GPT-5.6 Sol's spec says roughly 1.05 million tokens of context.
OpenAI's own Codex CLI has capped what you can actually use at 372k, then 272k, then 353k, then 258k, across four changes in two months, for cost and latency reasons.
The advertised number is a capability. The number you get is a business decision, and it can change on a Tuesday.