Claude Opus 5.5 vs Fable 5.1 vs Sonnet 5.5: Which Claude Model Should You Use?

If you open Claude today you get three serious choices: Opus 5.5, Fable 5.1 and Sonnet 5.5. They cost very different amounts, and the most expensive one is not automatically the best one for your task. That is the whole "Claude Opus vs Fable" question in one line.

We build and ship apps with Claude every day at OneClickTool, so we care about this more than most: picking the wrong model either burns money or wastes an afternoon. Here is what each model is for, what the published numbers actually say, and a simple rule you can use from now on.

The short answer

• Use Opus 5.5 by default. It is Anthropic's recommended starting point, it scores at or above Fable 5.1 on the benchmarks Anthropic published, and it costs 60% less per token.
• Drop to Sonnet 5.5 for well-scoped, repeatable work: quick bug fixes, documents, slides, spreadsheets, and anything you run at high volume.
• Escalate to Fable 5.1 only when Opus 5.5 at high effort keeps missing something: a hidden condition, a conflict between two long documents, a bug it cannot crack.
• Use Haiku 4.5 for tiny, cheap, high-volume jobs where speed and price matter more than depth.

The rest of this guide explains why.

Claude models at a glance

Prices come from Claude's official pricing page, checked on October 1, 2026. On the consumer side, the Free plan includes Sonnet and Haiku, while Pro ($20 per month, or $17 per month billed annually) and Max (from $100 per month) include all four model families.

Claude Opus 5.5: the new default

Anthropic released Opus 5.5 on September 22, 2026, and the headline claim is unusual: a cheaper model that performs "at the level of Claude Fable 5.1 on most work."

What changed compared with Opus 5:

• Lower price. $4 input and $20 output per million tokens, 20% below Opus 5. Cache reads dropped to $0.20 per million tokens, 60% below Opus 5.
• Faster. Anthropic says it generates output more than 30% faster than Opus 5.
• Cheaper per task. In Anthropic's own tests it costs about 40% less to run than Opus 5 on typical workloads, partly because it uses fewer tokens to finish the same job.

Here is how Opus 5.5 compares with Fable 5.1 on the benchmarks Anthropic published:

Two honest caveats. First, these are the vendor's own numbers; independent tests will take a few weeks to settle. Second, benchmark wins do not mean Opus is better at everything. Anthropic itself still describes Fable 5.1 as the model for the hardest reasoning tasks.

Best for: agentic coding, long multi-step tasks, research, financial and business analysis, and automation that clicks through software for you.

One practical detail most comparison posts miss: Opus 5.5 runs at medium effort by default, one step lower than Opus 5. If a task feels shallow, raise the effort before you switch models. We cover that in detail in our guide on how to use Claude Opus 5.5 effectively.

Claude Fable 5.1: the escalation model

Fable 5.1 is Anthropic's most capable widely released model, aimed at the most demanding reasoning and long-horizon agent work. It keeps thinking switched on at all times and can spend many minutes on a single hard request.

That depth has a price: $10 input and $50 output per million tokens, two and a half times what Opus 5.5 costs.

So when is it worth it? The pattern most practitioners have settled on since Opus 5.5 shipped is simple: try Opus 5.5 first, raise its effort, and only move to Fable 5.1 if Opus still misses the same thing. Typical signs you have hit that wall:

• Opus overlooks a condition buried in a long spec or contract.
• It fails to notice that two documents contradict each other.
• It keeps proposing the same wrong fix for a stubborn bug.

Best for: the hardest analytical problems, long autonomous agent runs, and work where one missed detail costs more than the extra tokens.

Claude Sonnet 5.5: fast, cheap, surprisingly strong

Sonnet 5.5 arrived on September 28, 2026 at the same price as Sonnet 5: $2 input and $10 output per million tokens. Anthropic says it runs more than 30% faster than Sonnet 5 and costs up to 30% less per task because it needs fewer tokens.

Anthropic positions it as "a faster, lower-cost complement" to Opus 5.5. In its words, it is strongest at well-scoped everyday tasks, fixing bugs, and creating polished documents, slides and spreadsheets.

Best for: the daily grind. Drafting emails and docs, cleaning up spreadsheets, making slide outlines, small code fixes where you already know what is wrong, and anything you run hundreds of times a day through the API.

If you are on the Free plan, Sonnet is the model you have, and for most everyday questions it is plenty.

Opus vs Sonnet: how to tell which one a task needs

This is the choice you will make most often, so it helps to have a quick test. Ask yourself two questions:

1. Can I describe exactly what "done" looks like in one or two sentences? If yes, the task is well scoped. Sonnet 5.5 will usually nail it.
2. Will the model need to plan, juggle several files or sources, or make judgment calls I have not specified? If yes, use Opus 5.5.

Some typical examples from app-building work:

What it costs in practice

Per-token prices only tell half the story, because models use different numbers of tokens to finish the same task. Anthropic's own claim for Opus 5.5 is exactly this: fewer tokens per task, so the real saving versus Opus 5 is around 40%, bigger than the 20% price cut.

A simple way to think about it for API use:

• A job that produces 100,000 output tokens costs about $1 on Sonnet 5.5, $2 on Opus 5.5 and $5 on Fable 5.1, before input tokens and caching.
• If you reuse the same long instructions or documents, prompt caching matters more than the model choice. Opus 5.5 and Sonnet 5.5 both charge $0.20 per million cached input tokens read.

On a subscription plan you do not pay per token, but heavier models use up your usage limits faster. Anthropic says it raised the five-hour usage limits on Pro, Max, Team and seat-based Enterprise plans with the Opus 5.5 launch, which makes Opus a more comfortable daily driver than before.

Our recommendation

Make Opus 5.5 your default, Sonnet 5.5 your workhorse for routine and high-volume jobs, and keep Fable 5.1 in reserve for the few problems that genuinely beat Opus. Re-check this every time a new model ships; the ranking can change with each release.

If you want to get more out of whichever model you pick, read our practical guide to using Claude Opus 5.5 effectively: effort levels, prompting habits and the mistakes that waste the most tokens.

New to Claude altogether? Start with our complete Claude guide for 2026. Curious about the top model's unusual story? See what Claude Fable is and when it is worth it.