Claude Code 2.1.299 compacts long idle sessions before the prompt cache expires

The 2.1.299 entry adds idle compaction in the terminal. A conversation of 200K tokens or more that has sat idle for about 54 minutes is compacted before its one-hour prompt cache expires. The changelog gives the reason: "so your next message doesn't reprocess it all." Two controls come with it. CLAUDE_CODE_IDLE_COMPACT_MIN_TOKENS changes the 200K-token minimum, and "idleCompaction": false in settings.json turns the behavior off. The changelog says both need v2.1.290 or later.

The same release adds /usage warnings for max effort, workflows, always-on context and large tool results, and a --disallow-tools option for claude plugin eval that keeps built-in tools such as Agent out of eval runs. We read the 2.1.299 and 2.1.300 sections of the page. The changelog does not say how much the compaction saves, how the summary is built, or whether it applies outside the terminal.

Why it matters: The cache window is the cost lever here. A long session that outlives the one-hour cache is expensive to resume, and Anthropic has chosen to trade that cost for a rewritten history. If a session carries state you need verbatim, such as a long spec or a half-finished plan, decide whether to set idleCompaction to false or write that state to a file first. Whether the compacted session behaves as well as the original is untested in the changelog.