whats-new-opus-5-5 changedmodels/opus-5-5/whats-new-opus-5-5
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3030"thinking.type.enabled" is not supported for this model. Use "thinking.type.adaptive" and "output_config.effort" to control thinking behavior.
3131```
3232
33The [effort parameter](https://platform.claude.com/docs/en/build-with-claude/effort) is the control for thinking depth, latency, and cost: lower it where you previously disabled thinking; [Optimizing for cost and intelligence](https://platform.claude.com/docs/en/about-claude/models/optimizing-for-cost-and-intelligence#tune-effort) has measured results for choosing a level. Because every response can begin with one or more `thinking` blocks (returned with an empty `thinking` field at the default `display: "omitted"`), select content blocks by their `type` field rather than by position, and pass `thinking` blocks back unmodified in tool-use loops. Code that already runs on Claude Opus 5 with thinking on needs no change. See [Thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) and the migration guide's [before and after](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#thinking-cant-be-disabled).
33The [effort parameter](https://platform.claude.com/docs/en/build-with-claude/effort) is the control for thinking depth, latency, and cost: lower it where you previously disabled thinking; [Optimizing for cost and intelligence](https://platform.claude.com/docs/en/about-claude/models/optimizing-for-cost-and-intelligence#tune-effort) has measured results for choosing a level. Because every response can start with one or more `thinking` blocks (returned with an empty `thinking` field at the default `display: "omitted"`), select content blocks by their `type` field rather than by position, and pass `thinking` blocks back unmodified in tool-use loops. Code that already runs on Claude Opus 5 with thinking on needs no change. See [Thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) and the migration guide's [before and after for thinking](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#thinking-cant-be-disabled).
3434
3535### Forced tool use is not supported
3636
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4040tool_choice: type "tool" and "any" are not supported for this model.
4141```
4242
43`tool_choice: {"type": "auto"}` (the default) and `{"type": "none"}` are supported, and the same validation applies to the [token counting](https://platform.claude.com/docs/en/build-with-claude/token-counting) endpoint. For schema-valid JSON, keep `tool_choice: {"type": "auto"}` and set `strict: true` with [strict tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use), or move the schema to [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs). To make the model call a tool rather than reply in text, say in the prompt when the tool applies. The migration guide shows the [before and after](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#forced-tool-use).
43`tool_choice: {"type": "auto"}` (the default) and `{"type": "none"}` are supported, and the same validation applies to the [token counting](https://platform.claude.com/docs/en/build-with-claude/token-counting) endpoint. For schema-valid JSON, keep `tool_choice: {"type": "auto"}` and set `strict: true` with [strict tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use), or move the schema to [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs). To make the model call a tool rather than reply in text, say in the prompt when the tool applies. The migration guide shows the [before and after for tool choice](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#forced-tool-use).
4444
4545### Thinking blocks are tied to the model and the conversation
4646
4747Every 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, from earlier Opus, Sonnet, and Haiku models, and, on the Claude API and Google Cloud, from Claude Sonnet 5.5 and Claude Haiku 5.5, but not 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; 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. One that moves from Claude Opus 5.5 to any model other than those two, or onto it from a Claude Fable or Claude Mythos model, runs the turns after the switch without the previous model's reasoning. 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).
4848
49The 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. As on Claude Fable 5.1, it enforces that check by default for accounts created on or after August 31, 2026, 00:00 UTC, on the Claude API and on cloud platforms. On those accounts, a request that replays a block after such a change returns a 400 error. To drop the affected blocks instead, send the `thinking-binding-controls-2026-08-01` beta header and set `thinking.block_binding.prefix_mismatch_behavior` to `"drop_block"`. On older accounts, setting that field to either value opts the request in. Keep the conversation append-only so the question never arises: 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. See [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) and the migration guide's [note on this change](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#thinking-blocks).
49The 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. As on Claude Fable 5.1, it enforces that check by default for accounts created on or after August 31, 2026, 00:00 UTC, on the Claude API and on cloud platforms. On those accounts, a request that replays a block after such a change returns a 400 error. To drop the affected blocks instead, send the `thinking-binding-controls-2026-08-01` beta header and set `thinking.block_binding.prefix_mismatch_behavior` to `"drop_block"`. On older accounts, setting that field to either value opts the request in. Keep the conversation append-only so the question never arises: 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. 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).
5050
5151### The `computer_20251124` computer use tool is not supported on the Claude API and Google Cloud
5252
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8181* **The default effort is `medium`.** A request that omits `effort` runs at `medium`; on Claude Opus 5 it ran at `high`. Set `effort` explicitly and re-run your sweep; see [Calibrate effort](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5-5#calibrate-effort).
