OpenAI Compaction Flaw Let Researchers Bypass Hidden Reasoning Protections

Swati KhandelwalOct 27, 2026Artificial Intelligence / Data Security

OpenAI's compaction feature, which shrinks a long conversation to save space, could be turned into a way to copy a model's hidden reasoning out in plain text, security firm Xint has shown.

OpenAI's reasoning models work through a problem before answering, and the service normally returns only the raw reasoning as an encrypted item the user cannot read. The method got past the protections OpenAI added in August against this kind of reasoning theft. OpenAI confirmed the problem and fixed it on October 23, according to Xint.

Xint ran the technique against gpt-5.6-sol, one of OpenAI's reasoning models, using math and algorithm puzzles its researchers wrote themselves. The reasoning it pulled out came from those same requests, and Xint did not claim the method could reach another user's reasoning.

Xint says it cannot prove the copied text is word-for-word the model's real reasoning, because it never had the original to compare against. Two cheaper models each repeated the item and produced the same text, which rules out mistakes in the copying step but leaves open the possibility that compaction reworded the text.

Compaction is an OpenAI feature that uses a model to squeeze a long conversation into a smaller, encrypted item that carries the context forward.