migration-guide changedmodels/sonnet-5-5/migration-guide
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845845### Thinking blocks are tied to the model and the conversation
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847Claude Sonnet 5.5 reads thinking blocks from Claude Sonnet 5, Claude Opus 4.8, Claude Haiku 4.5, and earlier models. It doesn't read blocks from Claude Opus 5, Claude Opus 5.5, or any Claude Fable or Claude Mythos model. The API drops blocks the model can't read. The request still returns 200, and dropped blocks aren't billed. See [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models).
847Claude Sonnet 5.5 reads thinking blocks from Claude Sonnet 5, Claude Opus 4.8, Claude Haiku 4.5, and earlier models, and, on the Claude API and Google Cloud, from Claude Haiku 5.5. It doesn't read blocks from Claude Opus 5, Claude Opus 5.5, or any Claude Fable or Claude Mythos model. The API drops blocks the model can't read. The request still returns 200, and dropped blocks aren't billed. See [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models).
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849849Each Claude Sonnet 5.5 thinking block is also signed over the conversation before it. For accounts created on or after August 31, 2026, 00:00 UTC, the API enforces this by default, on the Claude API, Amazon Bedrock, and Google Cloud. On those accounts, a request that replays a block after an edit to earlier history returns a 400 error. Keep conversations append-only, and change instructions or tools with [mid-conversation system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages). Thinking blocks that Claude Sonnet 5.5 produces 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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864864### The advisor tool accepts fewer advisors
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866With the [advisor tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool), a Claude Sonnet 5.5 executor needs one of these advisors: Claude Opus 5, Claude Opus 5.5, Claude Sonnet 5.5, Claude Fable 5, Claude Fable 5.1, Claude Mythos 5, or Claude Mythos 5.1. Claude Opus 4.8, Claude Opus 4.7, Claude Opus 4.6, Claude Sonnet 5, and Claude Sonnet 4.6 advisors return a 400 error. The advice comes back encrypted as an `advisor_redacted_result` block, so its text isn't readable in the response. See [Model compatibility](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool#model-compatibility).
866With the [advisor tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool), a Claude Sonnet 5.5 executor needs one of these advisors: Claude Opus 5, Claude Opus 5.5, Claude Sonnet 5.5, Claude Fable 5, Claude Fable 5.1, Claude Mythos 5, or Claude Mythos 5.1. Claude Opus 4.8, Claude Opus 4.7, Claude Opus 4.6, Claude Sonnet 5, Claude Sonnet 4.6, and Claude Haiku 5.5 advisors return a 400 error. The advice comes back encrypted as an `advisor_redacted_result` block, so its text isn't readable in the response. See [Model compatibility](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool#model-compatibility).
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868868### Text between tool calls is returned in thinking blocks
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