New model: Mythos 5.1 added to model registry

Mythos 5.1 joins the model catalog with a 1M context window and effort levels up to max.

Read from

Names in the bundleclaude-mythos-5-1xhighmax

What the documentation says

Documented inapi/about-claude/model-deprecations

Since it was published

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 since 15 Sep 2026 Anthropic's documentation has since written up claude-mythos-5-1, on Messages. - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more` api/beta/messages see the edit

Confirmed since 10 Sep 2026 Anthropic's documentation has since written up max, on Model configuration. Frontmatter effort applies when that skill or subagent is active, overriding the session level but not the environment variable. A [`maxEffortLevel`](/docs/en/settings-reference#maxeffortlevel) or [organization effort cap](#organization-ef… model-config see the edit

Confirmed since 10 Sep 2026 Anthropic's documentation has since written up xhigh, on Model configuration. * The model doesn't support `xhigh` effort model-config see the edit

Added since 10 Sep 2026 A small documentation edit on CLI reference touched a line naming max after this was published. | `--effort` | Set the [effort level](/docs/en/model-config#adjust-effort-level) for the current session. Options: `low`, `medium`, `high`, `xhigh`, `max`, or `ultracode`. Available levels depend on the model. `ultracode` requests `xhigh` … cli-reference see the edit

Added since 10 Sep 2026 A small documentation edit on Agent SDK reference - TypeScript touched a line naming xhigh 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/typescript see the edit

How sure we are

One source agreesOne thing we can check says the same as this entry.

Anthropic's documentation agreesA small documentation edit on Agent SDK reference - TypeScript touched a line naming xhigh after this was published.

See this entry in the whole of v2.1.257 →