Developer Ties Animated Short to Anthropic's Unreleased Fable 5.5

A developer posting on X as ChetasLua published an animated short on November 2, 2026 that he attributes to Anthropic's unreleased Fable 5.5 model, according to aitimes.com. The short, titled The Moss Keeper, shows a giant robot covered in moss and flowers waking in a forest and flying into a golden sky.

ChetasLua said the animation was made with purely code written by the LLM, without image or video generation models. He also published a separate animation starring a red fox, which he said was implemented in Python code written by AI, including its music and sound effects.

That second video, he said, runs 66 seconds at 1920×804 and 24fps, involved about 2 hours 40 minutes of autonomous AI work, and used no external images, audio or fonts.

Doubts over the model and the files

The GitHub files for The Moss Keeper contain code that plays a finished MP4 in a web page, not the source code that generated the frames. Some users also noted that the model name Opus 5.5 appeared on the released screen, questioning the Fable 5.5 claim.

Anthropic has not officially announced the existence or capabilities of Fable 5.5. ChetasLua has said he tested unreleased models from Anthropic and OpenAI in advance and has released game and animation results. LLMs such as Claude have recently been used to write code for animations and motion graphics, mostly letters, shapes and 2D and 3D graphics.