GPT 5 vs GPT OSS 20B vs GPT OSS 120B

Lightning AI describes evaluating whether to replace GPT-4o in a production fraud-detection agent with GPT-5, GPT-5 OSS models, Gemini variants, and smaller GPT-5 models, emphasizing that model upgrades should be based on task-specific evaluations rather than release hype. Its deterministic LitAI-based agent decomposes fraud assessments into explainable binary LLM decisions, enabling accuracy measurement, latency analysis, confusion matrices, and error-frequency review across 2,000 synthetic transactions. GPT-4o achieved the highest accuracy at 77.41% with a 1.59-second average response time, while GPT-5 was slightly less accurate at 76.37% and substantially slower at 8.61 seconds, making it unsuitable for an agent requiring multiple calls per case. GPT-5 OSS 20B emerged as a lower-cost alternative, delivering 73.42% accuracy at 1.68 seconds and costing far less than GPT-4o, whereas other tested models traded away too much accuracy or had less predictable latency. The company therefore retained GPT-4o because its approximately three-point accuracy advantage was considered important for business outcomes, while noting it could switch easily to GPT-5 OSS 20B if the performance gap narrowed.

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