whats-new-haiku-5-5 changedmodels/haiku-5-5/whats-new-haiku-5-5
Nearest release: v2.1.307, published an hour after this site recorded the change. Shown because the two are within 24 hours of each other. Nothing here says the release caused the edit.
Recorded here
Lines+6added
Lines−6removed
From line
4
where the diff opens
First seen
8 Oct 2026
this site's first read of the page
Recorded edits4to this page, all time
### Responses can start with thinking blocks ### Responses can begin with thinking blocks
The whole hunk
from line 4, old and new numbered
/
from line 4
44description: Overview of new capabilities, breaking changes, and behavior changes in Claude Haiku 5.5, with a link to each feature's guide and to the migration guide for code changes.
55---
66
7Claude Haiku 5.5 is built for high-volume, latency-sensitive work such as classification, routing, extraction, and subagent tasks. It supports adaptive thinking with the effort parameter, a 1M token context window, and up to 128k output tokens. It uses the same newer tokenizer as Claude 4.7 and later models, so the same text counts as approximately 30% more tokens than on Claude Haiku 4.5. Its thinking blocks work only in the account that produced them, or in an account linked to it.
7Claude Haiku 5.5 is built for high-volume, latency-sensitive work such as classification, routing, extraction, and subagent tasks. It supports adaptive thinking with the effort parameter, a 1M token context window, and up to 128k output tokens. It uses the same newer tokenizer as Claude 4.7 and later models, so the same text counts as approximately 30 percent more tokens than on Claude Haiku 4.5. Its thinking blocks work only in the account that produced them, or in an account linked to it.
88
99For code changes, see the [migration guide](https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide). For model IDs, pricing, and limits, see the [Claude Haiku 5.5 overview](https://platform.claude.com/docs/en/models/haiku-5-5/overview). For prompting guidance, see [Prompting Claude Haiku 5.5](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-haiku-5-5).
1010
from line 23
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. |
2424| [`computer_20250124` returns an error on the Claude API, Google Cloud, and Amazon Bedrock](https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide#computer-use-toolset) | Breaking | Replace it with `computer_toolset_20260801` on the Claude API and Google Cloud, or with `computer_20251124` on Amazon Bedrock. |
2525| [Changing earlier turns invalidates thinking blocks](https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide#keep-earlier-turns-unchanged) | Breaking | Keep conversations append-only if you send thinking blocks back. |
26| [Responses can begin with thinking blocks](https://platform.claude.com/docs/en/models/haiku-5-5/whats-new-haiku-5-5#responses-can-begin-with-thinking-blocks) | Changed | Select content blocks by `type`, not by position. |
26| [Responses can start with thinking blocks](https://platform.claude.com/docs/en/models/haiku-5-5/whats-new-haiku-5-5#responses-can-begin-with-thinking-blocks) | Changed | Select content blocks by `type`, not by position. |
2727| [Thinking text is omitted by default](https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide#configure-thinking) | Changed | To receive summarized thinking, set `thinking.display` to `"summarized"`. |
2828| [Same text counts as more tokens](https://platform.claude.com/docs/en/models/haiku-5-5/whats-new-haiku-5-5#same-text-counts-as-more-tokens) | Changed | Recount prompts and revisit `max_tokens` and cost estimates. |
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. |
from line 35
3535
3636### Adaptive thinking and effort
3737
38With [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking), Claude Haiku 5.5 determines when and how much to think. Adaptive thinking is on by default. While you can still turn thinking off with `thinking: {"type": "disabled"}` at `high` effort or below, the better way to trade response quality against speed and cost is to use the [effort parameter](https://platform.claude.com/docs/en/build-with-claude/effort).
38With [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking), Claude Haiku 5.5 determines when and how much to think. Adaptive thinking is on by default. Although you can still turn thinking off with `thinking: {"type": "disabled"}` at `high` effort or below, the better way to trade response quality against speed and cost is to use the [effort parameter](https://platform.claude.com/docs/en/build-with-claude/effort).
3939
4040### Larger context window and output
4141
from line 43
4343
4444## Behavior changes
4545
46### Responses can begin with thinking blocks
46### Responses can start with thinking blocks
4747
48Adaptive thinking is on by default, so a response can begin 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
from line 53
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% 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
5959
No line in this hunk matches that.