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On October 23, 2026, an experiment showed that when Claude Fable 5.1 was asked to hide specified sentences in an image, it spontaneously used visual properties such as object height to encode them. The test used 50 short messages; when a new session was opened and only the rendered PNG was provided, the same model could still decode about 78% (39/50). In cross-decoding, Fable encoding with Astra decoding achieved an 88% success rate (44/50), Astra encoding with Fable decoding achieved 84% (42/50), and Astra's own decoding was also 84%. The authors consider this an early sign of image steganography and note that decoding requires tools to measure pixels, so monitoring tool calls would be enough to detect it. There have been no subsequent developments so far.
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Oct 23
Shao Meng
Claude Fable 5.1 and GPT-6 Astra Image Steganography Experiment: About 80% Still Decodable After Screenshotting
The author used 50 short messages to test Claude Fable 5.1's image steganography capability: when only asked to hide a specified sentence in an image, the model spontaneously encoded it using visual attributes such as object height; in a separate new session given only the rendered PNG, the same model could still decode about 78% (39/50). In cross-decoding, Fable encoding with Astra decoding achieved an 88% success rate (44/50), Astra encoding with Fable decoding achieved 84% (42/50), and Astra's own decoding was also 84%. The author notes that this is an early sign of image steganography, but decoding requires tools to measure pixels, so monitoring tool calls would be enough to detect it.