Meta’s Muse Spark Open (Max) tops US open models in Code Arena WebDev

Meta has an open-weight model back near the top of a leaderboard. Muse Spark Open (Max) scored 1511 points in Arena’s Code Arena: WebDev rankings, making it the highest-ranked US open model on that board. That qualifier, “US,” is doing a lot of work. The model leads among American open releases. Global open-model rankings are a more crowded and more complicated story. What Meta actually shipped Meta Superintelligence Labs introduced Muse Spark Open on October 15, 2026. The lab is billing it as the most capable US open-weight model released so far. The headline spec is size. The model has 816 billion total parameters, but only 42 billion of them activate for any given token. That gap comes from its Mixture-of-Experts design, or MoE. Think of it as a large hospital staffed with specialists. Every patient doesn’t see every doctor; a triage system routes each case to the few experts who actually matter. The context window is one million tokens. That is enough room to feed in a sizable codebase or a long stack of documents in a single session. Muse Spark Open is natively multimodal on the input side. It accepts text, images, video and audio, though its outputs are primarily text. Meta s...