feat(claude): add Claude Fable 5.1 model

[q1]

Supersedes #9077 (closed by @juliusmarminge). The catalog commit here is #9077's verbatim, with his authorship; the follow-up commits add what it left for later.

What Changed

• Catalog entry claude-fable-5-1 ("Claude Fable 5.1") above Fable 5, gated on Claude Code v2.1.257 — the first published build with Fable 5.1 (the changelog entry is under 2.1.257; the docs' "2.1.255" never shipped to npm, releases jump 2.1.252 → 2.1.257). Same Fable-style effort ladder (low → max, ultracode, ultrathink, default high) and 200k / 1M context option (1M default). No fastMode — fast mode is Opus 5 / 4.8 only. xhigh is not downgraded to max. Older CLIs get the usual "too old, upgrade to v2.1.257" nudge first in the upgrade-message chain.
• Model manifest: claude-fable-5-1 is current and claude-fable-5 moves to legacy (per review — it is superseded, same treatment as Opus 4.8). Legacy classification is manifest-driven, so without this line the new model would render under "Legacy models". Note the remote manifest on main propagates to every install immediately, so between merge and a release that carries the catalog entry, installs without Fable 5.1 will see Fable 5 in the legacy section (still selectable).
• Slug aliases fable, fable-5.1, claude-fable-5.1 → claude-fable-5-1, matching Claude Code's own fable alias (/model fable resolves to Fable 5.1 as of 2.1.257). Existing aliases untouched.

No SDK bump: Claude Code is spawned from the user's install, and the pinned @anthropic-ai/claude-agent-sdk 0.3.170 drives 2.1.257 fine (verified below). Default model is unchanged: the picker's fallback order (isDefault → first listed → DEFAULT_MODEL_BY_PROVIDER) already had Fable 5 first, so the first-listed slot moving from Fable 5 to 5.1 is the same behavior as when Opus 5 / Fable 5 landed.

Why

Claude Code 2.1.257 makes Fable 5.1 the default Fable model. Without a catalog entry it cannot be selected from T3 Code, and users on a current CLI have no way to get to it. Fable 5.1 keeps Fable 5's per-token price with cache reads at $0.25/MTok, 1M context by default (Claude Code strips the [1m] suffix for it, so resolveClaudeApiModelId works unchanged), and the same effort ladder.

Testing

• ProviderRegistry.test.ts: shown at v2.1.257, hidden with the Fable 5.1 upgrade message at v2.1.256; existing Opus 5 / Fable 5 gating tests still pass.
• ClaudeAdapter.test.ts: xhigh reaches the SDK unchanged for claude-fable-5-1.
• ModelManifest.test.ts (5.1 current, 5 legacy) and packages/shared/src/model.test.ts (fable alias).
• tsgo --noEmit for apps/server, packages/contracts, packages/shared; vp lint and vp fmt --check clean on the touched files. CI green on the previous push.
• End to end on macOS with Claude Code 2.1.257 (Claude Max): booted the dev server from this branch against a scratch home, confirmed server.getConfig lists claude-fable-5-1 as non-legacy with status ready and no upgrade message, created a thread on it with effort: low / contextWindow: 1m, and ran a turn. The turn completed, the assistant replied, context-window.updated reported maxTokens: 1000000, and the Claude Code transcript records "model":"claude-fable-5-1".

Notes

• Pricing: usage cost for Fable 5.1 reads as unpriced until LiteLLM's model_prices_and_context_window.json gains claude-fable-5-1 (open: feat(models): add Claude Fable 5.1 across Anthropic, Bedrock, Vertex AI, and Azure AI BerriAI/litellm#39148, feat(anthropic): add Claude Fable 5.1 BerriAI/litellm#39151; $10 / $50 per MTok, cache read $0.25, cache write $12.50 per the model page). The table is fetched at runtime, so nothing to change here.
• No UI screenshots — data-only entries that render through the existing model picker rows.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes (n/a — no UI code changed)
• I included a video for animation/interaction changes (n/a)

Claude Fable 5.1 via Claude Code (catalog commit: gpt-5.6-sol via Codex, from #9077).

🤖 Generated with Claude Code

---

Note

Low Risk
Catalog, manifest, and alias changes only; no auth or session logic. Remote manifest may label Fable 5 legacy before releases ship the new catalog entry.

Overview
Adds Claude Fable 5.1 (claude-fable-5-1) to the Claude agent catalog so users on Claude Code v2.1.257+ can select it from the model picker, with the same reasoning ladder and 200k/1M context options as Fable 5. xhigh effort is passed through for this model (via normalizeClaudeCliEffort), and older CLIs see a Fable 5.1 upgrade nudge at the top of the existing version-message chain.

The bundled model manifest treats Fable 5.1 as current and Fable 5 as legacy (superseded). Slug aliases fable, fable-5.1, and claude-fable-5.1 resolve to claude-fable-5-1, aligned with Claude Code’s /model fable behavior.

Tests cover version gating, manifest legacy flags, alias normalization, and SDK effort forwarding for Fable 5.1.

^{Reviewed by Cursor Bugbot for commit 267b090. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Add claude-fable-5-1 model to Claude provider with version gating

• Registers a new built-in model claude-fable-5-1 in CLAUDE_MODEL_CATALOG with effort and contextWindow option descriptors, gated to CLI version >= 2.1.257 via supportsClaudeFable51.
• Extends normalizeClaudeCliEffort to preserve xhigh effort for claude-fable-5-1 instead of mapping it to max.
• Adds slug aliases fable, fable-5.1, and claude-fable-5.1 mapping to claude-fable-5-1 in MODEL_SLUG_ALIASES_BY_PROVIDER.
• Replaces claude-fable-5 with claude-fable-5-1 in the non-legacy models list in model-manifest.json, and checkClaudeProviderStatus emits an upgrade message when the CLI is too old.
• Behavioral Change: claude-fable-5 is no longer listed as a current model in the bundled manifest; claude-fable-5-1 takes its place, so systems relying on that manifest will treat claude-fable-5 as legacy.

^{Macroscope summarized 267b090.}

[juliusmarminge]

shouldn't 5 be demoted to legacy now that it's superseded by 5.1?

[q1]

Done in 267b090: claude-fable-5 is out of the current list, so it lands under Legacy models like Opus 4.8. One heads-up since the manifest is fetched live from main: between merge and a release that carries the catalog entry, installs without Fable 5.1 will show Fable 5 in the legacy section (still selectable). Also rebuilt the branch so your catalog change is a single commit under your authorship with the manifest/alias follow-ups on top.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — Adds a new user-facing Claude model with version-gated runtime behavior, new aliases, and changed legacy classification. Because it becomes the first fallback model for supported Claude installations and changes effective product defaults, human review is required.

^{You can add or adjust custom eligibility rules. Learn more.}

[q1]

Ready for another look.

• Addressed the review: claude-fable-5 is demoted to legacy in the manifest (267b090), test updated.
• Branch is three commits on main: the feat(claude): add Claude Fable 5.1 #9077 catalog change as a single commit under @juliusmarminge's authorship, then the manifest/alias follow-up, then the demotion. Net diff is the seven files in the description; none of the usage-pricing experiment from feat(claude): add Claude Fable 5.1 #9077's branch is included.
• Local: touched tests, typecheck, lint, and fmt pass on this exact tree. CI went green on the previous head; the new head is waiting on the first-contributor workflow approval again.

Happy to squash to one commit if you'd prefer that on merge.