What probably matters to youSection of the release
What
When switching to a model alias with a 1M-token context window (a [1m]-suffixed alias), Claude Code now checks for three specific problems before switching, each with its own clear error message telling you what to do instead:
The target model doesn't support a 1M-token context window.
The alias has no 1M variant for the current mode.
These checks run before the older Opus-1M-specific check that previously handled this.
Why
Previously these failures likely surfaced as a generic error; now each case is distinguished with its own message and telemetry reason, making it clear why a 1M-context switch didn't happen and what to try instead.
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
Confirmed sinceAnthropic's documentation has since written up CLAUDE_CODE_DISABLE_1M_CONTEXT, on Error reference.| `CLAUDE_CODE_DISABLE_1M_CONTEXT is set, but the 200K limit isn't enforced` | [Configuration warnings](#the-200k-limit-isnt-enforced) |errorssee the edit
Confirmed sinceAnthropic's documentation has since written up /model, on All settings.Cap the [effort level](/docs/en/model-config#adjust-effort-level) a session can use, leaving lower levels available. Any higher level runs at the cap instead, including one from `/effort`, the `/model` picker, `--effort`, [`CLAUDE_CODE_EFF…settings-referencesee the edit
Added sinceA small documentation edit on Agent SDK reference - TypeScript touched a line naming /model after this was published.`effortLevel` accepts an [effort level](/docs/en/model-config#adjust-effort-level) name. It also accepts `"ultracode"`, which requests `xhigh` effort with [ultracode](/docs/en/workflows#let-claude-decide-with-ultracode) on. `applyFlagSetti…agent-sdk/typescriptsee the edit
How sure we are
Two sources agreeTwo things we can check say the same as this entry.
Anthropic's documentation agreesA small documentation edit on Agent SDK reference - TypeScript touched a line naming /model after this was published.
Anthropic's release notes agreeFixed /model opusplan[1m] being rejected with "Model not found"
