I own the whole stack. Including the parts without an API.

Modern large language models increasingly advertise context windows approaching or exceeding one million tokens. GPT-5.6 Sol, for example, publicly supports a 1,050,000-token context window through the OpenAI API. Yet nominal context capacity and effective context utilization are not equivalent. OpenAI's own long-context evaluations show a substantial decline in GPT-5.6 Sol's retrieval performance between the 256K–512K and 512K–1M regimes: performance on the OpenAI MRCR v2 eight-needle benchmark falls from 91.5% to 73.8%. GraphWalks likewise declines from 90.7 F1 at 256K to 77.1 F1 at one million tokens.

This paper examines the hypothesis that the frequently observed deterioration of historical-context utilization after approximately half of the available context window has been consumed is related to Rotary Position Embedding, or RoPE, and the compromises required to operate RoPE-derived positional systems at extreme sequence lengths.

The central argument is not that RoPE contains a literal 50% cutoff. It does not. Rather, if a model's ability to discriminate and exploit relationships between tokens begins deteriorating once their relative positional separation reaches roughly half of the model's maximum supported context, then the conversational behavior of the system naturally produces an apparent half-window failure point. The first tokens in the conversation cross that critical relative distance precisely when the conversation itself reaches approximately half of the maximum context length. From that point onward, an increasing fraction of the historical context resides in the degraded positional regime.

For GPT-5.6 Sol specifically, this interpretation is consistent with publicly reported benchmark behavior, but it cannot presently be established as the unique causal explanation. OpenAI has not publicly specified the positional-encoding architecture of GPT-5.6 Sol in the materials considered here. OpenAI does publicly document the use of RoPE in its gpt-oss architecture, demonstrating its continued use within OpenAI model design, but this is not evidence that GPT-5.6 Sol uses an identical implementation.

The more defensible conclusion is therefore that GPT-5.6 Sol exhibits an empirically observable upper-half context degradation regime, and that the mathematics and known scaling limitations of RoPE provide a plausible architectural explanation for why such a regime would emerge.