public Builder setRetrieval(com.google.cloud.vertexai.api.Retrieval value) {
      if (retrievalBuilder_ == null) {
        if (value == null) {
          throw new NullPointerException();
        }
        retrieval_ = value;
      } else {
        retrievalBuilder_.setMessage(value);
      }
      bitField0_ |= 0x00000002;
      onChanged();
      return this;
    }
<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