Meta CEO Mark Zuckerberg returned to X for the first time since 2023 to announce Muse Spark 1.1, a new agentic and coding AI model, along with a new Meta Model API that lets outside developers build on it for the first time.

Zuckerberg posted the announcement on X rather than Threads, Meta's own platform, a move that quickly drew attention on its own.

Elon Musk responded by pointing out that Zuckerberg's post had already crossed 12 million views without any paid promotion. He said X remains the best platform for product announcements when done directly by a CEO, calling it more engaging than traditional press releases.

The launch came a day after Musk's company released Grok 4.5, also pitched as a low-cost model. Musk replied to Zuckerberg's post with a single word: "Jinx."

Meta priced Muse Spark 1.1 at $1.25 per million input tokens, $0.15 for cached input, and $4.25 for output, undercutting Grok 4.5. The model has a 1 million token context window and can delegate tasks to parallel sub-agents.

In short, Zuckerberg's rare appearance on a rival platform turned into a viral moment, with Musk using the engagement numbers to make a case for X over conventional PR channels.