whats-new-sonnet-5-5 changedmodels/sonnet-5-5/whats-new-sonnet-5-5
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2020| ----------------- | ----------------- | ---------------------------------------------- |
2121| Claude Sonnet 5.5 | claude-sonnet-5-5 | The best combination of speed and intelligence |
2222
23[Adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) is on by default, and the [effort parameter](https://platform.claude.com/docs/en/build-with-claude/effort) controls thinking depth. Its default on the Claude API is `high`. The tokenizer is the same as Claude Sonnet 5's, so the same text produces the same token counts. For the context window, output limits, knowledge cutoff, and prices, see the [Claude Sonnet 5.5 model page](https://platform.claude.com/docs/en/models/sonnet-5-5/overview).
23[Adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) is on by default, and the [effort parameter](https://platform.claude.com/docs/en/build-with-claude/effort) controls thinking depth. Its default on the Claude API is `high`. The tokenizer is the same as Claude Sonnet 5’s, so the same text produces the same token counts. For the context window, output limits, knowledge cutoff, and prices, see the [Claude Sonnet 5.5 model page](https://platform.claude.com/docs/en/models/sonnet-5-5/overview).
2424
2525For all current models, see the [models overview](https://platform.claude.com/docs/en/models/overview).
2626
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2828
2929### Turn off up-front thinking with `between_tools`
3030
31To turn off up-front thinking on Claude Sonnet 5.5, send `thinking: {"type": "between_tools"}` instead of `"disabled"`. It's the lowest thinking setting on this model. It's available on every platform that offers Claude Sonnet 5.5. It needs no beta header. The short [progress updates](https://platform.claude.com/docs/en/build-with-claude/thinking#progress-updates) the model writes between tool calls still come back as `thinking` blocks with their summary text. Pass those blocks back unchanged with the rest of the assistant turn. A progress-update block you send back gives the model the full note it wrote, not the summary. If your requests don't use tools, the response contains only text, as with `disabled` on Claude Sonnet 5.
31To turn off up-front thinking on Claude Sonnet 5.5, send `thinking: {"type": "between_tools"}` instead of `"disabled"`. It’s the lowest thinking setting on this model. It’s available on every platform that offers Claude Sonnet 5.5. It needs no beta header. The short [progress updates](https://platform.claude.com/docs/en/build-with-claude/thinking#progress-updates) the model writes between tool calls still come back as `thinking` blocks with their summary text. Pass those blocks back unchanged with the rest of the assistant turn. A progress-update block you send back gives the model the full note it wrote, not the summary. If your requests don’t use tools, the response contains only text, as with `disabled` on Claude Sonnet 5.
3232
3333On Claude Sonnet 5.5, a request that sends `thinking: {"type": "disabled"}` returns a 400 `invalid_request_error` whose message points to `between_tools`.
3434
35`between_tools` is accepted at `low`, `medium`, and `high` effort. At `xhigh` or `max` effort, a request with `between_tools` returns a 400 error. To run at `xhigh` or `max`, use adaptive thinking: omit the `thinking` field or send `thinking: {"type": "adaptive"}`, which is equivalent. With `between_tools`, effort can't change mid-conversation: a [per-message](https://platform.claude.com/docs/en/build-with-claude/effort#change-effort-mid-conversation-beta) `output_config.effort` that differs from the level in effect returns a 400 error. To vary effort for each turn, use adaptive thinking.
35`between_tools` is accepted at `low`, `medium`, and `high` effort. At `xhigh` or `max` effort, a request with `between_tools` returns a 400 error. To run at `xhigh` or `max`, use adaptive thinking: omit the `thinking` field or send `thinking: {"type": "adaptive"}`, which is equivalent. With `between_tools`, effort can’t change mid-conversation: a [per-message](https://platform.claude.com/docs/en/build-with-claude/effort#change-effort-mid-conversation-beta) `output_config.effort` that differs from the level in effect returns a 400 error. To vary effort for each turn, use adaptive thinking.
3636
3737`between_tools` takes no other field: `display`, `budget_tokens`, or `block_binding` sent with it returns a 400 error. Manual thinking budgets (`thinking: {"type": "enabled", "budget_tokens": N}`) return a 400 error. See [Thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) and [Turn off up-front thinking](https://platform.claude.com/docs/en/models/sonnet-5-5/migration-guide#turn-off-up-front-thinking) in the migration guide.
