Claude Code 2.1.286+: idle sessions get compacted on their own ("Compacted while idle, before the prompt cache expired") and lose their working context

发生场景

Source(s): https://github.com/anthropics/claude-code/issues/98747 Status: reported working by source, not reproduced by us.

问题

You leave a long Claude Code session idle (lunch, a meeting, overnight) and come back to the line above. The session now sees a summary instead of its working context: exact instructions, decisions and agreed methods are gone, although the session was nowhere near the context limit and you never ran /compact.

报错原文（已脱敏/规范化）

Compacted while idle, before the prompt cache expired

适用环境

Claude Code 2.1.286 to 2.1.289 (native install) on macOS, Windows 11 and WSL2 Ubuntu, large sessions; reported from 2026-10-01 to 2026-10-14.

如何确认是这个问题

1. The session shows Compacted while idle, before the prompt cache expired after a pause and you did not run /compact → expect: present.
2. Optional: in the session transcript, the compaction entry is recorded with trigger: "manual" on 2.1.286 to 2.1.289 (commenters); a collaborator said that from 2.1.290 it is recorded as auto.

不是这个问题时 / 边界

The context really filled up while you were working (normal threshold auto-compaction), or you ran /compact yourself.

根因

From 2.1.286 Claude Code compacts an idle session before its prompt cache expires. A collaborator said it only does this for larger contexts (200k+ tokens); a commenter measured 9 of 9 idle compactions at 55 to 56 minutes after the last response. On 2.1.289 a commenter who read the app found the idle path only checked autoCompactEnabled, so there was no separate off switch. 2.1.290 added the idleCompaction setting (collaborator).

修复步骤

1. Update Claude Code to 2.1.290 or later and check with claude --version → expect: 2.1.290 or higher.
2. Add "idleCompaction": false to your Claude Code settings.json (a commenter said user, project, local or managed settings all work) and start a new session → expect: the original poster confirmed on 2.1.291 that this stopped idle compaction. A commenter said threshold auto-compaction keeps working as long as autoCompactEnabled stays true.
3. Only if you cannot update yet: "autoCompactEnabled": false also stops it (a commenter said it seems to prevent it), but it turns off threshold auto-compaction too.

如何验证

Leave a large session idle for more than an hour, then send a message → expect: no Compacted while idle line and the earlier context is still there.

试过没用的办法

Using a PreCompact hook to protect the context: a commenter explained that hook output only adds instructions for the summary, it cannot keep files in context, and on these versions a hook cannot tell a subagent's compaction from the main session's.

风险提示

With autoCompactEnabled set to false a session can run into the context limit; turn it back on once you are on 2.1.290+ (our inference). A commenter said idleCompaction was still undocumented on 2026-10-10, so the name could change; re-check after updates (our inference).

验证范围

Claude Code 2.1.286 to at least 2.1.291, October 2026.

何时失效

Idle compaction becomes opt-in or the setting is renamed or documented differently. Open question: a commenter said CLAUDE_CODE_IDLE_COMPACT_MIN_TOKENS (default 200000, minimum 100000) sets the size threshold in 2.1.290; nobody else confirmed it.