Claude Code 2.1.257

• [changed] Artifact workflows are richer and clearer: multi-file artifacts, open actions, and an artifact toolset path are supported; watch/status/unwatch behaviour is spelled out so the agent does not claim to be watching when it is not; list rows distinguish (mine) vs (shared) and call out org/user defaults.
• [changed] Publishing guidance is stricter: do not publish pages that impersonate people or orgs, fabricated records/receipts/reviews, or credential/payment phishing flows — including when framed as a “prop” or “test.” Refusals should not be worked around with alternate hosting suggestions.
• [changed] Skill proposals (new and improvement) document when the skill should fire, require a complete SKILL.md body on save, and make clear that an improvement fully replaces the existing skill file — read first and keep what still matters.
• [removed] Artifact publish no longer takes a separate version note; version picker copy relies on the short label instead.
• [changed] Subagent guidance: background by default when work can continue in parallel; never invent pending-agent results; remote CCR isolation remains an option where available; resumed-from-transcript agents should not be treated as cancelled or relaunched.
• [new] CLI flag --system-prompt-snapshot for capturing the effective system prompt.
• [changed] On corrections, memory writes must happen in the same turn as the diagnosis/fix/offer — not deferred until later.
• [changed] Focus mode, brevity-over-narration, and continuous autonomous execution prompts are tightened: put what the user needs in the final message when focus mode is on; if the only remaining work is waiting on a background task or monitor, end the turn instead of polling.
• [changed] Plugin and connector discovery copy is clearer (SearchPlugins / list installed plugins, ListConnectors / resolve connector payloads from registry UUIDs).
• [changed] “Never delegate understanding” and related task-hand-off rules are reinforced: subagent prompts must carry concrete paths, line numbers, and change intent rather than “based on your findings…”.
• [changed] Git commit and PR attribution lines are turned off going forward (replaces earlier attribution guidance).
• [changed] Permission-denied tool results and several safety/classification preambles are tightened (block-first responses, no preamble before <block> where that path applies).
• [changed] Chrome/browser tool loading: when tools are deferred, batch needed tools into one ToolSearch rather than round-tripping one at a time.
• [changed] Cloud/session edge cases are called out more explicitly (no machine attached / file sync ended; artifact data read/edit unavailable in some cloud sessions; uploads must complete before files exist on the user’s machine).

Models & prompts

• Mid-conversation API betas are wired in: output config, system clear-at, tool changes, and thinking-binding controls (dated 2026-07 / 2026-08 headers).
• Fable naming/alias migration continues (Fable 5.1 / fable alias paths); stale “out of Fable 5 usage credits” copy is dropped in favour of current usage/model flows.
• Session naming, conversation/recent-segment summary, “at a glance” usage insights, workflow-script return-value behaviour (final text is the script result, not chat chit-chat), and SendUserMessage / talking-to-the-user guidance are refreshed.
• Optional model-catalog URL and subagent model-force overrides exist for controlled environments.

Under the hood

• Release manifests can carry manifestSignatureEnforcement; native auto-update paths can skip unverified releases and record signature/cache failures — groundwork for signed binary updates.
• New internal knobs and gates for artifact toolset surfaces, CCR/remote headless workers, per-turn effort, segmented auto-mode transcripts, narration toggles, gateway token via file descriptor, remote tool caller session limits, HIPAA history retention pruning, settings/agent frontmatter mode-widening guards, and related experiments (e.g. stellar drift / humble hammock feature gates).
• Script-hook acknowledgement and a CLAUDE_SCRIPT env group support workflow-script integration.
• Code-execution tool results are optional in the SDK typings when there is nothing to return.
• Assorted dead feature gates and env stubs removed (including old print-engine loop and permission-explainer event paths); unused third-party endpoint strings cleaned up.
• Large pass over computer-use / teach-mode / display-scope prompt fragments (access tiers, Space/off-screen window handling, clipboard, and related copy).

Upgrade notes

• If you automated artifact publishes that sent note, drop that field and put any short version title in label only.
• Skill-improvement flows should re-read the current SKILL.md and submit a full replacement body; partial “diff-only” drafts will overwrite incorrectly.
• Prefer --system-prompt-snapshot when you need to inspect the effective system prompt rather than scraping session logs.
• Environments that pin update behaviour should be aware that unverified native releases may be skipped when signature enforcement is active.