3838
3939### Forced tool use is not supported
4040
41Claude Sonnet 5.5 doesn't support forced tool use. `tool_choice` set to `{"type": "any"}` or `{"type": "tool", "name": "..."}` returns a 400 `invalid_request_error`:
41Claude Sonnet 5.5 doesn’t support forced tool use. `tool_choice` set to `{"type": "any"}` or `{"type": "tool", "name": "..."}` returns a 400 `invalid_request_error`:
4242
4343```text wrap
4444tool_choice: type "tool" and "any" are not supported for this model.
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4848
4949### Thinking blocks are tied to the model and the conversation
5050
51Every thinking block records which model produced it. Each model reads its own blocks and only some other models' blocks. Claude 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, but not from Claude Opus 5, Claude Opus 5.5, or any Claude Fable or Claude Mythos model. On the Claude API and Claude Models on Google Cloud (Google managed), Claude Opus 5.5 reads Claude Sonnet 5.5 thinking blocks; no other model does.
51Every thinking block records which model produced it. Each model reads its own blocks and only some other models’ blocks. Claude 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, but not from Claude Opus 5, Claude Opus 5.5, or any Claude Fable or Claude Mythos model. On the Claude API and Claude Models on Google Cloud (Google managed), Claude Opus 5.5 reads Claude Sonnet 5.5 thinking blocks; no other model does.
5252
53So a conversation that moves from Claude Sonnet 5 onto Claude Sonnet 5.5, or from Claude Sonnet 5.5 up to Claude Opus 5.5 on the Claude API and Claude Models on Google Cloud (Google managed), keeps its reasoning, and any other move away from Claude Sonnet 5.5 runs the turns after the switch without it. 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).
53So a conversation that moves from Claude Sonnet 5 onto Claude Sonnet 5.5, or from Claude Sonnet 5.5 up to Claude Opus 5.5 on the Claude API and Claude Models on Google Cloud (Google managed), keeps its reasoning, and any other move away from Claude Sonnet 5.5 runs the turns after the switch without it. 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).
5454
5555The API also checks whether anything before a Claude Sonnet 5.5 thinking block has changed since the block was produced: the `system` prompt, the `tools`, or an earlier message. It enforces that check by default for accounts created on or after August 31, 2026, 00:00 UTC, on the Claude API, Amazon Bedrock, and Claude Models on Google Cloud (Google managed). 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. `block_binding` works only with `thinking: {"type": "adaptive"}`. With `between_tools`, keep the history append-only, or strip the thinking blocks from the edited turn on.
5656
57Keep the conversation append-only so the check never fails: 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/sonnet-5-5/migration-guide#thinking-blocks).
57Keep the conversation append-only so the check never fails: 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/sonnet-5-5/migration-guide#thinking-blocks).
5858
5959### The `computer_20251124` computer use tool is not supported on the Claude API and Claude Models on Google Cloud (Google managed)
6060
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6666
6767On Amazon Bedrock, Claude Sonnet 5.5 accepts the earlier `computer_20251124` tool.
6868
69To move an existing integration on the Claude API or Claude Models on Google Cloud (Google managed), follow [Migrate from `computer_20251124`](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool#migrate-from-computer-20251124), which shows the request before and after. Drop the beta header, replace the `tools` entry with `{"type": "computer_toolset_20260801"}`, and update your agent loop for member `tool_use` blocks, batch actions, and `toolset_name` on results. The toolset is available on both the Claude API and Claude Models on Google Cloud (Google managed). For other platforms, see the computer use tool's [Compatibility](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool#compatibility) section. Integrations that already use the toolset, and the [browser use tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/browser-use-tool), need no change.
69To move an existing integration on the Claude API or Claude Models on Google Cloud (Google managed), follow [Migrate from `computer_20251124`](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool#migrate-from-computer-20251124), which shows the request before and after. Drop the beta header, replace the `tools` entry with `{"type": "computer_toolset_20260801"}`, and update your agent loop for member `tool_use` blocks, batch actions, and `toolset_name` on results. The toolset is available on both the Claude API and Claude Models on Google Cloud (Google managed). For other platforms, see the computer use tool’s [Compatibility](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool#compatibility) section. Integrations that already use the toolset, and the [browser use tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/browser-use-tool), need no change.
