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1919| --------------- | --------------- | -------------------------------------------------- |
2020| Claude Opus 5.5 | claude-opus-5-5 | For long-running agentic coding and knowledge work |
2121 
22[Adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) is always on, and the [effort parameter](https://platform.claude.com/docs/en/build-with-claude/effort) controls thinking depth; its default on this model is `medium`. For the context window, output limits, knowledge cutoff, and prices, see the [Claude Opus 5.5 model page](https://platform.claude.com/docs/en/models/opus-5-5/overview); for all current models, see the [models overview](https://platform.claude.com/docs/en/models/overview).
22[Adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) is always on, and the [effort parameter](https://platform.claude.com/docs/en/build-with-claude/effort) controls thinking depth; its default on this model is `medium`. For the context window, output limits, knowledge cutoff, and prices, see the [Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview) model page; for all current models, see [Models overview](https://platform.claude.com/docs/en/models/overview).
2323 
2424## Breaking changes
2525 
2626### Thinking can’t be disabled
2727 
28On Claude Opus 5, thinking is on by default and `thinking: {"type": "disabled"}` is accepted at effort `high` or below. On Claude Opus 5.5, thinking is always on: a request that sets `thinking: {"type": "disabled"}`, or a manual budget with `thinking: {"type": "enabled", "budget_tokens": N}`, returns a 400 `invalid_request_error`. Omit the `thinking` field, or send `thinking: {"type": "adaptive"}`, which is equivalent. No beta header is involved.
28On Claude Opus 5, thinking is on by default and `thinking: {"type": "disabled"}` is accepted at effort `high` or below. On Claude Opus 5.5, thinking is always on: a request that sets `thinking: {"type": "disabled"}`, or one that sets a manual budget with `thinking: {"type": "enabled", "budget_tokens": N}`, returns a 400 `invalid_request_error`. Omit the `thinking` field, or send `thinking: {"type": "adaptive"}`, which is equivalent.
2929 
3030The error messages are:
3131 
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5151 
5252### Thinking blocks are tied to the model and the conversation
5353 
54Every thinking block records which model produced it, and each model reads its own blocks and only some other models’. 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 blocks from Claude Sonnet 5.5 and Claude Haiku 5.5. It doesn’t read blocks from Claude Fable or Claude Mythos models. On the Claude API, Claude Fable 5.1 and Claude Mythos 5.1 read thinking blocks from Claude Opus 5.5. 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 conversation that moves from Claude Opus 5 onto Claude Opus 5.5, or from Claude Opus 5.5 up to Claude Fable 5.1 or Claude Mythos 5.1 on the Claude API, keeps its reasoning. So does one that moves from Claude Opus 5.5 up to Claude Fable 5.2 or Claude Mythos 5.2 on the Claude API. One that moves from Claude Opus 5.5 to any model other than those four runs the turns after the switch without the previous model’s reasoning. So does one that moves onto Claude Opus 5.5 from a Claude Fable or Claude Mythos model. When a request carries a block the target model can’t read, the API drops it before the model sees it: the request succeeds, and dropped blocks aren’t billed. With the `thinking-binding-controls-2026-08-01` beta header, the drop is reported in a top-level `input_transformations` array. See [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models).
54Every thinking block records which model produced it. A conversation that switches models keeps its earlier reasoning only if the model that it switches to can read the earlier model’s thinking blocks:
5555 
56The API also checks whether anything before a Claude Opus 5.5 thinking block (the `system` prompt, the `tools`, or an earlier message) has changed since the block was produced. Keep the conversation append-only: change instructions or tools with [mid-conversation system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) rather than edits. A request that replays a block 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. This applies on the Claude API and on cloud platforms. On accounts created before that date, the request gets the error only when it sets the field to `"error"`. To drop the affected blocks instead, send the `thinking-binding-controls-2026-08-01` beta header and set the field to `"drop_block"`. See [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) and the migration guide’s [note on thinking blocks](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#thinking-blocks).
