private void ensureContentsIsMutable() {
      if (!((bitField0_ & 0x00000040) != 0)) {
        contents_ = new java.util.ArrayList<com.google.cloud.vertexai.api.Content>(contents_);
        bitField0_ |= 0x00000040;
      }
    }
<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