💰 Same model, same task, but swap the software wrapper (harness) around it and the cost can jump 5x while success rate barely moves. Title: HarnessTax: How Much Does the Harness Matter for Coding Agents? URL: UC Berkeley and Arena compared three coding agent harnesses, Claude Code, Codex CLI, and Pi, across 7 models and 21 model-harness pairs. Here are 3 highlights. 📊 Statistically, harness barely moves success rate Out of 42 within-model harness comparisons, only 1 was statistically significant (chance alone predicts about 2), and none survived correction for multiple comparisons. 💸 Cost can differ 5x at the same success rate On GPT-5.6 Luna, Claude Code cost $0.15/task at 55.6% success, while Pi cost $0.03/task at 53.3% success, nearly identical accuracy, 5x the price. 🪶 The minimal open-source harness holds its own In 9 of 12 model comparisons, a non-vendor harness (Pi or Codex CLI) achieved the top success rate, showing a stripped-down harness can genuinely compete. The practical takeaway: prioritize cost and reliability over flashy add-on features first. #CodingAgents# #LLMCostOptimization#