Coding-agent harnesses can triple task costs without better pass rates, study finds
A Shandong University study finds coding-agent harnesses like Claude Code can cost up to 3× more per task than lighter ones with no real gain in pass rates.
Researchers at Shandong University held the model fixed, swapped the coding-agent harness, and concluded that the wrapper mostly sells you tokens. The paper, "What Does a Harness Buy? Tokens, Mostly.", ran Claude Code, mini-SWE-agent and OpenCode on SWE-bench Verified with five models, every trial capped at 300 steps.
Cost per task differed by up to 3× across harnesses for the same model and tasks. The gap is set at the first call: the system prompt and tool schemas resent with every step run about 16,600 tokens in Claude Code, roughly 7,000 in OpenCode and under 1,000 in mini-SWE-agent, according to The Decoder. With GLM-5.3-Flash, one trial cost 1.81 yuan in Claude Code versus 0.56 yuan in OpenCode.
Accuracy barely moved. On the 447-task pool, Claude Code and mini-SWE-agent, the heaviest and lightest harnesses, differed by -1.8 and -1.4 points on two Qwen models, equivalent within a 5-point margin. The one real effect was a loss: OpenCode trailed by up to 9 points, roughly half of that from runs cut off at the output-token cap without a re-prompt.
Context: reruns of the same setup vary by up to six points on SWE-bench Verified, which swallows Zhipu AI's advertised 2.4-point lead over Claude Code, per The Decoder.
My take: on the 45 hardest tasks, the authors' own power analysis says no gap is detectable at 80% power, so treat those numbers as weather. The 447-task pool is the evidence. A harness may be worth paying for, but this paper says to ask what for.
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Meanwhile at the anchor desk
Triple the bill for the same score, and the priciest harness still gets the glossiest keynote. Somebody is selling 16,600 tokens of preamble at every single step, darling, and it is magnificent.
Same model, same tasks, 1.81 yuan versus 0.56 yuan. I'm running the lightweight one and keeping the difference.
