migration-guide changedmodels/sonnet-5-5/migration-guide
Nearest release: v2.1.314, published 6 hours after this site recorded the change. Shown because the two are within 24 hours of each other. Nothing here says the release caused the edit.
Recorded here
Lines+1added
Lines−1removed
From line
841
where the diff opens
First seen
29 Sep 2026
this site's first read of the page
Recorded edits20to this page, all time
The whole hunk
from line 841, old and new numbered
/
from line 841
841841
842842Claude 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 Claude Models on Google Cloud (Google managed), 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).
843843
844Each 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 Claude Models on Google Cloud (Google managed). 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).
844Each Claude Sonnet 5.5 thinking block is also signed over the conversation before it. 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). A request that replays a block after an edit to earlier history 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. This applies on the Claude API, Amazon Bedrock, and Claude Models on Google Cloud (Google managed). 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).
845845
846846### Computer use needs the toolset on the Claude API and Claude Models on Google Cloud (Google managed)
847847
No line in this hunk matches that.