"Harness choice has little effect on task success rate, but can significantly affect the cost."
That is UC Berkeley's HarnessTax, which ran seven models through Claude Code, Codex and Pi on the same 60 tasks, three repetitions each, graded by the benchmarks' own evaluators rather than a model judge.
Claude Fable 5 resolves 96.7% of SWE-bench Lite attempts in Pi at $0.666 per attempt, and 97.8% in Claude Code at $1.329. Same model, 15.4 turns against 15.3, twice the bill. Databricks found the same shape in July on a multi-million-line internal codebase with held-out tests and no model judge.
Every one of those studies starts each harness out of the box. I have never once run one that way, and neither has anyone I work with.
So I rebuilt the experiment with my own two years of rules, memory and sub-agents loaded in. Nine arms, six SWE-bench Lite instances, seven of them on one model through one gateway deployment, graded by the official evaluator with the control run in both directions: gold patch resolves, empty patch does not.
My configuration costs 3.1x the vanilla setup on Claude Code and 8.1x on Prime Agent, the leanest harness I run. Loaded up, Prime Agent burned more input tokens across the six tasks than Claude Code does untouched.
Berkeley measured a tax on the harness you choose. There is a second one, it is larger, and you wrote it yourself.
The decomposition is the part worth stealing. My 109 skills cost 249 tokens, all of them together. My 22 standing-instruction files cost 31,191. The things I curate are nearly free and the prose I wrote is the entire bill.
I cannot show you a single resolved task it bought. At six instances the success column is noise, and the limitation that actually matters is that none of these tasks resemble the half-specified work I do on a Tuesday.
The post carries an interactive over Berkeley's published chart data: pick any of the seven models and watch the success intervals overlap while the cost intervals pull apart.
Full writeup in the link.