Inside vLLM's CPU backend: a new contributor's notes Most of the public technical writing about vLLM focuses on its GPU-side innovations — PagedAttention, continuous batching, the V1 engine. Less has been written about the CPU backend, which is where I spent the last couple of weeks: building vLLM from source, working through some rough edges, and shipping a small PR that clarifies three confusing error messages. This post is a writeup of what surprised me along the way. It's aimed at the next contributor who's going to spend time in the CPU paths — whether for dev work, CI testing, edge inference, or just because that's the entry point that fits their environment. Some of it is genuinely useful setup info that isn't well-documented elsewhere. Some of it is observations about how the project's GPU-first history shows up in the design of its CPU-side code. The setup story (or: prerequisites the docs don't make obvious) The official docs walk you through building vLLM from source on CPU. They're correct.…