What
Compaction is how Claude Code shrinks a long conversation by replacing older parts with a summary so it fits in the model's working memory. Previously the rebuilt context after compaction held only attachments and hook results. Two things are different now:
- Dropped tools are recorded: when the rebuild leaves out tools that had been included inline in the conversation, the summary gets a note about them.
- The cached start is kept: when Claude Code summarizes from a chosen message onward, it now keeps the earlier, already-cached part of the conversation as it is.
A bug in which direction a partial compaction ran was also fixed.
Why
This changes what the model sees after a compaction. Keeping the cached start can also let more of the conversation be reused from the cache, which helps speed and cost.