"Verified data.json is fetched live and all four requested metrics are shown." Muse Spark Open sent that line from inside Claude Code 16 seconds after we asked for a small dashboard page. The verification was a text search for a few expected words. Muse never rendered the result, and every bar on it was empty. DeepSeek V4.1 Flash needed 2 minutes 29 seconds, loaded its page in a headless browser (Chrome with no window, driven from a script), studied the screenshots and delivered a dashboard that worked.

Meta made the Muse Spark Open weights public on 15 October, and both models went through identical coding tests on the 16th. The full Muse Spark Open vs DeepSeek V4.1 Flash comparison has the video, every table and the run records. Here we look at which numbers fool you when you plug models into coding agents.

A 16-second "done" with an invisible bug

The bug survives a quick read of the code.…