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OpenAI's GPT-5.6 builder guide: Sol at 'low' reasoning beat GPT-5.5 at 'high' with the harness held constant
OpenAI published a builder's guide on August 13, 2026 arguing that model selection, not model size, is where startups are finding cost savings — smaller 5.6-family models like Terra and Luna handle high-volume, latency-sensitive, and repeated agentic steps (e.g. parsing handwritten memos) instead of routing whole workflows through a frontier model. The headline claim for anyone tuning effort settings: GPT-5.6 Sol at 'low' reasoning outperformed GPT-5.5 at 'high' when the harness was kept constant. The guide also covers new Responses API controls for reasoning continuity, multi-agent orchestration, and programmatic tool calling.
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