Claude Fable vs Opus 5.5 (and Sonnet 5.5): Which to Use

Fable vs Opus is the most common question about Claude's lineup right now, because Anthropic shipped Opus 5.5 three weeks after Fable 5.1 and said it matches Fable on most work for much less. Then Sonnet 5.5 landed six days later and nearly tied Opus 5.5 on knowledge work. Here is how the three compare, using Anthropic's own numbers, and when each one is the right pick.

Anthropic's model guide puts it plainly: "Most workloads start with Claude Opus 5.5." This comparison is kept by a Sumus AI Employee and updated when the lineup changes.

Fable vs Opus at a glance

Benchmarks are Anthropic-reported, from the Opus 5.5 and Sonnet 5.5 launch posts. All three share a June 2026 knowledge cutoff.

Opus 5.5 vs Fable 5.1

Opus 5.5 (September 22) was the first model of the Claude 5.5 family. Anthropic's launch post says it "performs at the level of Claude Fable 5.1 on most work and costs 40% less to run than Opus 5." Per token it is 60% cheaper than Fable 5.1 on input and output.

• Coding. Opus 5.5 leads on Terminal-Bench 4.0 (66.4% vs 55.8%), FrontierCode (54.4% vs 50.3%) and CursorBench (57.8% vs 51.8%). Translating HAProxy from C to Rust, both passed nearly all regression tests; Opus 5.5 took 9.5 hours vs 12 and cost 51% less.
• Research accuracy. In a sourced earnings-report test, 16 of 18 Opus 5.5 reports passed a zero-invented-figures bar; Fable 5.1 cleared it in no attempt.
• Safety. Opus 5.5 ties Fable 5.1 for the lowest prompt-injection success rate in Gray Swan's benchmark.
• Where Fable 5.1 still wins. Anthropic calls Fable 5.1 its most capable model open to all customers and recommends it for agent sessions that run for hours, multistep deep research, and analysis carried through to a finished document, spreadsheet or deck. Its guide says to move to Fable 5.1 when Opus 5.5 evals at xhigh or max effort still fall short.

Anthropic itself notes that "the gap between Opus 5.5 and Claude Fable 5.1 is narrower than these scores suggest." The practical rule for opus 5.5 vs fable 5.1: default to Opus, escalate to Fable for the few jobs where Opus at max effort isn't enough.

Sonnet 5.5 vs Opus 5.5

Sonnet 5.5 (September 28) is pitched as "a faster, lower-cost complement to Claude Opus 5.5." Per Anthropic's Sonnet 5.5 post, it runs 30%+ faster than Sonnet 5 and costs half of Opus 5.5 per token.

• Close on knowledge work. GDPval-AA 1844 vs 1846; OSWorld 80.1% vs 81.8%; AA-Briefcase 1811 vs 1822.
• Ahead on terminal coding. Terminal-Bench 4.0 70.6% vs Opus 5.5's 66.4%; within about two points on CursorBench.
• Behind on harder coding and reasoning. FrontierCode 46.2% (max) vs 54.4%; Humanity's Last Exam 64.5% vs 67.7%; Chartography no-tools 61.6% vs 64.4%.
• Best use. Well-scoped everyday tasks, bug fixes, polished documents, slides and spreadsheets, and design. Anthropic says Opus 5.5 "remains clearly stronger at complex, open-ended work requiring sustained judgment."

A pattern early testers describe: let Opus 5.5 set the architecture, then hand implementation to Sonnet 5.5. Anthropic's docs also support pairing them, with a cheaper executor consulting Opus through the advisor tool.

Fable vs Opus vs Sonnet: which to use

Before switching models, try the effort setting first: Anthropic says tuning effort "is often a better lever than switching models." Opus 5.5 defaults to medium; Fable 5.1 and Sonnet 5.5 default to high.

What about Fable 5.5?

Fable 5.5 is not out. Model IDs like claude-fable-5-5 have shown up in cloud catalogs and developer tools, but Anthropic has made no announcement as of October 24, 2026. Any "Fable 5.5 vs Opus" result today is speculation. We track the signals on our Fable 5.5 release date page, and will update this fable vs opus comparison the day it ships.

Try Fable, Opus and Sonnet side by side in Sumus

The fastest way to settle fable vs opus for your own work is to give the same task to both. In Sumus you can use Claude Opus 5.5 (the default), Sonnet 5.5, Fable 5.1 and Haiku 5.5 in one workspace, with reasoning effort adjustable up to max, 1M context and 128K output, and no separate Claude account or API key.

Run a task on Opus 5.5, rerun it on Fable 5.1 or Sonnet 5.5, and have a GPT-6 or Grok teammate review both. See the Claude in Sumus page.