Should I upgrade?
If you run long sessions or Scheduled Tasks, move to v2.1.311 or later: a missing Compaction summary is retried and a request over 200K tokens now compacts and continues instead of stopping. Short-session users have no urgent reason.
What does ultrathink do now?
Per the release notes, in the interactive terminal through Anthropic's API on Opus 5.5, Sonnet 5.5, Haiku 5.5 or Fable 5.1, a turn containing ultrathink runs at xhigh effort. The notes do not describe other environments such as Bedrock.
I used a settings file to turn DISABLE_COMPACT on and off. What now?
The notes say that in settings files and in a mod's $.env.set it can now only turn Compaction off, not back on. After upgrading, check that compaction still behaves as you expect in your setup.
I'm not an engineer. What is the one-line version?
It is like fixing the tires and the navigation on a car that makes long trips: no new engine, but fewer breakdowns halfway.
Claude Code v2.1.310 (2026-10-23) and v2.1.311 (2026-10-26) carry an unusual number of items about Compaction, effort and permissions. Everything below is from the official changelog, with the version marked on each item; the "what this means" section is my judgment and says so.
DISABLE_COMPACT in settings files and in a mod's $.env.set can now only turn compaction off, not back on; also fixed agent teammates compacting their history, and the rewind menu offering "Summarize from here", when DISABLE_COMPACT is set./teleport and claude --teleport warn when a compacted cloud session loads from its last compaction, so the messages it summarized are not in the local copy.-p, Agent SDK) during compaction also improved when the transcript file is very large.ultrathink in the interactive terminal now runs its turn at xhigh effort on Opus 5.5, Sonnet 5.5, Haiku 5.5 and Fable 5.1 through Anthropic's API; typed while Claude is working, it raises the rest of that turn.ultrathink rainbow appears only where the keyword can raise effort, not when the word is quoted or part of a file path./effort now warns about a prompt cache miss in plan mode when the plan model differs from the session model.effort-2025-11-24 beta value or mid-conversation system messages; Claude Code now retries without them./sandbox and the sandbox startup warning now name Ubuntu's AppArmor restriction on user namespaces, and how to lift it, when it keeps bubblewrap from starting the sandbox.SubagentStop hooks ran about twice a minute for each running Subagent; they now run once when it finishes.CLAUDE_CODE_MODEL_CATALOG=off to turn this off.claude -p and SDK sessions saving transcript messages out of order or keeping messages that were taken back.CLAUDE_CODE_RETRY_WATCHDOG), indefinitely.Neither version has one headline feature; the theme is closing the edges of long and unattended sessions: retrying a failed compaction summary, reading stale files after compaction, and the stop behavior past 200K tokens. If you run long sessions or Scheduled Tasks these are worth upgrading for. DISABLE_COMPACT becoming a one-way switch is good news for teams that use it to force compaction off, but anyone who relied on a settings file to turn it back on will find that no longer works; the notes do not say why it changed. CLAUDE_CODE_MODEL_CATALOG gives Claude Code on cloud providers one more startup dependency on downloads.claude.ai, which matters if your environment restricts outbound traffic; the notes say nothing about the file's contents or about behavior when the fetch fails, and I found no further official explanation.