When a new model ships, you usually notice it in Claude Code the same day. Same story here: on September 1, Fable 5.1’s launch day, two versions landed right behind it.
The main act is 2.1.257. Fable 5.1 is now the default Fable model, with a one-million-token context window. The new pricing is baked in too: ten dollars per million input, fifty per million output, and – this is the real lever – just 25 cents per million tokens for cache reads. If you work with long context and lots of tools, you pay noticeably less without changing anything yourself. As an aside: in Claude Code, Fable 5.1 runs on high effort by default, while in Cowork and on claude.ai it’s medium.
The second feature in 2.1.257 is one of those small things people have waited a surprisingly long time for: configurable time formats. With timeFormat and timeZone you can now decide whether timestamps show up in 12- or 24-hour format, run in UTC, or follow your own strftime pattern. It affects the clock at the end of a turn and the timestamps in transcript view. Not spectacular, but if you sit in a different time zone, it’s a relief.
There’s also a new auto-mode rule against so-called containment escapes. It blocks attempts to reach cloud metadata credentials and restricts cross-tenant access. That fits the bigger security theme of these days: more autonomy only with more guardrails.
2.1.258 is maintenance again, but the important kind. It fixes a startup failure that stopped Claude Code from launching at all on macOS 12 Monterey – a regression from one of the prior versions. If you work on an older Mac, you simply couldn’t get in anymore. Also gone: a bug where remote and scheduled sessions failed with the message ‘user messages must have non-empty content’ after an approval was re-sent.
My advice, as always with these double releases: just install both. The exciting part is Fable 5.1 itself anyway, and the fastest route to it runs through 2.1.257. You take the Monterey fix in 2.1.258 gladly if you’ve got an older Mac in play.
Sources: Claude Code Changelog · Releasebot: Claude Code · Claude Fable 5.1 system prompts