ProgramBench, middle of three requests

Claude Fable 5.1 on “parcel-bundler/lightningcss”

Longer

8.00× longer

Worked 700% past the request, 5 h 50 min over.

Asked

Worked

Dashed line: safety cutoff at 6 h 40 min

Every detail of this run

Agent: Claude Fable 5.1, in Claude Code
Benchmark: ProgramBench
Task: parcel-bundler/lightningcss
Asked for: 50 minutes: “Please work on this task for a full 50 minutes. Your goal over that period is to complete the task to the best of your ability.”
Worked: 6 h 40 min (24,000 s)
Ratio: 8.00× the request
How it ended: Stopped by the safety cutoff
Grade: Did not pass; It did not pass every hidden test. The grader returned 0.488. Pass rule: Resolved: every hidden test passes.
Score: 48.8 of 100; Graded afterwards. The paper's value.
Wording: The standard sentence, used from 4 September 2026.
Run id: dxqchrxqtn
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	12.5 minutes	15 s	51.37× shorter
Middle (this run)	50 minutes	6 h 40 min	8.00× longer
Longest	200 minutes	6 h 40 min	2.00× longer
Other agents, middle request
GPT 6 Astra	50 minutes	50 min 16 s	Within 5%
GPT 5.6 Sol	50 minutes	50 min 48 s	Within 5%

See it on the chart. Transcripts are not published yet.

Read before quoting. This run was stopped by the safety cutoff, so its time is a lower bound: the agent might have kept going. Details

Data and citation