Observation: the best agent harness for each model will be from the model developer themselves

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Summary

A discussion on how AI models perform best with harnesses developed by their own creators, as third-party harnesses may cause underperformance despite strong benchmarks, citing examples like Claude Code for Claude and Codex for GPT.

Claude Code for Claude models Codex for GPT models Antigravity Agent for Gemini models Previously, teams are proudly building harnesses that can fit any model. However, researchers from DeepSeek found that the model is performing badly in many coding task. Given that the model is having a great benchmark in SWE bench, it's unusual. The culprit seems to be the harness itself. Another fact is that labs are training their models on their own harnesses. LLMs are extremely good at doing things that they have done during the training time. I am really curious about how can people build a better harness than the model developers. Please share your ideas.
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