migration-guide changedmodels/haiku-5-5/migration-guide
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38388. If you replay stored conversations through a different account, replay each one through the account that produced it. See [Replay thinking blocks through the account that produced them](https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide#replay-thinking-blocks-through-the-producing-account).
39399. If your code changes `system`, `tools`, or earlier `messages` between requests in a conversation and sends thinking blocks back, keep the conversation append-only. See [Keep earlier turns unchanged](https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide#keep-earlier-turns-unchanged).
404010. Handle `stop_reason: "refusal"`. Claude Haiku 5.5 runs safety classifiers that can decline a request, and it has no server-side fallback. See [Safeguard refusals](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-haiku-5-5#safeguard-refusals).
4111. If you use [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs) (`output_config.format` or `strict: true` tools) on Amazon Bedrock, describe the format in the prompt or use a tool without `strict`, and validate the output in your code. Structured outputs aren't available for Claude Haiku 5.5 on Amazon Bedrock.
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4342If your organization has a [Priority Tier](https://platform.claude.com/docs/en/api/service-tiers#supported-models) commitment on Claude Haiku 4.5, plan capacity separately: Priority Tier is not supported on Claude Haiku 5.5.
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126125A prefill is a final assistant turn in `messages` that the model continues. Claude Haiku 4.5 accepts one when thinking is off. Claude Haiku 5.5 rejects it with a 400 error, even with thinking turned off. End `messages` with a user turn, and replace each prefill according to what it was for:
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128* **Output format:** use [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs), or tools with enum fields for classification. On Amazon Bedrock, structured outputs aren't available for Claude Haiku 5.5. There, describe the format in the prompt or use a tool without `strict`, and validate the output in your code.
127* **Output format:** use [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs), or tools with enum fields for classification.
129128* **Preambles:** ask in the system prompt for a direct answer.
130129* **Continuations:** move them to the user message, for example "Your previous response was interrupted and ended with `[previous_response]`. Continue from where you left off."
131130* **Context reminders:** put them in the user turn.
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