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322322* Remove any `thinking: {type: "disabled"}` configuration. Disabling thinking returns an error on `claude-mythos-5` and `claude-fable-5`.
323323* Remove `budget_tokens`. It has no direct replacement: thinking is adaptive, and the `effort` parameter is a separate output-level control, not a thinking budget.
324324* Verify any code that parses the `thinking` field treats it as display text only and passes thinking blocks back unchanged when continuing on the same model. `thinking.display` defaults to `"omitted"` on `claude-mythos-5` and `claude-fable-5`, the same as on Claude Mythos Preview. Set `display: "summarized"` to receive readable summaries. See [Thinking output on Claude Fable and Claude Mythos models](https://platform.claude.com/docs/en/build-with-claude/thinking#thinking-output-on-claude-fable-5-and-claude-mythos-5).
325* If you replay conversation history on an earlier model, strip `thinking` and `redacted_thinking` blocks from prior assistant turns first. Thinking blocks from `claude-fable-5` and `claude-mythos-5` are readable only by the model that produced them or a newer one: earlier models silently ignore them, while Claude Fable 5.1 and Claude Mythos 5.1 read them, so keep them when you move a conversation up to those models (see [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-for-model)). Stripping keeps requests to earlier models minimal and uniform.
325* If you replay conversation history on an earlier model, strip `thinking` and `redacted_thinking` blocks from prior assistant turns first. Thinking blocks from `claude-fable-5` and `claude-mythos-5` are readable only by the model that produced them or a newer one: earlier models silently ignore them, while Claude Fable 5.1 and Claude Mythos 5.1 read them, so keep them when you move a conversation up to those models (see [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models)). Stripping keeps requests to earlier models minimal and uniform.
326326* If you migrate to Claude Fable 5, handle `stop_reason: "refusal"` and read the `stop_details.category` field. Claude Fable 5 runs safety classifiers that Claude Mythos Preview and Claude Mythos 5 do not have. See [Refusals and fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback).
327327* Re-baseline token counts and costs on your own workloads. Token counts are roughly unchanged when migrating from `claude-mythos-preview`.
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696696* Update response parsing that reads content by position, such as `content[0].text`: with adaptive thinking always on, `thinking` blocks arrive before `text` blocks. Select content blocks by `type` instead, and pass `thinking` blocks back complete and unmodified in tool-use loops; modified blocks return a 400 error. See [Preserving thinking blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#preserving-thinking-blocks).
697697* If you removed manual extended thinking and assistant prefills during earlier migrations, no action is needed: both remain unsupported on `claude-fable-5` and `claude-mythos-5`.
698698* Verify any code that parses the `thinking` field treats it as display text only and passes thinking blocks back unchanged when continuing on the same model. `thinking.display` defaults to `"omitted"` on `claude-fable-5` and `claude-mythos-5`, the same as on Claude Opus 4.8. Set `display: "summarized"` to receive readable summaries. See [Thinking output on Claude Fable and Claude Mythos models](https://platform.claude.com/docs/en/build-with-claude/thinking#thinking-output-on-claude-fable-5-and-claude-mythos-5).
699* If you replay conversation history on an earlier model, strip `thinking` and `redacted_thinking` blocks from prior assistant turns first. Thinking blocks from `claude-fable-5` and `claude-mythos-5` are readable only by the model that produced them or a newer one: earlier models silently ignore them, while Claude Fable 5.1 and Claude Mythos 5.1 read them, so keep them when you move a conversation up to those models (see [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-for-model)). Stripping keeps requests to earlier models minimal and uniform. The exception is redeeming a [fallback credit](https://platform.claude.com/docs/en/build-with-claude/fallback-credit), which requires the request body echoed under that feature's exact rules.
699* If you replay conversation history on an earlier model, strip `thinking` and `redacted_thinking` blocks from prior assistant turns first. Thinking blocks from `claude-fable-5` and `claude-mythos-5` are readable only by the model that produced them or a newer one: earlier models silently ignore them, while Claude Fable 5.1 and Claude Mythos 5.1 read them, so keep them when you move a conversation up to those models (see [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models)). Stripping keeps requests to earlier models minimal and uniform. The exception is redeeming a [fallback credit](https://platform.claude.com/docs/en/build-with-claude/fallback-credit), which requires the request body echoed under that feature's exact rules.
700700* If you migrate to Claude Fable 5, handle `stop_reason: "refusal"` and read the `stop_details.category` field. To re-run refused requests on another model automatically, consider the opt-in `fallbacks` parameter (beta). See [Refusals and fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback).
701701* Re-evaluate your `effort` setting. Start at `high` for most tasks, including workloads that ran at `xhigh` on Claude Opus 4.8.
702702* Re-baseline cost and latency on your own workloads. Token counts are roughly unchanged when migrating from `claude-opus-4-8`; per-token pricing differs, and thinking tokens are billed as output tokens, so workloads that ran without thinking produce more output tokens per request.