.