56| Switch | Keeps its reasoning |
57| ------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------ |
58| To Claude Opus 5.5, from Claude Opus 5 or earlier, Claude Sonnet 5 or earlier, or Claude Haiku 4.5 or earlier | Yes |
59| To Claude Opus 5.5, from Claude Sonnet 5.5 or Claude Haiku 5.5 | Yes, on the Claude API and Claude Models on Google Cloud (Google managed) only |
60| To Claude Opus 5.5, from a Claude Fable or Claude Mythos model | No |
61| From Claude Opus 5.5, to Claude Fable 5.2 or Claude Mythos 5.2 | Yes, on the Claude API and Claude Models on Google Cloud (Google managed) only |
62| From Claude Opus 5.5, to Claude Fable 5.1 or Claude Mythos 5.1 | Yes, on the Claude API only |
63| From Claude Opus 5.5, to any other model | No |
5764 
65When a request carries a block the target model can’t read, the API drops it before the model sees it. The request succeeds, and dropped blocks aren’t billed. With the `thinking-binding-controls-2026-08-01` beta header, the drop is reported in a top-level `input_transformations` array. See [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models). For switches that don’t involve Claude Opus 5.5, see [Which model switches keep the reasoning on the Claude API](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#which-model-switches-keep-reasoning).
66 
67The API also checks whether anything before a Claude Opus 5.5 thinking block (the `system` prompt, the `tools`, or an earlier message) has changed since the block was produced. Keep the conversation append-only: change instructions or tools with [mid-conversation system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) rather than edits.
68 
69A request that replays a block after such a change, with `thinking.block_binding.prefix_mismatch_behavior` unset, gets a 400 error. This applies to accounts created on or after August 31, 2026, 00:00 UTC. [Preserved thinking availability](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#feature-availability) lists the platforms where this applies. On accounts created before that date, the request might or might not get the error. To drop the affected blocks instead, send the `thinking-binding-controls-2026-08-01` beta header and set the field to `"drop_block"`. See [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) and the migration guide’s [note on thinking blocks](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#thinking-blocks).
70 
5871### Some computer use tool versions are not supported
5972 
6073Claude Opus 5.5 supports [computer use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool) through the `computer_toolset_20260801` toolset. On Amazon Bedrock, Claude Opus 5.5 also accepts the `computer_20251124` tool with the `computer-use-2025-11-24` beta header. Claude Opus 5.5 returns a 400 error for any computer use tool version it doesn’t accept, such as `computer_20251124` on the Claude API. The Claude API’s `invalid_request_error` message names the rejected type and, after `Did you mean one of`, lists the tool types the model accepts. The message begins:
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6376'claude-opus-5-5' does not support tool types: computer_20251124.
6477```
6578 
66Integrations that use the toolset need no change. To move a `computer_20251124` integration to the toolset, follow [Migrate from `computer_20251124`](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool#migrate-from-computer-20251124). Drop the beta header and replace the `tools` entry with `{"type": "computer_toolset_20260801"}`. Then update your agent loop for member `tool_use` blocks, batch actions, and `toolset_name` on results. The migration guide shows the request [before and after](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#computer-use-toolset).
79Integrations that use the toolset need no change. To move a `computer_20251124` integration to the toolset, follow [Migrate from `computer_20251124`](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool#migrate-from-computer-20251124). Drop the beta header and replace the `tools` entry with `{"type": "computer_toolset_20260801"}`. Then update your agent loop for member `tool_use` blocks, batch actions, and `toolset_name` on results. The migration guide shows the [before and after for computer use](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#computer-use-toolset).
6780 
6881## Feature support
6982 
70Claude Opus 5.5 supports [per-message effort](https://platform.claude.com/docs/en/build-with-claude/effort#change-effort-mid-conversation-beta) (beta), [mid-conversation system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages), [task budgets](https://platform.claude.com/docs/en/build-with-claude/task-budgets), [prompt caching](https://platform.claude.com/docs/en/build-with-claude/prompt-caching) with a 512-token minimum cacheable prompt, [batch processing](https://platform.claude.com/docs/en/build-with-claude/batch-processing), the [Files API](https://platform.claude.com/docs/en/build-with-claude/files), [PDF support](https://platform.claude.com/docs/en/build-with-claude/pdf-support), [vision](https://platform.claude.com/docs/en/build-with-claude/vision), and server-side and client-side [tools](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview). See each feature’s page for model availability.
