Claude Code auto-compact replaces the whole conversation with a summary; /autocompact 400k lowers the trigger

Anthropic's Lydia Hallie says Claude Code's auto-compact does not keep the last 1M tokens of a conversation; instead it replaces the entire conversation with a short summary.

On 1M-context models, auto-compact triggers around 967K tokens, and each message before that point re-reads the full conversation, mostly from cache.

Users can run /autocompact 400k to make compaction trigger at 400K tokens instead, so the conversation never grows that large. This is Hallie's clarification, reported on X; the report does not describe any changes to default behavior.