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Claude Fable 5.1 (`claude-fable-5-1`, GA 2026-09-01) succeeds Claude Fable 5
at the same input/output prices, with a 1M context window and 128K max output.
The Mythos-class helpers added for Fable 5 already match it — `isMythosClassModel`
tests `claude-(?:fable|mythos)[-.]?\d`, so adaptive thinking, summarized-display
opt-in, sampling omission, 1M context, the 128K output clamp, prompt caching,
the Vertex multi-region gate, and the Bedrock PDF exemption all cover 5.1
without a code change. What was missing was registration and pricing.
Cache reads are the one rate that differs, and the substring matcher would have
silently gotten it wrong: `claude-fable-5-1` falls through to the `claude-fable-5`
key, which prices cache hits at the usual 0.1x base input ($1/MTok). Fable 5.1
and Mythos 5.1 price them at 0.025x ($0.25/MTok), so every cache read would have
billed 4x — on exactly the long agentic sessions this model is built for.
- Register `claude-fable-5-1` in the shared Anthropic list and
`global.anthropic.claude-fable-5-1` in the Bedrock list
- Add 1M context / 128K output entries for `claude-fable-5-1` and
`claude-mythos-5-1`
- Add $10/$50 rates and `{ write: 12.5, read: 0.25 }` cache rates for both
- Update the `.env.example` and Vertex `librechat.example.yaml` examples
- Test that 5.1 resolves to its own key rather than collapsing onto 5.0, across
token maps, pricing, cache rates, and Mythos-class detection
The three documented breaking changes from Fable 5 need no handling here:
LibreChat never sends a forced `tool_choice` on the Anthropic path (the agents
SDK omits the param when unset, so the API default `auto` applies); thinking
blocks an earlier model can't read are dropped server-side, unbilled; and
history is rebuilt from stored content parts rather than replayed signed
thinking blocks, so the prefix-binding check has nothing to reject.
Refs #15506
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| app | ||
| cache | ||
| config | ||
| db | ||
| models | ||
| server | ||
| strategies | ||
| test | ||
| utils | ||
| jest.config.js | ||
| jsconfig.json | ||
| package.json | ||
| typedefs.js | ||