Anthropic Drops Claude Fable 5 and Mythos 5 as OpenAI Acquires Ona and xAI Opens a Grok Plugin Marketplace

Friday was one of the densest days for frontier AI announcements in months — Anthropic shipped two new models, OpenAI made an acquisition, and xAI launched a developer ecosystem, all within hours.

Anthropic released Claude Fable 5 and Mythos 5, OpenAI acquired a company called Ona to bolster its Codex agent infrastructure, and xAI launched a plugin marketplace for Grok Build, all on the same day, as tracked by @oscarlau and @daogangtang. The simultaneous timing appears coincidental rather than coordinated, but the effect is a single news cycle that forces the industry to process three major strategic moves at once.

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