Boris Cherny: Opus 5.5 ported HAProxy to Rust faster and cheaper than Fable 5.1

Cherny says Opus 5.5 had been his daily driver for weeks. In an internal test both Opus 5.5 and Fable 5.1 ported HAProxy from C to Rust and passed nearly all of its tests, but Opus 5.5 took 9.5 hours versus 12 and cost 51% less, a figure repeated in TechCrunch and KDnuggets coverage. The same evening he posted that Opus 5.5 formally verified the Claude Agent SDK in Lean, producing 16 bug-fix PRs (x.com/bcherny/status/2102543349102338309). Verified via syndication: 2026-09-22T16:45:10Z.

Opus 5.5 is a really good model. It's been my daily driver the last few weeks.

We had Opus 5.5 and Fable 5.1 each port HAProxy from C to Rust. Both passed nearly all of HAProxy's tests, but Opus 5.5 finished in 9.5 hours compared to Fable 5.1's 12 hours, and for 51% less cost.

Quoting @claudeai: Introducing Claude Opus 5.5, the first model in our new Claude 5.5 family.

It performs at the level of Claude Fable 5.1 for most tasks, and costs 40% less to run than Opus 5. https://t.co/Q9C2VKQ79f

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Archived 2026-09-29 via syndication.