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Haiku 5.5 vs Haiku 4.5: What Changed and Should You Upgrade?

A side-by-side comparison of Claude Haiku 5.5 and Claude Haiku 4.5: context window, output limit, thinking, effort levels, tools and price — plus the breaking changes developers need to know.

Haiku55 Team
Haiku 5.5 vs Haiku 4.5: What Changed and Should You Upgrade?

Claude Haiku 5.5 replaces Claude Haiku 4.5 as Anthropic's fast, low-cost model. Here is what actually changed — for people chatting with it and for developers moving an app over.

Side-by-side comparison

Claude Haiku 5.5Claude Haiku 4.5
Context window1M tokens200K tokens
Max output128K tokens64K tokens
ThinkingAdaptive, on by defaultManual thinking budget
Effort levelslow · medium · high · xhigh · maxNot supported
Browser use toolSupportedNot supported
Input / output price per 1M tokens$0.10 / $0.50 (prompts ≤ 100K tokens)$1 / $5

The big upgrades

5× more context. One million tokens means whole documents and codebases fit in a single request instead of being chunked.

Twice the output. Up to 128K tokens of output for long drafts, reports and code.

Thinking you can dial up or down. Haiku 5.5 always decides how much to think, and the effort setting controls the depth. Use low for chat-speed answers and high when accuracy matters. In Haiku55's chat these are the Fast, Smart and Deep modes.

Much lower list price. For prompts up to 100K tokens, input is 10× cheaper and output is 10× cheaper than Haiku 4.5. Keep the newer tokenizer in mind: the same text produces roughly 30% more tokens.

Better agent behavior. Anthropic reports that Haiku 5.5 is substantially better at following instructions and at working as a sub-agent.

Breaking changes for developers

If you are migrating API code from claude-haiku-4-5 to claude-haiku-5-5:

  1. No manual thinking budgets. thinking: {type: "enabled", budget_tokens: N} returns a 400. Use adaptive thinking and set output_config.effort instead.
  2. No custom sampling. Non-default temperature, top_p and top_k values return a 400 — steer with the prompt or structured outputs.
  3. No assistant prefill. End messages with a user turn.
  4. Computer use goes through the new toolset on the Claude API and Google Cloud.
  5. Keep conversations append-only. Replayed thinking blocks become invalid if earlier turns, the system prompt or tools change.

Also new: responses can start with thinking blocks (read content blocks by type), and the model's safety classifiers can return stop_reason: "refusal".

Should you switch?

For almost everyone, yes. Haiku 5.5 is faster to work with on long inputs, thinks when it needs to, and costs a fraction of Haiku 4.5 at list price. The easiest way to feel the difference is to chat with Haiku 5.5 for free.

Haiku55 is an independent service and is not affiliated with Anthropic. Claude and Haiku are trademarks of Anthropic, PBC.