OpenAI Compaction Flaw Let Researchers Bypass Hidden Reasoning Protections

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.