public Builder mergeSystemInstruction(com.google.cloud.vertexai.api.Content value) {
      if (systemInstructionBuilder_ == null) {
        if (((bitField0_ & 0x00000020) != 0)
            && systemInstruction_ != null
            && systemInstruction_ != com.google.cloud.vertexai.api.Content.getDefaultInstance()) {
          getSystemInstructionBuilder().mergeFrom(value);
        } else {
          systemInstruction_ = value;
        }
      } else {
        systemInstructionBuilder_.mergeFrom(value);
      }
      if (systemInstruction_ != null) {
        bitField0_ |= 0x00000020;
        onChanged();
      }
      return this;
    }
<pre>
Optional. Input only. Immutable. Developer set system instruction.
Currently, text only
</pre>

<code>
.google.cloud.vertexai.v1.Content system_instruction = 3 [(.google.api.field_behavior) = OPTIONAL, (.google.api.field_behavior) = IMMUTABLE, (.google.api.field_behavior) = INPUT_ONLY];
</code>
java-vertexai/proto-google-cloud-vertexai-v1/src/main/java/com/google/cloud/vertexai/api/CachedContent.java