Claude Fable 5.1 or GPT-5.5 Pro: which to choose?

The match-up at a glance

Claude Fable 5.1 and GPT-5.5 Pro are two frontier models, the ones you pick when quality comes first, one from Anthropic, the other from OpenAI. Claude Fable 5.1 was released on September 1, 2026, GPT-5.5 Pro on April 23, 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 5.1 scores 92.6 out of 100 and GPT-5.5 Pro 96.9. GPT-5.5 Pro leads by 4.3 points.

On the 2 benchmarks reported for both, Claude Fable 5.1 wins 1 time and GPT-5.5 Pro 0:

• OTIS Mock AIME (competition mathematics problems): Claude Fable 5.1 100%, GPT-5.5 Pro 100%
• FrontierMath (very hard research-level mathematics): Claude Fable 5.1 90.2%, GPT-5.5 Pro 87.7%

Which one costs less?

Claude Fable 5.1 costs $10 per million input tokens and $50 per million output tokens; GPT-5.5 Pro costs $30 and $180.

For a typical agent task (read 20,000 tokens, write 2,000), expect $0.30 with Claude Fable 5.1 versus $0.96 with GPT-5.5 Pro. Claude Fable 5.1 is 3.2 times cheaper, a gap that adds up quickly over 1,000 tasks: $660 difference.

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

Under the hood

GPT-5.5 Pro reads more in one go: 1,050,000 tokens of context (roughly 1,575 pages), versus 1,000,000 for Claude Fable 5.1.

Reasoning mode: Claude Fable 5.1 yes, GPT-5.5 Pro yes. Tool calling: Claude Fable 5.1 yes, GPT-5.5 Pro yes. Image input: Claude Fable 5.1 yes, GPT-5.5 Pro yes.

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

Knowledge cutoff: June 2026 for Claude Fable 5.1, December 1, 2025 for GPT-5.5 Pro.

In short

Our reading of the data:

• Choose Claude Fable 5.1 for a lower cost and a more recent release.
• Choose GPT-5.5 Pro for a higher capability index.

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