"V cache quantization requires flash_attn" — the llama.cpp error that quietly halves your context window

I did not meet this error while debugging a crash. I met it while writing a calculator. llama_context: quantized V cache requires flash_attn to be enabled There is a second wording, thrown as an exception a little later in startup and surfacing as failed to initialize the context: quantized V cache was requested, but this requires Flash Attention and a third, older one — V cache quantization requires flash_attn — which is no longer in the tree but is what most of the search results still show you, because most of the world runs llama.cpp through something that vendors a build from six months ago. All three read like a configuration nag: you asked for one thing, turn on the other thing, move along. That framing is why almost nobody asks the interesting question, which is why those two settings are welded together. The answer is a memory-layout decision several levels below the flag you typed, and it is worth knowing, because it tells you precisely which half of th