overview changedrelease-notes/overview
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1515### November 9, 2026
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17* We’ve launched **Claude Fable 5.2** (`claude-fable-5-2`), built for demanding reasoning and long-horizon agentic work. **Claude Mythos 5.2** (`claude-mythos-5-2`) 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). Both models have a [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows), 128k max output tokens, and always-on [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking). They cost $8 USD per MTok of input and $40 USD per MTok of output (Claude Fable 5.1 and Claude Mythos 5.1 cost $10 USD per MTok of input and $50 USD per MTok of output). Claude Fable 5.2 is available on the Claude API, [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), and [Claude Models on Google Cloud (Anthropic managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/anthropic-managed). It’s also available on [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed). See [What’s new in Claude Fable 5.2](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2) for API changes and migration guidance. For model-specific prompting patterns, see [Prompting Claude Fable 5.2](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2).
17* We’ve launched **Claude Fable 5.2** (`claude-fable-5-2`), built for demanding reasoning and long-horizon agentic work. **Claude Mythos 5.2** (`claude-mythos-5-2`) 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). Both models have a [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows), 128k max output tokens, and always-on [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking). They cost $8 USD per MTok of input and $40 USD per MTok of output (Claude Fable 5.1 and Claude Mythos 5.1 cost $10 USD per MTok of input and $50 USD per MTok of output). Claude Fable 5.2 is available on the Claude API, [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), and [Claude Models on Google Cloud (Anthropic managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/anthropic-managed). It’s also available on [Claude in Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock). It’s also available on [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed). It’s also available on [Claude in Microsoft Foundry](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry). See [What’s new in Claude Fable 5.2](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2) for API changes and migration guidance. For model-specific prompting patterns, see [Prompting Claude Fable 5.2](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2).
1818* Claude Fable 5.2 and Claude Mythos 5.2 enforce the [prefix check](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#prefix-check) for every account, whatever its age. Claude Fable 5.1 returns the error by default on accounts created on or after August 31, 2026 (see [When `prefix_mismatch_behavior` is unset](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#enforcement)). If your code edits the `system` prompt, `tools`, or an earlier message and then sends back a Claude Fable 5.2 or Claude Mythos 5.2 thinking block from a later turn, the request returns a 400 error. Before you switch, [check whether your code edits the prefix](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-preserved-thinking).
1919* On the Claude API and Claude Models on Google Cloud (Google managed), a conversation that moves onto Claude Fable 5.2 or Claude Mythos 5.2 keeps the reasoning from other models, including Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5. When the conversation moves the other way, to any other model, including Claude Opus 5.5 and Claude Fable 5.1, the API drops the thinking blocks that Claude Fable 5.2 or Claude Mythos 5.2 produced, and the request still succeeds. See [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models).
20* Claude Fable 5.2 and Claude Mythos 5.2 support computer use through the `computer_toolset_20260801` toolset. A request that declares the earlier `computer_20251124` tool returns a 400 error. See [Migrate from `computer_20251124`](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool#migrate-from-computer-20251124).
20* Claude Fable 5.2 and Claude Mythos 5.2 support computer use through the `computer_toolset_20260801` toolset. A request that declares the earlier `computer_20251124` tool returns a 400 error on every platform except Amazon Bedrock, where the earlier tool still works. See [Migrate from `computer_20251124`](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool#migrate-from-computer-20251124).
2121* With the [advisor tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool), which is in beta, a Claude Fable 5.2 or Claude Mythos 5.2 executor accepts only Claude Fable 5.2 or Claude Mythos 5.2 as its advisor. Any other advisor returns a 400 error, so set the advisor’s `model` to one of these two models when you move an executor to them. Every executor model accepts Claude Fable 5.2 as its advisor. See [Model compatibility](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool#model-compatibility).
2222* Both models require 30-day data retention, like Claude Fable 5.1 and Claude Mythos 5.1. They aren’t available under zero data retention unless expressly authorized by Anthropic. See [Model-specific data retention requirements](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention#model-specific-data-retention-requirements).
2323* Thinking blocks that Claude Fable 5.2 and Claude Mythos 5.2 produce work only in the account that produced them, or in an account linked to it. See [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#account-bound-thinking).
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