whats-new-fable-5-1 changedmodels/fable-5-1/whats-new-fable-5-1
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### Only Claude Fable 5.1 and Claude Mythos 5.1 read Claude Fable 5.1 thinking blocks ### Earlier models can't read Claude Fable 5.1 thinking blocks
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77Claude Fable 5.1 extends Claude Fable 5 at the same input and output prices, with cache reads at a quarter of the cost, and brings stronger long-running agentic coding, multistep research, and document, spreadsheet, and slide work. For most workloads, start with Claude Opus 5.5 (see [Choosing a model](https://platform.claude.com/docs/en/about-claude/models/choosing-a-model)). Use Claude Fable 5.1 for demanding reasoning and long-horizon agentic work, or when your evals on Claude Opus 5.5 at higher effort still fall short. Claude Mythos 5.1 offers the same capabilities only to organizations verified through Anthropic's verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842).
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9If you already call Claude Fable 5, three changes are breaking: [forced tool use returns an error](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#forced-tool-use-is-not-supported), [earlier models can't read its thinking blocks](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#thinking-blocks-are-tied-to-the-model-that-produced-them), and [editing earlier turns invalidates thinking blocks](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#editing-earlier-turns-invalidates-thinking-blocks). Five are additive: [per-message effort](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#change-effort-mid-conversation-beta) (beta), [turn-scoped system messages](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#turn-scoped-system-messages-beta) (beta), [readable progress updates between tool calls](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#progress-updates-between-tool-calls-beta) (`display: "updates"`, beta), a [lower cache read price](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#pricing), and [content provenance](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#content-provenance).
9If you already call Claude Fable 5, three changes are breaking: [forced tool use returns an error](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#forced-tool-use-is-not-supported), [only Claude Fable 5.1 and Claude Mythos 5.1 can read Claude Fable 5.1's thinking blocks](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#thinking-blocks-are-tied-to-the-model-that-produced-them), and [editing earlier turns invalidates thinking blocks](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#editing-earlier-turns-invalidates-thinking-blocks). Five are additive: [per-message effort](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#change-effort-mid-conversation-beta) (beta), [turn-scoped system messages](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#turn-scoped-system-messages-beta) (beta), [readable progress updates between tool calls](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#progress-updates-between-tool-calls-beta) (`display: "updates"`, beta), a [lower cache read price](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#pricing), and [content provenance](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#content-provenance).
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1111## Models
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3939Thinking is always on for these models, and a forced tool call would skip it. The model would write its working-out into the tool arguments instead, which lowers argument quality. For schema-valid JSON, keep `tool_choice: {"type": "auto"}` and set `strict: true` with [strict tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use), or move the schema to [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs). To make the model call a tool rather than reply in text, state in the prompt when the tool applies (for example, "Use the `get_weather` tool to answer"). Claude Fable 5.1 follows explicit tool instructions reliably.
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41### Earlier models can't read Claude Fable 5.1 thinking blocks
41### Only Claude Fable 5.1 and Claude Mythos 5.1 read Claude Fable 5.1 thinking blocks
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43Every thinking block records which model produced it, and it's preserved in one direction only: Claude Fable 5.1 reads earlier models' thinking blocks, and no earlier model reads Claude Fable 5.1's. A conversation that moves onto Claude Fable 5.1 (from Claude Opus 5, Claude Fable 5, or any earlier Claude model) keeps its reasoning. A conversation that moves from Claude Fable 5.1 to any of those models loses it for the turns that run there.
43Every thinking block records which model produced it. Claude Fable 5.1 reads earlier models' thinking blocks, but only Claude Fable 5.1 and Claude Mythos 5.1 read Claude Fable 5.1's. A conversation that moves onto Claude Fable 5.1 (from Claude Opus 5, Claude Fable 5, or any earlier Claude model) keeps its reasoning. A conversation that moves from Claude Fable 5.1 to any other model loses it for the turns that run there.
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4545When a request carries a block the target model can't read (a router or fallback that switches models mid-conversation, for example), the API drops the block before the model sees it. Dropped blocks don't count toward `input_tokens` and aren't billed. With the `thinking-binding-controls-2026-08-01` beta header, the drop is reported in a top-level `input_transformations` array. Without it, the drop is silent. See [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models).
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413413| ---------- | --------------- | --------------- | ------------ | ---------- |
414414| $10 / MTok | $12.50 / MTok | $20 / MTok | $0.25 / MTok | $50 / MTok |
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416Cache reads (hits and refreshes) cost 0.025 times the base input price on these models, compared with 0.1 on other Claude models. Long agentic sessions that re-read a cached prefix pay a quarter of the Claude Fable 5 rate. Cache writes and the [512-token minimum cacheable prompt length](https://platform.claude.com/docs/en/build-with-claude/prompt-caching#cache-limitations) are unchanged.
416Cache reads (hits and refreshes) cost 0.025 times the base input price on these models, compared with 0.05 on Claude Opus 5.5 and Claude Sonnet 5.5, and 0.1 on other Claude models. Long agentic sessions that re-read a cached prefix pay a quarter of the Claude Fable 5 rate. Cache writes and the [512-token minimum cacheable prompt length](https://platform.claude.com/docs/en/build-with-claude/prompt-caching#cache-limitations) are unchanged.
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418418[Batch processing](https://platform.claude.com/docs/en/build-with-claude/batch-processing) is $5 USD per million input tokens and $25 USD per million output tokens. See [Pricing](https://platform.claude.com/docs/en/about-claude/pricing) for data residency and tool pricing.
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