Or, for Claude Mythos 5.1, which offers the same capabilities to verified organizations: changedmodels/fable-5-1/migration-guide
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12081208 Dropping thinking blocks once, at a compaction boundary for example, has little effect. An integration that invalidates prior thinking on every request restarts the prompt cache each time, which can raise cost per task (see [Keep the conversation history append-only](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#keep-the-conversation-history-append-only)).
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12104. **The advisor tool accepts fewer advisors:** With the [advisor tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool) (beta), the advisor must be at least as capable as the executor. With a `claude-fable-5-1` executor, a less capable advisor, such as `claude-fable-5` or `claude-opus-5-5`, returns a 400 `invalid_request_error`. Choose an advisor from the executor’s row in the [compatibility table](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool#model-compatibility).
12104. **The advisor tool accepts fewer advisors:** With the [advisor tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool) (beta), the advisor must be at least as capable as the executor. With a `claude-fable-5-1` executor, a less capable advisor, such as `claude-fable-5` or `claude-opus-5-5`, returns a 400 `invalid_request_error`. Choose an advisor that [Model compatibility](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool#model-compatibility) lists for the executor.
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12121212### Behavior changes
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