Long-Context LLM Benchmarks 2026: Which Model Actually Holds Accuracy Past 200K Tokens?

Every frontier LLM in 2026 advertises a 1M-token context window, but RULER, MRCR v2, and NoLiMa scores prove that "advertised" and "effective" diverge by 30-60 points for multi-fact retrieval past 200K tokens. Gemini 3.1 Pro is the only model whose 1M window holds for single-needle retrieval; Claude Opus 4.6 leads multi-needle MRCR at 1M; GPT-5.5 wins single-needle precision; DeepSeek V4 Pro lands surprisingly close at one-thirteenth the cost. Pick your model by the retrieval shape of your job, not by the headline number.