Every OSS prompt injection guard needs a GPU, a sidecar, or 7 GB of disk. PromptPurify is 14 MB, CPU-only, runs inside your Node process. Zero network calls. $0/call. Two layers: deterministic structural firewall + 14 MB ML classifier. Benchmarked against Meta Prompt-Guard, ProtectAI, deepset same inputs, same scoring code, reproducible on your laptop. Battle-tested by 2,458 red teamers before release. MIT. Drop in with 3 lines of code.