Home / What a language model is, without the jargon

One family of models, a choice between speed and depth

The Claude models, Fable, Opus, Sonnet and Haiku, come out of the same base training and are mainly distinguished by one trade-off: how fast the answer comes against how deep the reasoning goes. Fast models suit short, repeated tasks, deeper ones suit files that need cross checking, and the price follows the same axis.

Claude exists at four model tiers, Fable, Opus, Sonnet and Haiku, each renewed from one generation to the next. All four come out of the same base training and are mainly distinguished by a single setting, the trade-off between how fast the answer comes and how deep the reasoning applied before answering goes.

A common foundation, a different slider

Choosing a model amounts to moving a slider. At one end, a model answers within seconds and suits short, repeated tasks, an acknowledgement of receipt, a rewording, a sort of messages. At the other end, a model takes longer because it chains more reasoning steps before concluding, which matters for a file that needs cross checking or a plan that needs verifying on several points. The documentation published by Anthropic recommends, as of October 24, 2026, starting with Opus 5.5 for most uses, and reserving Fable 5.1 for tasks that call for demanding reasoning or that stretch across many chained steps.

Here are two requests the same volunteer might make on the same day.

Short request: rewrite this welcome message in three warmer sentences.
Long request: compare the twelve quotes received this quarter, flag the most
significant price differences and propose a reasoned ranking.

The first task is handled within seconds by a fast model. The second benefits from being given to a model that takes the time to cross check the figures before concluding.

The price follows the same trade-off

The price charged per million tokens exchanged (a token is roughly a piece of a word) varies strongly from one model to another, and this variation follows exactly the same axis as speed, the deepest model costs the most, the fastest costs the least. The first two figures reproduce these published prices, input then output, with their source and date, rather than copying them here where they would go out of date without anyone noticing.

Sonnet 5.5 and Haiku 5.5 since late September

Claude Sonnet 5.5 (identifier claude-sonnet-5-5) came out on September 28, 2026 and Claude Haiku 5.5 (claude-haiku-5-5) on October 7, 2026. The three figures in this lesson date from October 24, 2026 and cover the four current models; the last one compares them price aside. The Claude Code changelog makes these two models the default Sonnet and Haiku models on the Anthropic API, and the starting recommendation on the Models overview page should be reread on the date you read this.

A deeper model answers better the requests that need several reasoning steps, not every request. The choice of model, covered here, acts on a first lever. The next lesson shows a second one, independent of this one: the order of words within a single message.