migration-guide changedmodels/haiku-5-5/migration-guide
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134134## Replay thinking blocks through the account that produced them
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136Thinking blocks from Claude Haiku 5.5 work only in the account that produced them, or in an account linked to it. When another account sends one of these blocks, the API drops the block before the model receives it, and the request succeeds without that reasoning. This affects code that stores conversations and replays them through a different account, for example a service that serves several customers from one conversation store. Replay each conversation through the account that produced it. See [Thinking blocks stay with the account that produced them](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#account-bound-thinking).
136Thinking blocks from Claude Haiku 5.5 work only in the account that produced them or in an account linked to it. When another account sends one of these blocks, the API drops the block before the model receives it, and the request succeeds without that reasoning. This affects code that stores conversations and replays them through a different account, for example a service that serves several customers from one conversation store. Replay each conversation through the account that produced it. See [Thinking blocks stay with the account that produced them](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#account-bound-thinking).
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138138## Keep earlier turns unchanged
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140A Claude Haiku 5.5 thinking block stays valid only while everything sent before it is unchanged: a request that sends a thinking block back after a change to `system`, `tools`, or earlier `messages` returns a 400 error. Claude Haiku 4.5 doesn’t run this check. Keep conversations append-only. On accounts created before August 31, 2026, 00:00 UTC, the error comes only on requests that set `thinking.block_binding.prefix_mismatch_behavior`. For the changes that trigger the error and what to do instead, see [Who needs to change anything](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#who-is-affected).
140A Claude Haiku 5.5 thinking block stays valid only while everything sent before it is unchanged: the `system` prompt, the `tools`, and every earlier message. Claude Haiku 4.5 doesn’t run the prefix check. Keep conversations append-only. A request that sends a block back after such a change and leaves `thinking.block_binding.prefix_mismatch_behavior` unset gets a 400 error on accounts created on or after August 31, 2026, 00:00 UTC. On accounts created before that date, the request gets the error only when it sets the field to `"error"`. For the changes that trigger the error and what to do instead, see [Who the prefix check affects](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#who-is-affected).
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142142## Migrating to Claude Haiku 5.5 from Claude Haiku 3.5 and earlier Haiku models
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