public com.google.cloud.vertexai.api.RetrievalOrBuilder getRetrievalOrBuilder() {
      if (retrievalBuilder_ != null) {
        return retrievalBuilder_.getMessageOrBuilder();
      } else {
        return retrieval_ == null
            ? com.google.cloud.vertexai.api.Retrieval.getDefaultInstance()
            : retrieval_;
      }
    }
<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