This says 400K context window: https://platform.openai.com/docs/models/compare?model=gpt-5

They say:

In the API, all GPT‑5 models can accept a maximum of 272,000 input tokens and emit a maximum of 128,000 reasoning & output tokens, for a total context length of 400,000 tokens.

So it's only 270k for input and 400k in total considering reasoning & output tokens.

They do, but if you look at the graphs...what is the point of the large context window if accuracy drops off waaaaay before context window is maxed?