7070
7171### Some advisor tool pairings are not supported
7272
73With the [advisor tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool) (beta), a Claude Sonnet 5.5 executor needs Claude Mythos 5.1, Claude Fable 5.1, Claude Mythos 5, Claude Fable 5, Claude Opus 5.5, or Claude Opus 5 as its advisor, or Claude Sonnet 5.5 itself. Claude Opus 4.8, Claude Opus 4.7, Claude Sonnet 5, and Claude Haiku 5.5 advisors work with a Claude Sonnet 5 executor, but with a Claude Sonnet 5.5 executor they return a 400 `invalid_request_error`. Every advisor that Claude Sonnet 5.5 accepts returns its advice encrypted, as an `advisor_redacted_result` block, so your client can't read the advice text. See the advisor tool's [Model compatibility](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool#model-compatibility) and [Result variants](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool#result-variants).
73With the [advisor tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool) (beta), a Claude Sonnet 5.5 executor needs Claude Mythos 5.1, Claude Fable 5.1, Claude Mythos 5, Claude Fable 5, Claude Opus 5.5, or Claude Opus 5 as its advisor, or Claude Sonnet 5.5 itself. Claude Opus 4.8, Claude Opus 4.7, Claude Sonnet 5, and Claude Haiku 5.5 advisors work with a Claude Sonnet 5 executor, but with a Claude Sonnet 5.5 executor they return a 400 `invalid_request_error`. Every advisor that Claude Sonnet 5.5 accepts returns its advice encrypted, as an `advisor_redacted_result` block, so your client can’t read the advice text. See the advisor tool’s [Model compatibility](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool#model-compatibility) and [Result variants](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool#result-variants).
7474
7575## Feature support
7676
77Claude Sonnet 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), [mid-conversation tool changes](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#mid-conversation-tool-changes) (beta), [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). Per-message effort, mid-conversation system messages, and mid-conversation tool changes aren't available on Claude Sonnet 5, whose minimum cacheable prompt is 1,024 tokens. On the Claude API and Claude Models on Google Cloud (Google managed), computer use requires the `computer_toolset_20260801` toolset (see the [computer use breaking change](https://platform.claude.com/docs/en/models/sonnet-5-5/whats-new-sonnet-5-5#computer-20251124-is-not-supported)). See each feature's page for model availability.
77Claude Sonnet 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), [mid-conversation tool changes](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#mid-conversation-tool-changes) (beta), [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). Per-message effort, mid-conversation system messages, and mid-conversation tool changes aren’t available on Claude Sonnet 5, whose minimum cacheable prompt is 1,024 tokens. On the Claude API and Claude Models on Google Cloud (Google managed), computer use requires the `computer_toolset_20260801` toolset (see the [computer use breaking change](https://platform.claude.com/docs/en/models/sonnet-5-5/whats-new-sonnet-5-5#computer-20251124-is-not-supported)). See each feature’s page for model availability.
7878
7979### Compact on demand (beta)
8080
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8686
8787### Thinking blocks stay with the account that produced them
8888
89Thinking blocks that Claude Sonnet 5.5 produces 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. On the Claude API and Claude Models on Google Cloud (Google managed), with the `thinking-binding-controls-2026-08-01` beta header, the response lists each dropped block in `input_transformations` with `reason: "organization_binding_mismatch"`. Blocks from earlier models aren't affected. See [thinking blocks from another account](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#account-bound-thinking) in Preserved thinking.
89Thinking blocks that Claude Sonnet 5.5 produces 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. On the Claude API and Claude Models on Google Cloud (Google managed), with the `thinking-binding-controls-2026-08-01` beta header, the response lists each dropped block in `input_transformations` with `reason: "organization_binding_mismatch"`. Blocks from earlier models aren’t affected. See [thinking blocks from another account](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#account-bound-thinking) in Preserved thinking.
9090
9191## Behavior differences
9292
9393Claude Sonnet 5.5 differs from Claude Sonnet 5 in several ways that show up without any code change. [Prompting Claude Sonnet 5.5](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-sonnet-5-5) has guidance for each:
9494
95* **Effort levels are recalibrated.** An [effort](https://platform.claude.com/docs/en/build-with-claude/effort) level doesn't produce the same amount of thinking as it did on Claude Sonnet 5. Re-run your effort sweep rather than carrying a setting over. Start at `high` unless your workload is agentic or latency-sensitive. For agentic coding and multistep tool use, start at `medium` for well-specified tasks and move to `high` for harder or longer ones. For chat and other latency-sensitive work, start at `medium` or `low`.
