Claude Opus 5.5 vs Claude Fable 5.1

Pyyan / Compare / Claude Opus 5.5 vs Claude Fable 5.1

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Language Models · verified 24 Sept 2026

Claude Opus 5.5Anthropiccurrent

Claude Fable 5.1Anthropiccurrent

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Specification	Claude Opus 5.5	Claude Fable 5.1
Summary	Fable 5.1's level of work on most tasks, at 40% less to run than Opus 5.	Same price as Fable 5, and cache reads cost a quarter of what they did.
Context	1M	1M
Input ($/Mtok)	$4	$10
Output ($/Mtok)	$20	$50
Licence	Proprietary	Proprietary
Max output	128K	128K
Parameters	Not published	Not published
Released	22 Sep 2026	1 Sep 2026
Model ID	claude-opus-5-5	claude-fable-5-1
Category	Language Models	Language Models
Official	Anthropic ↗	—

Highlighted rows are where these differ.

66.4% on Terminal-Bench 4.0 against 52.3% for Opus 5, on Anthropic's own harness
Cache reads cut 60% to $0.20, and output more than 30% faster than Opus 5
Attempts to work around its boundaries 85% less often than Opus 5, Anthropic reports

Best for agentic coding at flagship quality.

Matches or beats Fable 5 at low and medium effort, and pulls clear at high effort
Cache reads cut to 25% of the old price, so re-reading a long context got much cheaper
Identical model to Mythos 5.1 underneath; the difference is the safeguards

Best for coding and long-running work.