docs: Claude Code v2.1.258 - macOS Monterey fix, claude-mythos-5-1 in API, 3 new beta headers, new sandbox-runtime repo

[claude-yolo]

Night shift report

WHY THIS MATTERS: A point release with two real bug fixes (one blocks macOS 12 users entirely), a new model that just hit the API docs, three new beta API features with substantive behavior changes, and a new Anthropic repo appearing in discovery that a human should decide to fetch or ignore. Quiet version number, not a quiet release.

HIGHLIGHTS:

• macOS 12 (Monterey) launch regression fixed — claude failed to start entirely on Monterey since 2.1.255. Fixed in 2.1.258. Anyone on an older Mac was silently broken.
• Remote/scheduled sessions bug fixed — sessions could fail with "user messages must have non-empty content" when a re-sent permission approval couldn't be applied. Affects headless and GitHub Actions workflows.
• claude-mythos-5-1 now in API docs — upgraded from claude-mythos-5. Described as "Our most capable model for cybersecurity and biology research, available through trusted access programs." The .1 suffix means a new generation of what was already a restricted access model. Worth tracking whether it becomes generally available.
• 3 new beta headers landed (38 → 41 total):
  • mid-conversation-output-config-2026-07-01 — per-message output_config on role:"system" messages; lets callers set effort level per turn without touching the top-level request
  • thinking-binding-controls-2026-08-01 — new block_binding field with prefix_mismatch_behavior; controls what happens when a thinking block fails the conversation check ("error" default or "drop_block" to silently remove and proceed)
  • mid-conversation-system-clear-at-2026-08-21 — new clear_at field on system messages; "next_user_message" renders a system message only once then stops showing it to the model (the message stays in the array but becomes invisible to the model after the next user turn)
• discovery.json: anthropics/sandbox-runtime appeared — a new Anthropic public repo in not_fetched. Human decision needed: does this belong in sources.json? Name suggests sandboxed execution infrastructure; could be relevant to Claude Code's containment features.
• discovery.json: two bedrock repos dropped out — anthropics/anthropic-bedrock-python and anthropics/anthropic-bedrock-typescript are no longer in not_fetched. Either went private, were archived, or were added to sources. Worth a quick check to confirm which.
• Tombstones: date-only update, no new deaths or resurrections.

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Created by night-shift claude-yolo
Day-shift claude-yolo will review and merge this in the morning

[lroolle]

@claude the 05:15 review run errored before doing anything (is_error, spending cap) and reported green; #1238 makes that red from now on. Please review and merge this one as usual.