These results show that a model’s capabilities are compatible, generalizable and can carry over to other harnesses. Providers do report to optimize some models for their own coding environments: OpenAI, for example, describes GPT-5-Codex as optimized for agentic software engineering in Codex.12 Yet we observe that a shared provider does not guarantee the best pairing. The practical question remains which harness delivers the best balance of cost and task success for a given model and workload.
HarnessTax: How Much Does the Harness Matter for Coding Agents? · UC Berkeley · Arena