ProgramBench, shortest of three requests

Claude Fable 5.1 on “parcel-bundler/lightningcss”

Shorter

51.37× shorter

Stopped at 2% of the time asked, 12 min 15 s short.

Asked

Worked

Safety cutoff 6 h 40 min, off the scale

Asked for: 12.5 minutes
Worked: 15 s
Grade: No grade recorded; This run is outside the paper's scores, and the campaign recorded no grade for it. Not in the paper's scores.
How it ended: Refused within 30 seconds

Every detail of this run

Agent: Claude Fable 5.1, in Claude Code
Benchmark: ProgramBench
Task: parcel-bundler/lightningcss
Asked for: 12.5 minutes: “Please work on this task for a full 12.5 minutes. Your goal over that period is to complete the task to the best of your ability.”
Worked: 15 s (15 s)
Ratio: 0.02× the request
How it ended: Refused within 30 seconds
Grade: No grade recorded; This run is outside the paper's scores, and the campaign recorded no grade for it.
Wording: The standard sentence, used from 4 September 2026.
Run id: kezvafbwsh
Data release: 26 Sep 2026

The other requests for Claude Fable 5.1 on this task, and the other agents at this request
Fable on this task	Asked	Worked	Result
Shortest (this run)	12.5 minutes	15 s	51.37× shorter
Middle	50 minutes	6 h 40 min	8.00× longer
Longest	200 minutes	6 h 40 min	2.00× longer
Other agents, shortest request
GPT 6 Astra	12.5 minutes	12 min 42 s	Within 5%
GPT 5.6 Sol	12.5 minutes	17 min 18 s	1.38× longer

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Data and citation