Comment by furyofantares

OpenAI has some magic they do on their standalone endpoint (/responses/compact) just for compaction, where they keep all the user messages and replace the agent messages or reasoning with embeddings.

> This list includes a special type=compaction item with an opaque encrypted_content item that preserves the model’s latent understanding of the original conversation.

Some prior discussion here https://openai.com/index/unrolling-the-codex-agent-loop/