Humanlayer · 2026-07-27 · notable
Opus 5 scores 24% on SlopCodeBench — Humanlayer flags 93% of the code
Dex Horthy, founder of Humanlayer, benchmarks Claude Opus 5 on SlopCodeBench and finds a 24% strict pass rate, nearly 5× more code than Opus 4.8, and quality warnings on 93% of the output.
An independent third-party benchmark of Claude Opus 5's coding quality lands with a critical read.
What is it?
Humanlayer's Dex Horthy ran Claude Opus 5 through SlopCodeBench, a benchmark that feeds a coding agent requirements in sequential checkpoints instead of dumping the full spec up front. Opus 5 cleared 4 of the 17 checkpoints across three problems — a 24% strict pass rate.
How does it work?
SlopCodeBench measures how well a model absorbs new requirements without dragging past defects forward. Each checkpoint adds a change on top of the previous version of the code, so a small regression in checkpoint 2 fails everything after it. Horthy also tracked complexity metrics, warning counts, and total code volume per problem.
Why does it matter?
The Opus 5 launch page led on higher scores at coding, so the SlopCodeBench read is a useful counterweight. Opus 5 wrote almost five times as much code as Opus 4.8 per problem, and 93% of that code triggered quality warnings — with complexity climbing across checkpoints. For developers deciding whether to make Opus 5 their default coding agent, the concrete failure mode is a starting point.
Who is it for?
Developers evaluating Opus 5 for agentic coding, benchmark authors.