TL;DR

Frontier LLMs are now capable of generating artistic images using SVG. We ask – if these LLMs had to hide a message in an image – how would they do so? And if they hid such messages, could they be successfully decoded?

We show interesting results from testing Fable 5.1 on these questions, and briefly compare with GPT-6 Astra on the same task.

Figure 1: The stars encode a message, read left to right. Each star's height represents its place in the alphabet. Hidden message: The monster acts on the moonshine module.

Introduction

Frontier LLMs can now capably generate images. Whenever asked for an image, Claude Fable 5.1 writes an SVG, which renders as an ordinary image.

The model has full control over every attribute (such as shape, height, width, colour, rotation, etc.). We ask – how does a model hide a message in the image it draws? Additionally, can an instance of the same model read the message from the image without the SVG source code and without any scheme agreed on in advance?

We tested this with Claude Fable 5.1 at maximum reasoning effort:

What encoding scheme does Fable reach for on its own?
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Outline:
(00:14) TL;DR
(00:57) Introduction
(02:05) Types of encodings
(02:50) Height encoding
(03:43) How does the decoding model do it?
(05:08) Patching gaps in encoding style
(07:19) Method
(09:38) Results: What survives a screenshot
(10:08) What does GPT-6 Astra do differently?
(10:57) Implications
(11:34) Prior work
(12:29) Citation
(12:39) Acknowledgments

The original text contained 1 footnote which was omitted from this narration.
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First published:
October 21st, 2026

Source:
https://www.lesswrong.com/posts/WHEQps5PK8LrrcTAb/how-does-fable-5-1-hide-messages-in-images

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Narrated by TYPE III AUDIO.

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“How does Fable 5.1 hide messages in images?” by Nikita Kezin, 13ryanC, hronir, James Chua

LessWrong (30+ Karma)

“How does Fable 5.1 hide messages in images?” by Nikita Kezin, 13ryanC, hronir, James Chua