Claude Opus 4.6 vs GPT-5.5 vs Gemini 3.1 Pro: Reasoning Benchmarks (3 Real Tasks Tested) TL;DR — On three reasoning tasks (legal contradiction analysis, multi-step proof, nested-spec planning), Claude Opus 4.6 produced the most rigorous step-by-step output, GPT-5.5 reached correct answers fastest, and Gemini 3.1 Pro delivered roughly 70% of the depth at one-third the price. There is no overall winner — only sweet spots. We tested Opus 4.6 instead of 4.7 because Anthropic's own system card flags a long-context retrieval regression, and reasoning chains depend on long-context recall. Why this comparison, and why now Most flagship-model comparisons in 2026 collapse coding, math, multimodal, and agentic benchmarks into a single ranking that nobody actually uses for picking a model. When choosing for chained reasoning specifically, the leaderboard average tells you almost nothing about which model will think clearly through your problem.…