migration-guide changedmodels/opus-5-5/migration-guide
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280280* Set `effort` explicitly: the default is `medium`, where Claude Opus 5’s is `high`.
281281* Replace `tool_choice` types `any` and `tool` with `auto` plus strict tool use or structured outputs.
282282* If you use an earlier computer use tool version, declare `computer_toolset_20260801` (no beta header) instead and update your agent loop for the toolset. On Amazon Bedrock, `computer_20251124` also works.
283* If a router or fallback can move a conversation from Claude Opus 5.5 to another model, expect that model to run without Claude Opus 5.5’s thinking blocks. Claude Fable 5.1 and Claude Mythos 5.1 on the Claude API are the exception and keep them. Claude Opus 5.5 itself reads thinking from Claude Opus 5 and from earlier Opus, Sonnet, and Haiku models. On the Claude API and Claude Models on Google Cloud (Google managed), it also reads thinking from Claude Sonnet 5.5 and Claude Haiku 5.5. It doesn’t read thinking from Claude Fable or Claude Mythos models.
283* If a router or fallback can move a conversation from Claude Opus 5.5 to another model, expect that model to run without Claude Opus 5.5’s thinking blocks. Claude Fable 5.1 and Claude Mythos 5.1 on the Claude API are the exception and keep them. So do Claude Fable 5.2 and Claude Mythos 5.2 there. On Claude Models on Google Cloud (Google managed), Claude Fable 5.2 and Claude Mythos 5.2 read the same blocks as on the Claude API. Claude Opus 5.5 itself reads thinking from Claude Opus 5 and from earlier Opus, Sonnet, and Haiku models. On the Claude API and Claude Models on Google Cloud (Google managed), it also reads thinking from Claude Sonnet 5.5 and Claude Haiku 5.5. It doesn’t read thinking from Claude Fable or Claude Mythos models.
284284* Read content blocks by `type`, and pass `thinking` blocks back unmodified in tool-use loops.
285285* If your interface renders text between tool calls, set `display: "updates"` (beta) or `"summarized"` and render the non-empty `thinking` blocks.
286286* If your code edits earlier turns, the `system` prompt, or `tools` mid-conversation, follow [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking).
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877877#### Thinking blocks are tied to the model and the conversation
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879On the Claude API, Claude Fable 5.1 and Claude Mythos 5.1 read Claude Opus 5.5 thinking blocks. No other model does. A router or fallback that moves a conversation from Claude Opus 5.5 to any other model runs those turns without them. In the other direction, Claude Opus 5.5 reads thinking blocks from Claude Opus 5 and from earlier Opus, Sonnet, and Haiku models. On the Claude API and Claude Models on Google Cloud (Google managed), it also reads them from Claude Sonnet 5.5 and Claude Haiku 5.5. It doesn’t read them from Claude Fable or Claude Mythos models. Keep the conversation append-only (no edits to the `system` prompt, `tools`, or earlier messages mid-conversation) so the blocks stay valid; Claude Code, claude.ai, Claude Managed Agents, and the Claude Agent SDK already do. On accounts created on or after August 31, 2026, 00:00 UTC, on the Claude API and on cloud platforms, a request that replays a thinking block after such an edit and leaves `thinking.block_binding.prefix_mismatch_behavior` unset gets a 400 error. There is no code change for append-only integrations. See [Thinking blocks are tied to the model and the conversation](https://platform.claude.com/docs/en/models/opus-5-5/whats-new-opus-5-5#thinking-blocks-are-tied-to-the-model-that-produced-them) and [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking).
879On the Claude API, Claude Fable 5.1 and Claude Mythos 5.1 read Claude Opus 5.5 thinking blocks. So do Claude Fable 5.2 and Claude Mythos 5.2 on the Claude API. On Claude Models on Google Cloud (Google managed), Claude Fable 5.2 and Claude Mythos 5.2 read the same blocks as on the Claude API. No other model does. A router or fallback that moves a conversation from Claude Opus 5.5 to any other model runs those turns without them. In the other direction, Claude Opus 5.5 reads thinking blocks from Claude Opus 5 and from earlier Opus, Sonnet, and Haiku models. On the Claude API and Claude Models on Google Cloud (Google managed), it also reads them from Claude Sonnet 5.5 and Claude Haiku 5.5. It doesn’t read them from Claude Fable or Claude Mythos models. Keep the conversation append-only (no edits to the `system` prompt, `tools`, or earlier messages mid-conversation) so the blocks stay valid; Claude Code, claude.ai, Claude Managed Agents, and the Claude Agent SDK already do. On accounts created on or after August 31, 2026, 00:00 UTC, on the Claude API and on cloud platforms, a request that replays a thinking block after such an edit and leaves `thinking.block_binding.prefix_mismatch_behavior` unset gets a 400 error. There is no code change for append-only integrations. See [Thinking blocks are tied to the model and the conversation](https://platform.claude.com/docs/en/models/opus-5-5/whats-new-opus-5-5#thinking-blocks-are-tied-to-the-model-that-produced-them) and [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking).
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881881#### Some computer use tool versions are not supported
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