Native harnesses don't always solve more coding tasks
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Running agentic coding tasks on vendor-native SDKs yields a negligible average performance difference of within 1.25 percentage points compared to neutral, third-party harnesses on the same models. This means you can safely bypass vendor lock-in and build on unified, multi-model agent frameworks without sacrificing coding capability. However, you must carefully monitor your API spend, as third-party orchestrators can increase raw per-task inference costs by 20% to 60% compared to native solutions.
Same-model harness swaps on 256 private coding tasks showed no reliable overall win for vendor-native harnesses: Opus 4.8 was 48.8% vs 50.0%, and GPT-5.5 was 55.6% vs 54.4%, both within wide confidence intervals. For production agent teams, the harness choice should be treated as workload- and cost-dependent rather than assumed native-best: repository vs contest tasks diverged sharply for Opus, timeouts sometimes contained passing patches, and the neutral harness cost about 1.2–1.6× more per solved task on observed usage.