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Claude Code 2.1.299 Release Notes Add Idle Compaction Ahead of One-Hour Prompt-Cache Expiry and List Several Sandbox Fixes

Anthropic's 2.1.299 release notes for Claude Code add idle compaction for long idle terminal sessions and list fixes to sandbox symlink, glob and plugin-directory handling.

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Overview

Claude Code 2.1.299, published on October 14, 2026 at 02:49 UTC according to its GitHub release page, adds a terminal behavior called idle compaction and lists a group of fixes to how the agent’s sandbox handles paths and settings. The items below are Anthropic’s own descriptions from the release notes; the notes do not say whether any of the fixed behaviors was exploited, and they carry no severity ratings.

What We Know

Idle compaction

The release notes list this entry: “Added idle compaction in the terminal: a 200K+ token conversation left idle for about 54 minutes is compacted before its one-hour prompt cache expires, so your next message doesn’t reprocess it all”, according to the release notes on GitHub. The same entry appears in the Claude Code changelog.

A separate entry says CLAUDE_CODE_IDLE_COMPACT_MIN_TOKENS changes the 200K-token minimum, and that "idleCompaction": false in settings.json turns the feature off. That entry ends: “Both need v2.1.290 or later”, per the release notes. Idle compaction is listed as newly added in 2.1.299, so it is unclear from the notes alone how that version floor relates to the feature itself.

The reference to a one-hour cache relates to Anthropic’s prompt caching, which its documentation says has a default lifetime of 5 minutes, refreshed each time the cached content is used, with “a 1-hour cache duration at additional cost” also offered. The release notes do not say how Claude Code chooses between the two durations, and the 54-minute figure is Anthropic’s description rather than something measured independently.

Several entries in the 2.1.299 notes concern the sandbox that confines shell commands run by the agent. Anthropic’s sandboxing documentation says the sandbox uses the built-in Seatbelt framework on macOS and relies on bubblewrap and socat on Linux and WSL2. The fixes listed in the release notes include:

  • “Fixed a sandboxed shell command being able to write into a --plugin-dir folder, whose code Claude Code reloads and runs outside the sandbox”
  • “Fixed sandboxed commands on macOS being able to read or write a denied path by its real location when the denied path passes through a symlink”
  • “Fixed sandbox glob allowRead and allowWrite entries not being re-checked when the directory they start from is replaced by a symlink”
  • “Fixed sandbox denyRead and allowRead globs with braces, parentheses or backslashes in a folder’s name being read differently by Claude Code’s settings checks than by the sandbox”
  • “Fixed /sandbox reporting success when its setting could not be saved because the project folder is not writable; it now says nothing changed and why”
  • “Fixed sandboxed commands failing on WSL2 with bwrap: Can't mkdir /mnt/c/Program Files/ClaudeCode when the sandbox can write to the Windows drive”
  • “Fixed sign-in, MCP servers and project trust appearing lost on Linux after a sandboxed command left an empty .config.json in the config directory”

The same notes also say: “Fixed worktree-isolated sessions accepting some awk programs the isolation guard should refuse”. That entry concerns the isolation guard for worktree sessions rather than the sandbox, according to the release notes.

Other additions

Per the release notes, 2.1.299 also adds a CLAUDE_CODE_DISABLE_FILE_MENTIONS environment variable, after which “Claude gets the paths of @ file mentions to read with its file tools, not the files’ contents”. It adds /usage warnings for “max effort, workflows, always-on context and large tool results”. One entry changes shell-command prompting: “An rm built at run time in a script Claude Code can’t read no longer prompts unless CLAUDE_CODE_STRICT_INLINE_SHELL_RM_PROMPT=1”.

What We Don’t Know

  • The release notes do not say how long the listed sandbox behaviors existed, how many installations were affected, or whether any of them was used against a real system. No advisory or CVE is cited in the notes.
  • The notes describe each fix in one line and do not say how it was verified; the behavior described has not been independently reproduced here.
  • The notes do not state how idle compaction decides that a conversation is idle, or what a user sees when it runs.
  • The notes list no GitHub release for versions 2.1.297 and 2.1.298; their contents, if they were distributed, are not covered here.

Context

The release follows the hook-enforcement changes in 2.1.294 to 2.1.296, which The Machine Herald previously reported. That coverage concerned hooks that failed to block actions; the 2.1.299 sandbox entries concern a different mechanism, the filesystem confinement of shell commands, and the notes do not link the two sets of changes.