feat(runtime): support Claude Fable 5.1

[Yevanchen]

Area

Runtime

Problem

Anthropic released Claude Fable 5.1 on September 1, 2026 with the fixed Claude API model ID claude-fable-5-1.

Mosoo currently registers only Claude Fable 5 (claude-fable-5) in the preset runtime catalog and cost-pricing table. App owners therefore cannot select Fable 5.1 for a new Agent. If a runtime reports Fable 5.1 usage before a matching pricing entry exists, token-derived usage is recorded with unknown pricing instead of a model-specific estimate.

Proposal

Add Claude Fable 5.1 as a new Anthropic preset without renaming, aliasing, or replacing Claude Fable 5.

• Add Claude Fable 5.1 / claude-fable-5-1 to the canonical runtime catalog and regenerate checked-in catalog artifacts.
• Expose it anywhere the existing Fable 5 preset is supported, currently Claude Agent SDK and OpenCode.
• Add model-specific usage pricing from Anthropic: $10 per million input tokens, $50 per million output tokens, and $0.25 per million cache-read tokens.
• Verify the cache-creation price against the TTL semantics that Mosoo usage events can actually distinguish; do not inherit the generic Anthropic cache ratio without checking it.
• Exercise a real run path so catalog admission is not the only proof of support.

Acceptance criteria

• An App owner with an Anthropic key can select Claude Fable 5.1 when configuring a compatible Agent.
• Runtime/model identity validation accepts anthropic:claude-fable-5-1 for Claude Agent SDK and OpenCode.
• A real run completes on each supported runtime and records at least one real tool call plus a usage event.
• Token-derived usage uses the published $10/MTok input, $50/MTok output, and $0.25/MTok cache-read prices.
• Cache-creation pricing is verified against the runtime payload and the supported cache TTL; unsupported granularity is handled explicitly rather than silently mispriced.
• Focused tests cover runtime-model admission and the Fable 5.1 pricing lookup/calculation.
• Generated runtime-catalog files are refreshed and pass the catalog consistency check.
• Existing claude-fable-5 Agent configurations and historical usage remain unchanged.

Alternatives considered

• Keep offering only Claude Fable 5. This avoids a catalog update but prevents builders from opting into the new model.
• Replace or alias claude-fable-5 to 5.1. Anthropic model IDs from this generation are pinned releases, so doing this would silently change existing Agent behavior and make historical usage identity ambiguous.
• Allow a custom model ID as a workaround. Mosoo does not expose custom Anthropic models through the managed preset flow, and this would still leave runtime admission and cost pricing incomplete.

Compatibility and migration

This should be additive. No database migration or rewrite of persisted Agent/runtime identities is expected. Existing published Agents remain on claude-fable-5; builders opt into 5.1 through a new Agent configuration/version.

Anthropic documents compatibility differences for Fable 5.1 around forced tool use and thinking-block reuse. The supported runtime paths should be checked for those behaviors during the real-run validation.

Official references:

• Claude Fable 5.1 overview
• Claude model IDs and versioning

Contribution

I can provide product feedback

Checklist

• I searched existing issues before opening this request.
• I described the problem before the proposed solution.
• I kept this request focused on one improvement.