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77**Legacy.** Released February 17, 2026.
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9Although Claude Sonnet 4.6 is still available, you should consider migrating to Claude Sonnet 5 for improved performance. [See Claude Sonnet 5](https://platform.claude.com/docs/en/models/sonnet-5/overview) · [Migrate to Claude Sonnet 5](https://platform.claude.com/docs/en/about-claude/models/migration-guide#migrating-from-claude-sonnet-4-6-to-claude-sonnet-5)
9Although Claude Sonnet 4.6 is still available, you should consider migrating to Claude Sonnet 5 for improved performance. [See Claude Sonnet 5](https://platform.claude.com/docs/en/models/sonnet-5/overview) · [Migrate to Claude Sonnet 5](https://platform.claude.com/docs/en/models/sonnet-5/migration-guide#migrating-from-claude-sonnet-4-6-to-claude-sonnet-5)
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1111Model ID: `claude-sonnet-4-6`
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8080## Resources
8181 
8282<CardGroup cols={3}>
83 <Card title="Migrate to Claude Sonnet 5" icon="arrows-left-right" href="https://platform.claude.com/docs/en/about-claude/models/migration-guide#migrating-from-claude-sonnet-4-6-to-claude-sonnet-5">
83 <Card title="Migrate to Claude Sonnet 5" icon="arrows-left-right" href="https://platform.claude.com/docs/en/models/sonnet-5/migration-guide#migrating-from-claude-sonnet-4-6-to-claude-sonnet-5">
8484 What changes when moving from Claude Sonnet 4.6 to Claude Sonnet 5.
8585 </Card>
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1313Context window: 1M tokens · Max output: 128K tokens · Input pricing: $2 / MTok · Output pricing: $10 / MTok
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15[Announcement](https://www.anthropic.com/news/claude-sonnet-5) · [What’s new](https://platform.claude.com/docs/en/models/sonnet-5/whats-new-sonnet-5) · [Migration guide](https://platform.claude.com/docs/en/about-claude/models/migration-guide#migrating-to-claude-sonnet-5)
15[Announcement](https://www.anthropic.com/news/claude-sonnet-5) · [What’s new](https://platform.claude.com/docs/en/models/sonnet-5/whats-new-sonnet-5) · [Migration guide](https://platform.claude.com/docs/en/models/sonnet-5/migration-guide)
1616 
1717## Overview
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1891892. **Extended thinking:** if you still set `budget_tokens`, migrate to [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking). Manual extended thinking (`thinking: {type: "enabled"}`) is not supported and returns a 400 error.
1901903. **Sampling parameters:** requests that set sampling parameters (`temperature`, `top_p`, `top_k`) to a non-default value return a 400 error; remove them when migrating. Tool definitions and response shapes are unchanged, and assistant message prefilling was already unsupported on Claude Sonnet 4.6.
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192See the [Claude Sonnet 5 section of the migration guide](https://platform.claude.com/docs/en/about-claude/models/migration-guide#migrating-from-claude-sonnet-4-6-to-claude-sonnet-5) for details.
192See [Migrating to Claude Sonnet 5 from Claude Sonnet 4.6](https://platform.claude.com/docs/en/models/sonnet-5/migration-guide#migrating-from-claude-sonnet-4-6-to-claude-sonnet-5) for details.
193193 
194194## Next steps
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6868* [Effort](https://platform.claude.com/docs/en/build-with-claude/effort) is the primary control for steering Claude Opus 5: the model supports the full ladder (`low`, `medium`, `high`, `xhigh`, `max`), with `max` for capability-critical work.
6969* Mid-conversation tool changes are now in beta on Claude Fable 5, Claude Mythos 5, Claude Opus 4.8, and Claude Opus 5: add or remove tools between turns of a conversation while preserving the prompt cache. Include the `mid-conversation-tool-changes-2026-07-01` beta header in your requests.
7070* The `fallbacks` parameter now supports a `"default"` mode, which applies Anthropic's recommended fallback models by refusal category. Server-side fallback is in beta, and the `"default"` mode requires the `server-side-fallback-2026-07-01` beta header. See [Refusals and fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback).
