Or, for the Project Glasswing model with the same capabilities: changedmodels/fable-5-1/migration-guide
Nearest release: v2.1.291, published 3 hours after this site recorded the change. Shown because the two are within 24 hours of each other. Nothing here says the release caused the edit.
Recorded here
Lines+0added
Lines−1removed
From line
33
where the diff opens
First seen
1 Sep 2026
this site's first read of the page
Recorded edits16to this page, all time
The whole hunk
from line 33, old and new numbered
/
from line 33
3333Where the two models diverge:
3434
3535* **Availability:** Claude Fable 5.1 doesn't require access approval. Claude Mythos 5.1 is available only to approved customers in [Project Glasswing](https://anthropic.com/glasswing). Contact your Anthropic account team for access.
36* **Safety classifiers:** Claude Fable 5.1 runs safety classifiers covering the same `stop_details` categories as Claude Fable 5. A declined request returns `stop_reason: "refusal"` with a `stop_details.category`, and can fall back to another model with the `fallbacks` parameter or a client-side retry. See [Refusals and fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback).
3736* **Priority Tier:** Neither model is supported on [Priority Tier](https://platform.claude.com/docs/en/api/service-tiers#supported-models). Claude Fable 5 is.
3837
3938## Migrating to Claude Fable 5.1 from Claude Fable 5
No line in this hunk matches that.