private com.google.protobuf.SingleFieldBuilderV3<
            com.google.cloud.vertexai.api.Retrieval,
            com.google.cloud.vertexai.api.Retrieval.Builder,
            com.google.cloud.vertexai.api.RetrievalOrBuilder>
        getRetrievalFieldBuilder() {
      if (retrievalBuilder_ == null) {
        retrievalBuilder_ =
            new com.google.protobuf.SingleFieldBuilderV3<
                com.google.cloud.vertexai.api.Retrieval,
                com.google.cloud.vertexai.api.Retrieval.Builder,
                com.google.cloud.vertexai.api.RetrievalOrBuilder>(
                getRetrieval(), getParentForChildren(), isClean());
        retrieval_ = null;
      }
      return retrievalBuilder_;
    }
<pre>
Optional. Retrieval tool type.
System will always execute the provided retrieval tool(s) to get external
knowledge to answer the prompt. Retrieval results are presented to the
model for generation.
</pre>

<code>
.google.cloud.vertexai.v1.Retrieval retrieval = 2 [(.google.api.field_behavior) = OPTIONAL];
</code>
java-vertexai/proto-google-cloud-vertexai-v1/src/main/java/com/google/cloud/vertexai/api/Tool.java