Meta spent two years as AI's loudest open-source champion, building millions of developers into the Llama ecosystem and publishing manifestos about openness as a moral imperative. Today it shipped Muse Spark — its most competitive model yet — as fully proprietary, and the stock market rewarded the reversal with a 9% surge. For anyone who built their stack on the assumption that Big Tech open-source AI would stay open, this is the week to revisit that bet.