Or, for Claude Mythos 5.1, which offers the same capabilities to verified organizations: changedmodels/fable-5-1/migration-guide
Nearest release: v2.1.309, published 3 hours before this site recorded the change. Shown because the two are within 24 hours of each other. Nothing here says the release caused the edit.
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Recorded edits28to this page, all time
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2828* **Prefill:** Prefilling the assistant message returns a 400 error, unchanged from Claude Fable 5. Use system prompt instructions instead.
2929* **Tool choice:** `{type: "auto"}` (the default) and `{type: "none"}` are supported. Forcing a tool call with `{type: "any"}` or `{type: "tool", name: "..."}` returns a 400 error. See [Breaking changes](https://platform.claude.com/docs/en/models/fable-5-1/migration-guide#fable-5-1-breaking-changes).
3030* **Preserved thinking across models:** Claude Fable 5.1 reads thinking blocks from Claude Opus 5, Claude Fable 5, Claude Mythos 5, and earlier Claude models. None of those models can read Claude Fable 5.1's blocks. See [Breaking changes](https://platform.claude.com/docs/en/models/fable-5-1/migration-guide#fable-5-1-breaking-changes).
31* **Context window and output:** A [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows) by default, and up to 128k output tokens per request.
31* **Context window and output:** A [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows) by default, and up to 128k output tokens for each request.
3232* **Pricing:** $10 USD per million input tokens and $50 USD per million output tokens, the same as Claude Fable 5. Prompt cache reads are $0.25 USD per million tokens, a quarter of the Claude Fable 5 rate. See [Claude pricing](https://platform.claude.com/docs/en/about-claude/pricing).
33* **Data retention:** Both models require 30-day data retention, aren't available under zero data retention (ZDR) arrangements unless expressly authorized by Anthropic, and are designated Covered Models, the same as Claude Fable 5 and Claude Mythos 5. On the Claude API, a request from an organization or workspace without 30-day retention returns a 400 `invalid_request_error`. Organizations with a ZDR arrangement should contact their Anthropic account team, or configure retention per workspace. See [Model-specific data retention requirements](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention#model-specific-data-retention-requirements) for per-platform details.
33* **Data retention:** Both models require 30-day data retention, aren't available under zero data retention (ZDR) arrangements unless expressly authorized by Anthropic, and are designated Covered Models, the same as Claude Fable 5 and Claude Mythos 5. On the Claude API, a request from an organization or workspace without 30-day retention returns a 400 `invalid_request_error`. Organizations with a ZDR arrangement should contact their Anthropic account team, or configure retention for individual workspaces. See [Model-specific data retention requirements](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention#model-specific-data-retention-requirements) for per-platform details.
3434* **Priority Tier:** Neither model is supported on [Priority Tier](https://platform.claude.com/docs/en/api/service-tiers#supported-models). Claude Fable 5 is.
3535
3636Where the two models diverge:
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254254 messages: [
255255 ['role' => 'user', 'content' => 'Summarize: The meeting moved to Thursday.']
256256 ],
257 model: 'claude-fable-5',
257 model: Model::CLAUDE_FABLE_5,
258258 toolChoice: ['type' => 'tool', 'name' => 'record_summary'],
259259 tools: [
260260 [
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