You'll notice
Compaction summaries now flag when the oldest messages had to be dropped
What
When a long conversation gets compacted (summarized to save space) and the very beginning of the conversation was too large to include, the resulting summary now adds an explicit note saying the earliest part isn't covered and instructing the model not to guess about it.
Why
Without this note, the model could unknowingly make things up about what happened at the start of a conversation it never actually saw. The warning makes that gap explicit instead.