Claude Haiku 5.5's 100K line: one extra token can make the whole request 5x

Claude Haiku 5.5 is cheap at $0.10 input and $0.50 output per 1M tokens, and it has a 1M-token context window. The price sheet also has a step at 100K prompt tokens, and it applies to the whole request. If your agent or batch job sits near that line, the step matters more than the headline price. Here's the math, using Anthropic's prices checked on 2026-10-30.

Once the prompt crosses 100K, everything in that request is billed at the higher row: output, cache reads and cache writes too. US-only inference costs 1.1x either row. The Batch API is half price.

An agent turn with 90% of the prompt cached and 4K output tokens:

2% more prompt, about 5x the cost of the turn.

100M input tokens and 10M output tokens, Anthropic list prices:

Putting 30% of the tokens over the line multiplies the bill by 2.2x at every cache-hit rate.

Disclosure: This is IteraCompute, an OpenAI-compatible API for many models. We list Claude Haiku 5.5 at $0.05 input, $0.005 cached input and $0.25 output per 1M tokens for prompts up to 100K, and $0.25 / $0.025 / $1.25 above that (checked 2026-10-30). That's half of the list price on both sides of the step, but the step itself is still there, so the advice above applies to us too. Current prices are on the model page: Model Page