96* **Text between tool calls comes back in thinking blocks.** Between tool calls, notes longer than a sentence or two come back as [progress-update `thinking` blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#progress-updates). Shorter remarks stay `text`. At the default `display: "omitted"`, the progress-update blocks' text is empty, so an application that streams those notes to its users goes quiet between tool calls, with no error. If you [turn off up-front thinking with `between_tools`](https://platform.claude.com/docs/en/models/sonnet-5-5/whats-new-sonnet-5-5#turn-off-up-front-thinking), the text comes back. To receive it, see [Text between tool calls is returned in thinking blocks](https://platform.claude.com/docs/en/models/sonnet-5-5/migration-guide#text-between-tool-calls) in the migration guide.
97* **Safeguard categories.** The model's safeguards can decline a request in five [`stop_details` categories](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#refusal-response). `"cyber"` means the request could enable cyber harm. `"bio"` means it could enable biological harm. `"frontier_llm"` means it could help with the development of competing AI models. `"reasoning_extraction"` means it asks the model to reproduce its internal reasoning in the response text. `"general_harms"` means it falls under another usage-policy area. See [Refusals, fallback, and billing](https://platform.claude.com/docs/en/models/sonnet-5-5/whats-new-sonnet-5-5#refusals-fallback-and-billing).
95* **Effort levels are recalibrated.** An [effort](https://platform.claude.com/docs/en/build-with-claude/effort) level doesn’t produce the same amount of thinking as it did on Claude Sonnet 5. Re-run your effort sweep rather than carrying a setting over. Start at `high` unless your workload is agentic or latency-sensitive. For agentic coding and multistep tool use, start at `medium` for well-specified tasks and move to `high` for harder or longer ones. For chat and other latency-sensitive work, start at `medium` or `low`.
96* **Text between tool calls comes back in thinking blocks.** Between tool calls, notes longer than a sentence or two come back as [progress-update `thinking` blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#progress-updates). Shorter remarks stay `text`. At the default `display: "omitted"`, the progress-update blocks’ text is empty, so an application that streams those notes to its users goes quiet between tool calls, with no error. If you [turn off up-front thinking with `between_tools`](https://platform.claude.com/docs/en/models/sonnet-5-5/whats-new-sonnet-5-5#turn-off-up-front-thinking), the text comes back. To receive it, see [Text between tool calls is returned in thinking blocks](https://platform.claude.com/docs/en/models/sonnet-5-5/migration-guide#text-between-tool-calls) in the migration guide.
97* **Safeguard categories.** The model’s safeguards can decline a request in five [`stop_details` categories](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#refusal-response). `"cyber"` means the request could enable cyber harm. `"bio"` means it could enable biological harm. `"frontier_llm"` means it could help with the development of competing AI models. `"reasoning_extraction"` means it asks the model to reproduce its internal reasoning in the response text. `"general_harms"` means it falls under another usage-policy area. See [Refusals, fallback, and billing](https://platform.claude.com/docs/en/models/sonnet-5-5/whats-new-sonnet-5-5#refusals-fallback-and-billing).
9898
9999## Refusals, fallback, and billing
100100
101Everything in [Refusals and fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback) applies to Claude Sonnet 5.5. A declined request returns HTTP 200 with `stop_reason: "refusal"` and a [`stop_details`](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#refusal-response) object naming the policy area. Handle refusals and configure fallback. [Server-side fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#server-side-fallback) (`fallbacks: "default"`), in beta on the Claude API and on [Claude Models on Google Cloud (Anthropic managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/anthropic-managed), retries `"cyber"` and `"frontier_llm"` declines on Claude Sonnet 5. It doesn't retry `"bio"`, `"reasoning_extraction"`, or `"general_harms"` declines. You can also use the [SDK middleware](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#client-side-fallback) or your own retry. Whether a refusal that arrives before any output is billed depends on its refusal category, and it counts against your rate limits either way. See [How refusals are billed](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#how-refusals-are-billed).
101Everything in [Refusals and fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback) applies to Claude Sonnet 5.5. A declined request returns HTTP 200 with `stop_reason: "refusal"` and a [`stop_details`](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#refusal-response) object naming the policy area. Handle refusals and configure fallback. [Server-side fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#server-side-fallback) (`fallbacks: "default"`), in beta on the Claude API and on [Claude Models on Google Cloud (Anthropic managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/anthropic-managed), retries `"cyber"` and `"frontier_llm"` declines on Claude Sonnet 5. It doesn’t retry `"bio"`, `"reasoning_extraction"`, or `"general_harms"` declines. You can also use the [SDK middleware](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#client-side-fallback) or your own retry. Whether a refusal that arrives before any output is billed depends on its refusal category, and it counts against your rate limits either way. See [How refusals are billed](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#how-refusals-are-billed).
102102
103103## Pricing
104104
No line in this hunk matches that.