Claude Haiku 5.5 is out. Code written for Haiku 4.5 can break on it.
By Lior AharonovFounder, 7IT Solutions ·
On Wednesday 7 October 2026, Anthropic launched Claude Haiku 5.5 (claude-haiku-5-5), with a 1M token context window and up to 128k output tokens. Requests with budget_tokens, non-default temperature, top_p or top_k, or an assistant prefill now return errors. The same text counts as about 30% more tokens than on Haiku 4.5.
Picture the swap: one model ID changed in a config file, and the cheap classifier behind your support inbox starts answering 400. Or it quietly counts the same prompt as more tokens. A small max_tokens can even stop after the thinking and before the answer.
Search your code for budget_tokens, temperature, top_p, top_k and assistant prefills before you switch to claude-haiku-5-5. Then read content blocks by type, not position, and recount your prompt costs.
A cheaper model with a breaking change is still a breaking change. I'd ship it behind a flag, never as a one-line swap.
Lior Aharonov · my takeSources
- Claude Platform release notes (7 October 2026) platform.claude.com
- What's new in Claude Haiku 5.5 platform.claude.com
- Claude Haiku 5.5 migration guide platform.claude.com
Researched with AI tools; every fact is checked against the linked sources.