Claude Code v2.1.284: Sonnet 5.5 and auto mode as the default#

Published September 28, 2026 by Alex Sorokoletov

Part of the Claude Code Version Tracker series. | Official Env Vars | Official Changelog

Claude Code v2.1.284 adds Claude Sonnet 5.5 (claude-sonnet-5-5) as the default Sonnet model on the Anthropic API, with a 1M-token context window. Coming from v2.1.283, interactive terminal and VS Code sessions now start in auto mode on every plan and provider, and auto mode's prompt before a read outside the working directories gains a "Yes, but ask again next time" answer.^[1]

Sonnet 5.5 on the Anthropic API#

Sonnet 5.5 arrives as the default Sonnet model on the Anthropic API, priced at $2/$10 per Mtok with $0.20/Mtok cache reads and a 1M-token context window.^[1]

Auto mode as the default, with a softer outside-read prompt#

The default permission mode keeps widening. v2.1.283 started third-party-provider and telemetry-off sessions in auto mode when no permission mode was configured; v2.1.284 extends that to interactive terminal and VS Code sessions on every plan and provider.^[1] permissions.defaultMode still overrides it, so confirm that setting matches what you expect.

Auto mode's prompt before a read outside the working directories now offers a fourth answer, "Yes, but ask again next time." It allows that one read and leaves the later ones still gated, rather than blocking the read or opening the whole directory for the session.^[1]

Ultracode splits from xhigh effort#

Ultracode is now its own toggle in /effort (Tab, or /effort ultracode [on|off]) and no longer forces xhigh effort.^[1] Three new keybinding actions, effortSlider:decreaseEffort, effortSlider:increaseEffort and effortSlider:toggleUltracode, make the slider's arrow and Tab keys rebindable in keybindings.json. VS Code gets a matching Ultracode on/off switch under the Effort slider.

Fixes and smaller changes#

The window keeps a long list of reliability fixes. A "Prompt is too long" error that persisted after compacting now triggers one more compaction that keeps less of the recent conversation; a damaged response stream that surfaced raw errors like "JSON Parse error" or "undefined is not an object" is handled; and an overloaded or server error arriving right after a thinking block is now retried instead of ending the turn.^[1] A session whose model is unavailable with no fallback left shows the model-unavailable notice and its Learn more link again rather than a bare message.^[1]

On the operational side, /mcp reconnect all retries every MCP server that failed to connect or needs authentication at once, /rate-limit-options joins /help for claude.ai subscribers, and the Claude apps gateway spend limit now shows dollar amounts in /usage and the status line.^[1]

What these tell us#

Auto mode is now the resting default across every plan and provider, and the new "ask again next time" answer narrows the all-or-nothing choice on outside-directory reads to a single-read allowance. Anyone who relied on the old default should set permissions.defaultMode explicitly.

Splitting Ultracode from xhigh effort, plus the new rebindable slider actions, points at finer per-session control over how hard the model works.

This analysis is conducted for educational and research purposes under fair use principles. All trademarks and software referenced belong to their respective owners.

Sources#

1. Claude Code Official Changelog, v2.1.284 release notes

Related Versions#

• Claude Code v2.1.283: exact model allowlists, model deny rules, and grouped gateway requests. Exact model matching, deny list, gateway prompt-id.
• Claude Code v2.1.269: plugin eval reports and Bash command diffs. Plugin eval scoring, Bash edit diffs.
• Claude Code v2.1.266: Fable 5.1 becomes the default Fable model and a fullscreen /diff panel. Default Fable model, fullscreen diff.
• Claude Code v2.1.252: model-switch hooks and a symlink escape closed on file tools. Model-switch hooks, symlink fix.
• Claude Code v2.1.247: /claude-api cost tooling and org spinner tips loaded from a file. Admin API cost coverage, file-based spinner tips.