🤖 feat: promote Claude Fable 5.1 as the fable model

[ibetitsmike]

Summary

Prepares Xum for the Claude Fable 5.1 drop: promotes Fable 5.1 (anthropic:claude-fable-5-1) to the FABLE known model, so the fable alias, the /fable command, tokenizer warming, and the default refusal-fallback chain all route to the new model. Fable 5 stays usable as the custom model string anthropic:claude-fable-5 and keeps its metadata entry and tokenizer approximation.

Important

Merge gate: hold until Anthropic officially announces Fable 5.1, then verify the assumptions below against the announcement before merging. Merging early would break fable-alias sends (the API id would not exist yet).

Background

Fable 5.1 is not yet announced; leak reports point at an imminent launch. This PR is prepared ahead of the drop (same play as the Opus 5 promotion in #3750), with assumptions to confirm on release day:

• API id: claude-fable-5-1 (dash form, dateless, matching the 5-generation convention of claude-sonnet-5 / claude-opus-5 and the dash style of claude-haiku-4-5).
• Pricing: unchanged from Fable 5, $10/M input, $50/M output (cache write 1.25x input, cache read 0.1x input).
• Envelope: unchanged, native 1M context, 128K max output, full effort ladder with native xhigh and max, adaptive thinking only (disabled thinking rejected), safeguard classifiers retained so the refusal fallback stays relevant.

Implementation

• knownModels.ts: FABLE -> claude-fable-5-1 (keeps the fable alias and tokenizer warming; tokenizer override stays on the Opus 4.5 approximation).
• models-extra.ts: new claude-fable-5-1 pricing/limits entry; the Fable 5 entry is retained.
• models.ts: claude-fable-5-1 added to the native-1M patterns (the existing claude-fable-5 pattern only tolerates date suffixes, not -1).
• No wire-format changes needed for thinking/effort: the Mythos-class wildcard matchers (anthropicSupportsNativeXhigh, anthropicRejectsDisabledThinking) and native web-fetch id parsing already match the new id; tests now pin that.
• Fallback seed migration: DEFAULT_MODEL_FALLBACKS is keyed by the FABLE id and the one-time seed is guarded by migrations.defaultModelFallbacksSeeded, so already-seeded configs would never get a chain for the new source key. A second one-shot flag (defaultModelFallbacksSeededFable51) re-runs the gap-check once: pre-5.1 configs get the Fable 5.1 -> Opus chain, an existing 5.1 chain is never overwritten, and the legacy Fable 5 chain is left byte-identical on disk.
• Downgrade safety (Codex round 1): completing the original seed pass claims defaultModelFallbacksSeeded, which pre-5.1 builds trust for their own claude-fable-5 key. The original seed pass (and the fresh-install default config) now also carries a LEGACY_DEFAULT_MODEL_FALLBACKS chain for anthropic:claude-fable-5, so a downgrade still finds its expected default instead of silently losing refusal fallback.
• Legacy tokenizer overrides (Codex round 1): TOKENIZER_MODEL_OVERRIDES is derived from current KNOWN_MODELS ids, so retiring an id dropped its approximate-tokenizer mapping. A LEGACY_TOKENIZER_MODEL_OVERRIDES map retains anthropic:claude-fable-5 and the identical pre-existing case anthropic:claude-opus-4-8; exact-id lookup resolves both to the intended Opus 4.5 approximation instead of warning and falling back to the generic per-provider tokenizer.
• Docs model table and built-in skill content regenerated.

Validation

• Behavioral coverage added for the new id: native-1M classification, display formatting (Fable 5.1), 5-level thinking policy with off-clamp, provider options (adaptive + summarized display, no disabled thinking), native web-fetch support, and the config seeding semantics (legacy chain carried by the original seed pass, 5.1 chain seeded once for pre-5.1 configs, existing 5.1 chains and deletions respected).
• The WorkflowRunner alias-mapping test caught the alias flip (fable -> 5.1) and was updated; remaining claude-fable-5 fixtures were audited and stay valid as explicit custom model strings.

Risks

Low: additive registry/metadata changes plus config migration flags. The main user-visible effect is that the fable alias and /fable route to Fable 5.1. Until the model exists at the API, alias sends would fail, which is why the merge gate above matters. Existing Fable 5 selections and user-edited fallback chains are unaffected; note that with current ai-tokenizer data the legacy tokenizer fix changes no token counts (both approximations share the claude encoding), it removes warning spam and future drift risk.

---

Generated with xum • Model: anthropic:claude-fable-5 • Thinking: xhigh

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[ibetitsmike]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: e288521a98

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[ibetitsmike]

@codex review

Addressed both round-1 findings: legacy Fable 5 chain now seeded during the original seed pass for downgrade safety, and legacy tokenizer overrides retained for the retired fable-5/opus-4-8 ids.

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Breezy!

Reviewed commit: 116066c324

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