So nice of them to provide a list of workarounds right in the article!

Content generated by Claude may not carry a detectable mark if, for example:

• It was generated by a model released before marking was supported;
• The text has been heavily edited, paraphrased, translated, or mixed into other writing;
• The passage is very short, leaving too little text for a reliable signal;
• A file’s metadata was stripped through format conversion, re-saving, screenshots, or other means;
• It was produced through a platform, feature, or file type where a particular marking type wasn’t supported.