public Builder clearRetrieval() {
      bitField0_ = (bitField0_ & ~0x00000002);
      retrieval_ = null;
      if (retrievalBuilder_ != null) {
        retrievalBuilder_.dispose();
        retrievalBuilder_ = null;
      }
      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