71* We've removed [fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode) for Claude Opus 4.7. Requests to `claude-opus-4-7` with `speed: "fast"` now return an error; unlike Claude Opus 4.6, they do not fall back to standard speed. Claude Opus 4.7 itself remains available at standard speed. To continue using fast mode, migrate to [Claude Opus 5](https://platform.claude.com/docs/en/about-claude/models/migration-guide#migrating-from-claude-opus-47) or Claude Opus 4.8. Read more in [Fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode#supported-models).
71* We've removed [fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode) for Claude Opus 4.7. Requests to `claude-opus-4-7` with `speed: "fast"` now return an error; unlike Claude Opus 4.6, they do not fall back to standard speed. Claude Opus 4.7 itself remains available at standard speed. To continue using fast mode, migrate to [Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5/migration-guide#migrating-from-claude-opus-47) or Claude Opus 4.8. Read more in [Fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode#supported-models).
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7373### July 22, 2026
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157157* Claude Fable 5 and Claude Mythos 5 use the tokenizer introduced with Claude Opus 4.7. Compared to models before Claude Opus 4.7, the same text produces roughly 30% more tokens. The exact increase depends on the content and workload shape. Use the [token counting API](https://platform.claude.com/docs/en/build-with-claude/token-counting#token-counts-on-claude-fable-5) with `model: "claude-fable-5"` to measure your prompts under the new tokenizer.
158158* Claude Fable 5 runs safety classifiers on requests and during response generation. When a classifier declines a request, the Messages API returns `stop_reason: "refusal"`. You are not billed for a request refused before any output is generated. An opt-in `fallbacks` parameter (in beta on the Claude API and Claude Platform on AWS; not supported on the Message Batches API) re-runs refused requests on another model, billed at the fallback model's rates. See [Handling stop reasons](https://platform.claude.com/docs/en/build-with-claude/handling-stop-reasons).
159159* The [`stop_details.category`](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#refusal-response) field on refusal responses now includes `"reasoning_extraction"` on Claude Fable 5, returned when a request is blocked under Anthropic's Terms of Service restrictions on reverse engineering or duplicating model outputs. The existing `"cyber"` and `"bio"` categories are unchanged. No beta header is required.
160* On Claude Fable 5 and Claude Mythos 5, [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) is the only thinking mode: `thinking: {"type": "disabled"}` is not supported, and manual extended thinking budgets and assistant prefill are not supported (both return a 400 error). See [Migrating from Claude Mythos Preview to Claude Mythos 5](https://platform.claude.com/docs/en/about-claude/models/migration-guide#migrating-from-claude-mythos-preview).
160* On Claude Fable 5 and Claude Mythos 5, [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) is the only thinking mode: `thinking: {"type": "disabled"}` is not supported, and manual extended thinking budgets and assistant prefill are not supported (both return a 400 error). See [Migrating from Claude Mythos Preview to Claude Mythos 5](https://platform.claude.com/docs/en/models/fable-5/migration-guide#migrating-from-claude-mythos-preview).
161161* On Claude Fable 5 and Claude Mythos 5, `thinking.display` defaults to `"omitted"`, the same as Claude Opus 4.8, Claude Opus 4.7, and Claude Mythos Preview; set `display: "summarized"` to receive readable thinking summaries. The raw chain of thought is never returned; pass thinking blocks back unchanged in multi-turn conversations on the same model. See [Thinking output on Claude Fable 5 and Claude Mythos 5](https://platform.claude.com/docs/en/build-with-claude/thinking#thinking-output-on-claude-fable-5-and-claude-mythos-5).
162162* Claude Fable 5 requires 30-day data retention and is not available under zero data retention. See [Model-specific data retention requirements](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention#model-specific-data-retention-requirements).
163163* Claude Managed Agents now supports [scheduled deployments](https://platform.claude.com/docs/en/managed-agents/scheduled-deployments), letting you run sessions on a cron schedule without managing your own scheduler.
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