whats-new-haiku-5-5 changedmodels/haiku-5-5/whats-new-haiku-5-5
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1717| [Adaptive thinking and effort](https://platform.claude.com/docs/en/models/haiku-5-5/whats-new-haiku-5-5#adaptive-thinking-and-effort) | New | Optional: set `effort` to trade response quality against speed and cost. |
1818| [Larger context window and output](https://platform.claude.com/docs/en/models/haiku-5-5/whats-new-haiku-5-5#larger-context-window-and-output) | New | None. Existing `max_tokens` values stay valid, but thinking tokens count toward them. Long prompts are billed at higher prices; see [Long context pricing](https://platform.claude.com/docs/en/about-claude/pricing#long-context-pricing). |
1919| [Browser use tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/browser-use-tool) | New | None. Available on the Claude API and Claude Models on Google Cloud (Google managed). |
20| [Safety classifiers can decline a request](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#refusal-response) | New | Handle `stop_reason: "refusal"` in your client. Server-side fallback isn't available. |
20| [Safety classifiers can decline a request](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#refusal-response) | New | Handle `stop_reason: "refusal"` in your client. Server-side fallback isn’t available. |
2121| [Manual extended thinking returns an error](https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide#configure-thinking) | Breaking | Replace `budget_tokens` with adaptive thinking. |
2222| [Sampling parameters can return an error](https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide#remove-sampling-parameters) | Breaking | Omit `temperature`, `top_p`, and `top_k`. |
2323| [Assistant message prefill returns an error](https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide#replace-assistant-prefill) | Breaking | End `messages` with a user turn. |
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2929| [Replaying thinking blocks across accounts](https://platform.claude.com/docs/en/models/haiku-5-5/whats-new-haiku-5-5#replaying-thinking-blocks-across-accounts) | Changed | If you replay stored conversations through a different account, replay each through the account that produced it. |
3030| [Earlier thinking blocks stay in context](https://platform.claude.com/docs/en/models/haiku-5-5/whats-new-haiku-5-5#earlier-thinking-blocks-stay-in-context) | Changed | Expect more input tokens in multi-turn conversations. To remove older blocks, use [thinking block clearing](https://platform.claude.com/docs/en/build-with-claude/context-editing#thinking-block-clearing). |
3131| [Shorter prompts can be cached](https://platform.claude.com/docs/en/build-with-claude/prompt-caching#cache-limitations) | Changed | None. The minimum cacheable prompt length is 512 tokens, down from 4,096 on Claude Haiku 4.5, so a prompt in between that sets `cache_control` is cached on Claude Haiku 5.5. |
32| [Context awareness tags aren't injected](https://platform.claude.com/docs/en/build-with-claude/context-windows#context-awareness) | Changed | To give the model an explicit budget for an agentic loop, use [task budgets](https://platform.claude.com/docs/en/build-with-claude/task-budgets) (beta). |
32| [Context awareness tags aren’t injected](https://platform.claude.com/docs/en/build-with-claude/context-windows#context-awareness) | Changed | To give the model an explicit budget for an agentic loop, use [task budgets](https://platform.claude.com/docs/en/build-with-claude/task-budgets) (beta). |
3333
3434## New capabilities
3535
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4545
4646### Responses can start with thinking blocks
4747
48Adaptive thinking is on by default, so a response can start with one or more `thinking` blocks even when the request doesn't mention thinking. Code that reads the first content block as the answer needs to select blocks by their `type` field. See [Configure thinking](https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide#configure-thinking) in the migration guide.
48Adaptive thinking is on by default, so a response can start with one or more `thinking` blocks even when the request doesn’t mention thinking. Code that reads the first content block as the answer needs to select blocks by their `type` field. See [Configure thinking](https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide#configure-thinking) in the migration guide.
4949
5050### Earlier thinking blocks stay in context
5151
52Claude Haiku 5.5 keeps thinking blocks from all earlier assistant turns in context, and they count as input tokens. Claude Haiku 4.5 kept only the latest turn's blocks. Multi-turn conversations therefore carry more input tokens than the tokenizer change alone explains. To remove older blocks, use [thinking block clearing](https://platform.claude.com/docs/en/build-with-claude/context-editing#thinking-block-clearing). See [Thinking block preservation by model](https://platform.claude.com/docs/en/build-with-claude/thinking#thinking-block-preservation-by-model).
52Claude Haiku 5.5 keeps thinking blocks from all earlier assistant turns in context, and they count as input tokens. Claude Haiku 4.5 kept only the latest turn’s blocks. Multi-turn conversations therefore carry more input tokens than the tokenizer change alone explains. To remove older blocks, use [thinking block clearing](https://platform.claude.com/docs/en/build-with-claude/context-editing#thinking-block-clearing). See [Thinking block preservation by model](https://platform.claude.com/docs/en/build-with-claude/thinking#thinking-block-preservation-by-model).
5353
5454### Same text counts as more tokens
5555
56Claude Haiku 5.5 uses the same newer tokenizer as Claude 4.7 and later models. As with all models that use this tokenizer, the same input text produces approximately 30 percent more tokens on Claude Haiku 5.5 than on Claude Haiku 4.5. The exact increase depends on the content. The shape of requests and responses doesn't depend on the tokenizer, but anything you measure or budget in tokens changes. See [Recount tokens](https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide#recount-tokens) in the migration guide.
56Claude Haiku 5.5 uses the same newer tokenizer as Claude 4.7 and later models. As with all models that use this tokenizer, the same input text produces approximately 30 percent more tokens on Claude Haiku 5.5 than on Claude Haiku 4.5. The exact increase depends on the content. The shape of requests and responses doesn’t depend on the tokenizer, but anything you measure or budget in tokens changes. See [Recount tokens](https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide#recount-tokens) in the migration guide.
5757
5858### Replaying thinking blocks across accounts
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