A new model, Mythos 5.1 (claude-mythos-5-1), has been added to the model catalog. It ships with a 1M context window, support for effort levels up to xhigh/max, and new capability flags including a dedicated prompt bundle.
Documented inapi/about-claude/model-deprecations
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.
Added since
A small documentation edit on claude-on-vertex-ai touched a line naming claude-mythos-5-1 after this was published.
| Claude Mythos 5.1 ([limited availability](https://anthropic.com/glasswing)) | `claude-mythos-5-1` |build-with-claude/claude-on-vertex-ai see the edit
Confirmed since
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
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
Anthropic's documentation has since written up xhigh, on Model configuration.
* The model doesn't support `xhigh` effortmodel-config see the edit
Added since
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
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
One source agreesOne thing we can check says the same as this entry.
Anthropic's documentation agrees
A small documentation edit on Agent SDK reference - TypeScript touched a line naming xhigh after this was published.