8282* **More thinking per turn at a given effort level.** At the same [effort](https://platform.claude.com/docs/en/build-with-claude/effort) setting the model tends to think more per turn than Claude Opus 5, most of all at `xhigh` and `max`. Re-run your effort sweep rather than carrying a setting over, and leave room in `max_tokens` for the thinking. See [Calibrate effort](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5-5#calibrate-effort).
8383* **Text between tool calls comes back in thinking blocks.** The short notes the model writes between tool calls arrive as [progress-update `thinking` blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#progress-updates) rather than `text` blocks, so at the default `display: "omitted"` an application that streams them to its users goes quiet between tool calls, with no error. The [migration guide](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#text-between-tool-calls) has the fix for receiving them, and [User-facing progress updates](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5-5#user-facing-progress-updates) covers how to ask for more of them.
84* **More safeguard categories.** The model runs a biology safety classifier in addition to the cybersecurity one. As on Claude Opus 5, requests that push it to reproduce its internal reasoning in the response text may be declined with the `reasoning_extraction` category. See [Refusals and fallback](https://platform.claude.com/docs/en/models/opus-5-5/whats-new-opus-5-5#refusals-and-fallback) and [Safeguard refusals](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5-5#safeguard-refusals).
85* **Sharper reading of charts, diagrams, and screenshots.** The model reads values off dense charts and layout-dependent visuals much more precisely without tools, so prompt-side vision workarounds built for earlier models may no longer be needed; image tools still add accuracy on the densest inputs. See [Tools for complex visual inputs](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5-5#tools-for-complex-visual-inputs).
84* **More safeguard categories.** The model runs a biology safety classifier in addition to the cybersecurity one. As on Claude Opus 5, requests that push it to reproduce its internal reasoning in the response text might be declined with the `reasoning_extraction` category. See [Refusals and fallback](https://platform.claude.com/docs/en/models/opus-5-5/whats-new-opus-5-5#refusals-and-fallback) and [Safeguard refusals](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5-5#safeguard-refusals).
85* **Sharper reading of charts, diagrams, and screenshots.** The model reads values off dense charts and layout-dependent visuals much more precisely without tools, so prompt-side vision workarounds built for earlier models might no longer be needed; image tools still add accuracy on the densest inputs. See [Tools for complex visual inputs](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5-5#tools-for-complex-visual-inputs).
8686
87If your Claude Opus 5 integration ran with thinking disabled, see [Prompts written for thinking disabled](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5-5#prompts-written-for-thinking-disabled) alongside the [breaking change](https://platform.claude.com/docs/en/models/opus-5-5/whats-new-opus-5-5#thinking-cant-be-disabled). For the capability gains in agentic coding and code review, knowledge work, communication, visual inputs, and computer use, see [Capabilities relevant to prompting](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5-5#capability-improvements).
87If your Claude Opus 5 integration ran with thinking disabled, see [Prompts written for thinking disabled](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5-5#prompts-written-for-thinking-disabled) alongside the breaking change [Thinking can't be disabled](https://platform.claude.com/docs/en/models/opus-5-5/whats-new-opus-5-5#thinking-cant-be-disabled). For the capability gains in agentic coding and code review, knowledge work, communication, visual inputs, and computer use, see [Capabilities relevant to prompting](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5-5#capability-improvements).
8888
8989## Refusals and fallback
9090
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9292
9393## Pricing
9494
95Claude Opus 5.5 costs $4 USD per million input tokens and $20 USD per million output tokens, below Claude Opus 5's $5 and $25, with 5-minute cache writes at $5, 1-hour cache writes at $8, and cache reads at $0.20 per million tokens (0.05x the base input price). [Batch processing](https://platform.claude.com/docs/en/build-with-claude/batch-processing) is half price: $2 and $10. See [Pricing](https://platform.claude.com/docs/en/about-claude/pricing) for data residency and tool pricing.
95Claude Opus 5.5 costs $4 USD per million input tokens and $20 USD per million output tokens, below Claude Opus 5's $5 and $25 USD, with 5-minute cache writes at $5 USD, 1-hour cache writes at $8 USD, and cache reads at $0.20 USD per million tokens (0.05x the base input price). [Batch processing](https://platform.claude.com/docs/en/build-with-claude/batch-processing) is half price: $2 and $10 USD. See [Pricing](https://platform.claude.com/docs/en/about-claude/pricing) for data residency and tool pricing.
9696
9797## Availability
9898
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