# Claude Fable 5.1: Benchmark results and reasoning-effort experiments

DevFeed: [Claude Fable 5.1: Benchmark results and reasoning-effort experiments](<https://devfeed.tech/articles/claude-fable-5-1-made-me-a-really-nice-animated-pelican-30506.md>)

Original publisher: [Read original article](<https://simonwillison.net/2026/Sep/1/claude-fable-5-1/>)

Author: Simon Willison

Published: 2026-09-01T23:57:28Z

Content type: article

Language: en

Sources: [Simon Willison](<https://devfeed.tech/sources/simon-willison.md>)

Topics: [Fable](<https://devfeed.tech/topics/fable.md>), [Benchmark](<https://devfeed.tech/topics/benchmark.md>), [coding](<https://devfeed.tech/topics/coding.md>), [Large Language Model](<https://devfeed.tech/topics/llm.md>), [SVG](<https://devfeed.tech/topics/svg.md>)

Tags: [ai](<https://devfeed.tech/tags/ai.md>), [ai-2-235](<https://devfeed.tech/tags/ai-2-235.md>), [anthropic](<https://devfeed.tech/tags/anthropic.md>), [anthropic-336](<https://devfeed.tech/tags/anthropic-336.md>), [benchmark](<https://devfeed.tech/tags/benchmark.md>), [benchmarks](<https://devfeed.tech/tags/benchmarks.md>), [claude](<https://devfeed.tech/tags/claude.md>), [claude-310](<https://devfeed.tech/tags/claude-310.md>), [coding](<https://devfeed.tech/tags/coding.md>), [fable](<https://devfeed.tech/tags/fable.md>), [generative-ai](<https://devfeed.tech/tags/generative-ai.md>), [generative-ai-1-981](<https://devfeed.tech/tags/generative-ai-1-981.md>), [llm](<https://devfeed.tech/tags/llm.md>), [llm-reasoning](<https://devfeed.tech/tags/llm-reasoning.md>), [llm-reasoning-103](<https://devfeed.tech/tags/llm-reasoning-103.md>), [llm-release](<https://devfeed.tech/tags/llm-release.md>), [llm-release-231](<https://devfeed.tech/tags/llm-release-231.md>), [llms](<https://devfeed.tech/tags/llms.md>), [llms-1-947](<https://devfeed.tech/tags/llms-1-947.md>), [models](<https://devfeed.tech/tags/models.md>), [pelican-riding-a-bicycle](<https://devfeed.tech/tags/pelican-riding-a-bicycle.md>), [pelican-riding-a-bicycle-142](<https://devfeed.tech/tags/pelican-riding-a-bicycle-142.md>), [reasoning](<https://devfeed.tech/tags/reasoning.md>), [svg](<https://devfeed.tech/tags/svg.md>)

## AI overview

The article examines Claude Fable 5.1 through benchmark results and a pelican SVG-generation experiment across its five reasoning-effort levels. It reports that low and medium settings appeared to skip reasoning for this prompt, while higher effort used more tokens, time, and cost.

## Source excerpt

Today is Claude Fable (and Mythos) 5.1 day. Anthropic say that Fable 5.1 "sets a new standard for coding, knowledge work, and long-running problem-solving tasks". Their announcement spends a notable amount of time on scientific research, boasting of a 52.6% score on the brand new Terminal-Bench-Science 0.1 benchmark (first announced on August 27th), up from 24.7% for Fable 5, 29.0% for Opus 5 and 22.4% for GPT-5.6 Sol. Other benchmarks show slightly improved scores, but none as impressive as the Science one. But how well can it pelican? Back in July I wrote about how I was losing faith in the pelican benchmark - its connection to how good the models were at other tasks didn't seem to hold as strongly as it did back in 2025. The most interesting insights I get from it now are comparisons within model families, and particularly comparisons for the same prompt at different reasoning effort levels. Fable 5.1 has five reasoning levels: low, medium, high, xhigh, max - and no option to turn off reasoning entirely. I fixed an issue in llm-anthropic which caused reasoning traces not to be correctly recorded, then ran some prompts. Here's the full set of pelicans for all of the reasoning levels, each with the full reasoning transcript. I'll replicate them here: Low and medium, both without reasoning? Next, a bit of a mystery. This is what I got for effort low: The transcript doesn't show any summarized reasoning tokens, and the output token count is 1,998. With Claude that output token count includes reasoning tokens. It took 23.8 seconds and cost 10.017 cents. I bumped that up to medium and got this: Weirdly, that one also shows no reasoning text and used 1,977 output tokens - 21 tokens less than low. It took 23 seconds and cost 9.912 cents. So for this particular prompt ("Generate an SVG of a pelican riding a bicycle") Fable 5.1 appeared to skip reasoning entirely at both low and medium settings. High Here's high - 29.6 seconds, 2,612 output tokens, 13.087 cents: This one d