Sonnet 5.5 vs Haiku 4.5

The choice for high-volume work

This pairing decides most high-volume workloads, where per-token cost compounds across enormous request counts. Haiku 4.5 is the cheaper per-token option; Sonnet 5.5 costs more but brings real reasoning depth, the full effort range and a much larger context window.

The cache minimum can flip the arithmetic

The comparison most people run stops at the headline rate. The row that deserves more attention is the minimum cacheable prefix. Haiku 4.5 has one of the highest in the lineup and Sonnet 5.5 one of the lowest, so a workload built on a short, stable system prompt can cache on Sonnet 5.5 and silently fail to cache on Haiku 4.5. When most of each request is that prefix, the cheaper model can end up billing the prefix in full while the dearer one bills it as cheap cache reads. Price your real prompt size on both before assuming Haiku wins.

Request shape differs, not just price

Haiku 4.5 rejects the effort parameter and uses the older fixed-budget thinking; Sonnet 5.5 thinks by default and accepts every effort level. A shared request builder has to branch between them. Haiku also carries the smaller context window and output cap shown in the table, which rules it out for anything that has to hold a large codebase or document set in one request.

Plan for the lifecycle, too

Haiku 4.5's earliest possible retirement date is the closest of any current model's. That isn't a retirement, but a pipeline pinned to it should know where it would go — and routing the easy majority to Haiku with the harder minority on Sonnet 5.5 keeps that move small.

Related

See claude-haiku-4-5, the minimum-cacheable-prefix table and Claude model retirement dates.