83Claude Opus 5.5 supports [per-message effort](https://platform.claude.com/docs/en/build-with-claude/effort-per-message) (in beta), [mid-conversation system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages), [task budgets](https://platform.claude.com/docs/en/build-with-claude/task-budgets), [prompt caching](https://platform.claude.com/docs/en/build-with-claude/prompt-caching) with a 512-token minimum cacheable prompt, [batch processing](https://platform.claude.com/docs/en/build-with-claude/batch-processing), the [Files API](https://platform.claude.com/docs/en/build-with-claude/files), [PDF support](https://platform.claude.com/docs/en/build-with-claude/pdf-support), [vision](https://platform.claude.com/docs/en/build-with-claude/vision), and server-side and client-side tools (see [Tool use with Claude](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview)). See each feature’s page for model availability.
7184 
7285### Fast mode
7386 
74[Fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode) is available for Claude Opus 5.5 on the Claude API, on [Claude Models on Google Cloud (Anthropic managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/anthropic-managed), and on [Claude in Microsoft Foundry](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry) for deployments hosted on Azure. On the Claude API and on Claude Models on Google Cloud (Anthropic managed), set `speed: "fast"` on your request. Claude Opus 5.5 doesn’t need a beta header. See [Fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode) for supported models, rate limits, and pricing.
87[Fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode) is available for Claude Opus 5.5 on the Claude API, on [Claude Models on Google Cloud (Anthropic managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/anthropic-managed), and on [Claude in Microsoft Foundry](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry) for deployments hosted on Azure. On the Claude API and on Claude Models on Google Cloud (Anthropic managed), set `speed: "fast"` on your request. See [Fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode) for supported models, rate limits, and pricing.
7588 
7689### Define tools in a message (beta)
7790 
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106119Claude Opus 5.5 is available on:
107120 
108121* **Claude API:** all customers, as `claude-opus-5-5`
109* **AWS:** [Claude in Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock), as `anthropic.claude-opus-5-5`, and [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), as `claude-opus-5-5`
122* **AWS:** [Claude in Amazon Bedrock (`bedrock-mantle`)](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock), as `anthropic.claude-opus-5-5`, and [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), as `claude-opus-5-5`
110123* **Google Cloud:** [Claude Models on Google Cloud (Anthropic managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/anthropic-managed), as `claude-opus-5-5`, and [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed), as `claude-opus-5-5`
111124* **Microsoft Foundry:** [Claude in Microsoft Foundry](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry), as `claude-opus-5-5`
112125 
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151164 ```
152165</CodeGroup>
153166 
154Then remove any `thinking: {"type": "disabled"}` or `thinking: {"type": "enabled", ...}` settings and choose an [effort](https://platform.claude.com/docs/en/build-with-claude/effort) level instead. Replace `tool_choice` types `any` and `tool` with `auto` plus [strict tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use). If you use computer use through `computer_20251124`, [move to the toolset](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool#migrate-from-computer-20251124). On Amazon Bedrock, `computer_20251124` also works. If your interface shows the text between tool calls, also set `thinking.display`; see [Text between tool calls is returned in thinking blocks](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#text-between-tool-calls). See the [migration guide](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide) for step-by-step instructions from Claude Opus 5 and earlier models, and the full checklist.
167Then remove any `thinking: {"type": "disabled"}` or `thinking: {"type": "enabled", ...}` settings and choose an [effort](https://platform.claude.com/docs/en/build-with-claude/effort) level instead. Replace `tool_choice` types `any` and `tool` with `auto` plus [strict tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use). If you use computer use through `computer_20251124`, [move to the toolset](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool#migrate-from-computer-20251124). On Amazon Bedrock, `computer_20251124` also works. If your interface shows the text between tool calls, also set `thinking.display`; see [Text between tool calls is returned in thinking blocks](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#text-between-tool-calls). See the [Claude Opus 5.5 migration guide](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide) for step-by-step instructions from Claude Opus 5 and earlier models, and the full checklist.
155168 
156169## Next steps
157170 
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