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Add Claude Sonnet 5.5, Fable 5.1, and Mythos 5.1 model support - #206
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Register claude-sonnet-5-5 with adaptive thinking and max effort. Non-default sampling parameters are rejected, and forced tool choice is rejected on every request. Co-authored-by: Florian Reifschneider <florian@omnifact.ai>
Share one forced-tool-choice rejection list across Opus 5.5, Sonnet 5.5, Fable 5.1, and Mythos 5.1. Fable 5.1 and Mythos 5.1 use always-on adaptive thinking, max effort, and always-restricted sampling. Co-authored-by: Florian Reifschneider <florian@omnifact.ai>
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September 29, 2026 19:57
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Summary
Registers three Claude models and shares one forced-tool-choice rule across the models that reject it on every request.
claude-sonnet-5-5uses adaptive thinking and acceptsmaxeffort. Omittingreasoningleaves the API default ofhigh. Up-front thinking can be turned off, soreasoning.modestaysoptional.claude-fable-5-1andclaude-mythos-5-1use always-on adaptive thinking, acceptmaxeffort, and default tohigh. Mythos 5.1 matches Fable 5.1.claude-opus-5-5,claude-sonnet-5-5,claude-fable-5-1, andclaude-mythos-5-1reject forced tool choice (requiredand{ type: 'tool' }) on every request, including whenreasoningis omitted.'auto'and'none'stay valid. Claude Fable 5 and Claude Mythos 5 still allow forced tool choice whenreasoningis omitted.temperature,topP, andtopKon every request.Test plan
npm run test -w @core-ai/anthropicnpm run lint -w @core-ai/anthropicnpm run check-types -w @core-ai/anthropic