claude-mythos-5-1
A Claude Code model, read out of the shipped bundle. No mined inventory carries it: everything below comes from what this site has written about it.
A claude-fable-5-1 model id is recognized and becomes the fable family default, with UI text updated to Fable 5.1.
Not Anthropic's. This is the line from the earliest changelog entry here that named it, v2.1.257.
In the changelogs
3Releases whose published page names claude-mythos-5-1.
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v2.1.257
Fable 5.1 model introduced
A claude-fable-5-1 model id is recognized and becomes the fable family default, with UI text updated to Fable 5.1.
found in this entry's text, named in this entry
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v2.1.257
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.
found in this entry's text, named in this entry
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v2.1.257
'Fable 5' branding shortened to 'Fable' in usage-limit messages
Usage-limit text now says Fable limit, and fable-5-1 and mythos-5-1 aliases were added.
found in this entry's text
First cited
1The earliest release whose published evidence quoted claude-mythos-5-1 was v2.1.257, 1 Sep 2026. That is the oldest release this project wrote about it, so it is a floor on the name's age and not the release that introduced it. It is not a presence reading: which builds carried the name is the table below.
Presence across releases
0The miner has never read a build containing this name. It is here because a changelog entry of ours names it, which is evidence that it existed and not evidence of which releases carried it. The inventory starts at v2.1.138 and everything older than that was never mined.
Read out of the published npm bundle release by release, and out of Anthropic's own documentation as this site captured it. Nothing on this page is a description anybody here wrote about what the model does. All models.