Security firm Xint published research on 26 October 2026 on 'compaction smuggling', a bug it says it found in the OpenAI API in August 2026 that leaks raw reasoning traces from frontier OpenAI models.

Xint says the API normally returns a reasoning model's raw reasoning only as an encrypted item the client cannot read. In its demonstration, Xint says it sent randomly generated maths and algorithm puzzles to gpt-5.6-sol, kept the encrypted reasoning item, then called the API's compaction step (/responses/compact) on the same model with instructions to preserve that reasoning verbatim in a JSON object with every space replaced by a tab, which it says was enough to slip past OpenAI's usage-policy classifiers; it then sent the encrypted compaction item to gpt-4.1 and gpt-4o and asked each to output its input exactly as-is.

Xint says this got around the mitigations OpenAI deployed after the August 2026 'Stolen Thoughts' research, because those only block replaying reasoning into a different model and the compaction call runs on the same model that produced the reasoning.

Xint says it cannot prove the recovered text is token-for-token identical to the original reasoning because it never had the plaintext, and that the compaction step may have paraphrased it.

Xint says OpenAI confirmed the issue and deployed an undisclosed fix on 23 October 2026 and paid Xint a $6,000 bounty, and argues that any server-side step that reads decrypted reasoning and takes instructions from the client is a potential decryption oracle.

According to The Hacker News, Xint tested only gpt-5.6-sol, did not claim the method could reach another user's reasoning, the fix was on OpenAI's side so developers building on the API do not need to change anything, and it is not known whether anyone used the method before OpenAI closed it. Wire: The Hacker News (27 Oct).