Migrating from Haiku 4.5 to Claude Haiku 5.5: 6 breaking changes and when the price goes up 5x

Claude Haiku 5.5 (claude-haiku-5-5) was released on October 7, 2026. It is the first Haiku with adaptive thinking and effort, has a 1M-token context window and 128K output, and its API shape now matches Opus 5.5 / Sonnet 5.5.

Pricing is $0.10 / $0.50, one tenth of Haiku 4.5, but it is also the first Haiku where the price goes up 5x once the prompt exceeds 100K tokens.

Haiku 4.5 was the last generation that accepted budget_tokens, temperature, and prefill, so changing only model breaks more things than it does on the other 5.5 models. Based on the official migration guide (as of October 12, 2026), here are the 6 changes that return 400 and the things that change silently.

What I verified

On October 12, 2026, I checked items 1 and 2, plus the effort restriction on disabled, against the live API. All requests were short, with max_tokens: 64, and the three 400s are not billed.

• 1. thinking: {type: "enabled", budget_tokens: 2000} HTTP 400 "thinking.type.enabled" is not supported for this model. Use "thinking.type.adaptive" and "output_config.effort" to control thinking behavior.
• 2. temperature: 0.2 HTTP 400 temperature is deprecated for this model.
• 3. thinking: {type: "disabled"} + effort xhigh HTTP 400 output_config.effort 'xhigh' is not supported when thinking is disabled on this model. Use effort 'high' or below, or enable thinking.
• 4. thinking: {type: "disabled"} + effort low HTTP 200 stop_reason=end_turn content=['text'] input_tokens=15 output_tokens=3 thinking_tokens=0

Case 4 also shows that disabled is accepted with low, and that usage.output_tokens_details.thinking_tokens comes back as 0.

For the other items (prefill, computer_20250124, history edits, the organization boundary), I am relying on the official migration guide and did not test them against the live API.

In my own code, the only place that used Haiku 4.5 was one worker in a crawling agent. It passed neither thinking nor temperature, so changing the model ID was all it needed.

6 changes that return 400

1. budget_tokens returns 400

# Haiku 4.5
thinking={"type": "enabled", "budget_tokens": 8000}
# Haiku 5.5
thinking={"type": "adaptive"}, output_config={"effort": "medium"}

Thinking amount is controlled by effort (low to max). The default is medium.

thinking: {"type": "disabled"} turns thinking off, but it works only with high or below; combining it with xhigh / max returns 400. While thinking is disabled, changing effort per message also returns 400.

2. temperature / top_p / top_k return 400

Any non-default value returns 400. Remove them and steer with the prompt instead.

3. A trailing assistant turn (prefill) returns 400

End messages with a user turn. If you used prefill to force JSON, switch to structured outputs (output_config.format).

Note that Haiku 5.5 on Amazon Bedrock has neither structured outputs nor strict. On Bedrock, specify the format in the prompt and validate the output yourself.

4. computer_20250124 returns 400 on every platform

On the Claude API and Google Cloud, switch to computer_toolset_20260801 (no beta header needed). On Amazon Bedrock, switch to computer_20251124 (computer-use-2025-11-24). Remove the computer-use-2025-01-24 header.

The browser tool (browser_toolset_20260801) is newly available on the Claude API and Google Cloud.

5. Editing conversation history returns 400

This check did not exist on Haiku 4.5. If you change system, tools, or earlier turns and then send thinking blocks back, you get 400.

The check is on by default for accounts created on or after August 31, 2026; for older accounts it applies only when you send prefix_mismatch_behavior. Keep history append-only, and make instruction changes with a mid-conversation role: "system" message.

I covered the details in a separate article (Invalid signature in thinking block, in Japanese).

6. Thinking blocks can only be read by the organization that created them

If you send them from a different organization, they are silently dropped without an error, and the model answers without that reasoning. A service that carries conversations for multiple customers in one store should replay each conversation in the organization that created it.

What changes without an error

Thinking is on by default. Code that never set thinking will start thinking on Haiku 5.5. Because max_tokens is a cap on thinking plus the answer combined, a small value can be used up by the thinking block alone and leave the text empty.

Select the block by type, not content[0].text. The default for display is omitted.

Tokens increase by about 30%. This is the same new tokenizer as 4.7 and later. Images also use the high-resolution tier (about 2.5x for 2000x1500). Re-measure with count_tokens.

Thinking blocks from every turn stay in the input. Haiku 4.5 kept only the previous turn's. Multi-turn input grows by more than 30%. To remove them, use thinking block clearing in context editing (deleting them yourself triggers item 5).

The minimum cacheable prompt is 512 tokens (4,096 on 4.5). Short prompts you had given up on caching now work.

Refusals were added. cyber / frontier_llm / bio / general_harms. You get HTTP 200 with stop_reason: "refusal". There is no server-side fallback, so handle it yourself. Resending usually gets refused again.

Priority Tier is not supported. Context-awareness tags are also not injected (use task budgets for budgeting).

Forced tool_choice still works. Unlike Opus 5.5 / Sonnet 5.5, it does not return 400.

When the price goes up 5x

"Prompt length" is the total input tokens of a single request, including cache reads and cache writes. Even if caching makes the request cheaper, once the total crosses the threshold that request is billed at the higher rate. The decision is made per request, and earlier requests are not re-billed.

So an agent that runs a long conversation on Haiku 5.5 needs to either shrink it with compaction as input approaches 100K tokens, or compare against Sonnet 5.5 ($2 / $10), whose price does not depend on length. Haiku 5.5 above 100K tokens ($0.50 / $2.50) is still 1/4 of Sonnet 5.5, so it is often still cheaper, but an estimate built on "Haiku is 1/20" will be off.

Where to start with effort

The official starting points: medium for most work (including agentic coding); low for chat, short tool tasks, and simple high-volume processing; high for knowledge work, long agentic work, and strict instruction following.

At low, long agentic tasks more often skip searches, stop early, or skip checks. Asking in the prompt to "answer directly" does not reduce thinking; lower the effort instead.

If you give the model a search tool, put today's date in the system prompt so it answers based on recent search results. Render it once at the start of the conversation and keep sending the same system. If the day rolls over, put the date in the latest turn.

Finding these statically

Before changing model, I updated a tool that lists the affected places as file:line. The check for Haiku 5.5 is --target haiku-5-5.

git clone https://github.com/volvox-ltd/claude5-migration-kit
python claude5-migration-kit/scan.py path/to/your/app --target haiku-5-5

Summary

The Opus 5.5 / Sonnet 5.5 changes (no disabled, no forced tools, and so on) are covered in the Japanese article, which includes a section on those two models.

https://zenn.dev/persimmoq/articles/claude-opus-5-5-sonnet-5-5-breaking-changes

Official: https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide / https://platform.claude.com/docs/en/about-claude/pricing

I wrote this with help from Claude, Anthropic's AI model. It drafted the post from the official Haiku 5.5 migration guide and pricing page, and I checked it before publishing.