public Builder mergeRetrieval(com.google.cloud.vertexai.api.Retrieval value) {
      if (retrievalBuilder_ == null) {
        if (((bitField0_ & 0x00000002) != 0)
            && retrieval_ != null
            && retrieval_ != com.google.cloud.vertexai.api.Retrieval.getDefaultInstance()) {
          getRetrievalBuilder().mergeFrom(value);
        } else {
          retrieval_ = value;
        }
      } else {
        retrievalBuilder_.mergeFrom(value);
      }
      if (retrieval_ != null) {
        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