Claude Fable 5.1 and Mythos 5.1 are Anthropic's first models to watermark text outputs; a detection API is available to eligible groups as required under EU law (Ben Patterson/PCWorld)

Anthropic has launched Claude Fable 5.1, now generally available, and Claude Mythos 5.1 for trusted partners with cybersecurity and life-sciences safeguards, marking the first Anthropic models that embed watermarks in their text outputs. A detection API for those watermarks is being offered to eligible groups to comply with EU regulations.