Claude Fable 5 or Claude Sonnet 5.5: which to choose?

Side-by-side comparison

Criterion	Claude Fable 5	Claude Sonnet 5.5
Developer	Anthropic	Anthropic
Released	June 9, 2026	September 28, 2026
Capability index	90.8 / 100	95.9 / 100
Input price	$10	$2
Output price	$50	$10
Cost of a typical task	$0.30	$0.06
Context window	1,000,000 tokens	1,000,000 tokens
Maximum output	50,000 tokens	128,000 tokens
Reasoning mode	Yes	Yes
Tool calling	Yes	Yes
Accepted inputs	text, image, PDF	text, image, PDF
Open weights	No	No
Knowledge cutoff	January 31, 2026	June 2026

What are we comparing?

This comparison pits two frontier models, the ones you pick when quality comes first against each other: Claude Fable 5 and Claude Sonnet 5.5, both built by Anthropic. The first dates from June 9, 2026, the second from September 28, 2026.

All figures below come from public sources checked on October 22, 2026: models.dev and OpenRouter for prices and capabilities, Epoch AI for scores.

What the benchmarks say

On our capability index, Claude Fable 5 scores 90.8 out of 100 and Claude Sonnet 5.5 95.9. Claude Sonnet 5.5 leads by 5.1 points.

On the 5 benchmarks reported for both, Claude Fable 5 wins 1 time and Claude Sonnet 5.5 3:

GPQA Diamond (PhD-level science questions): Claude Fable 5 85.9%, Claude Sonnet 5.5 95.6%
OTIS Mock AIME (competition mathematics problems): Claude Fable 5 100%, Claude Sonnet 5.5 100%
FrontierMath (very hard research-level mathematics): Claude Fable 5 87%, Claude Sonnet 5.5 88.8%
FrontierCode (advanced programming tasks): Claude Fable 5 53.5%, Claude Sonnet 5.5 52.1%
APEX-Agents (professional tasks carried out by an agent): Claude Fable 5 63.6%, Claude Sonnet 5.5 75.5%

Which one costs less?

Claude Fable 5 costs $10 per million input tokens and $50 per million output tokens; Claude Sonnet 5.5 costs $2 and $10.

For a typical agent task (read 20,000 tokens, write 2,000), expect $0.30 with Claude Fable 5 versus $0.06 with Claude Sonnet 5.5. Claude Sonnet 5.5 is 5 times cheaper, a gap that adds up quickly over 1,000 tasks: $240 difference.

Run the numbers with your own volumes in the agent cost calculator.

Under the hood

Both models have the same context window: 1,000,000 tokens, roughly 1,500 pages.

Reasoning mode: Claude Fable 5 yes, Claude Sonnet 5.5 yes. Tool calling: Claude Fable 5 yes, Claude Sonnet 5.5 yes. Image input: Claude Fable 5 yes, Claude Sonnet 5.5 yes.

Neither publishes its weights: both are used through an API.

Knowledge cutoff: January 31, 2026 for Claude Fable 5, June 2026 for Claude Sonnet 5.5.

In short

Our reading of the data:

Choose Claude Sonnet 5.5 for a higher capability index, a lower cost and a more recent release.

This verdict relies on public figures; it does not replace a trial on your own tasks.

Frequently asked questions

Claude Fable 5 or Claude Sonnet 5.5: which is better?

On our index (Epoch AI data), Claude Fable 5 scores 90.8 and Claude Sonnet 5.5 95.9 out of 100. The best choice also depends on price and on your use case.

Which one is cheaper?

Claude Sonnet 5.5. To read 20,000 tokens and write 2,000, expect $0.30 with Claude Fable 5 and $0.06 with Claude Sonnet 5.5 (prices as of October 22, 2026).

Which one has the larger context window?

Neither: both accept 1,000,000 tokens.

Can I run Claude Fable 5 or Claude Sonnet 5.5 on my own servers?

No: both are closed models, available through an API.

Claude Fable 5 versus Claude Sonnet 5.5