An unannounced model named Obsidian 3 appeared in the public data behind Cognition's FrontierCode 2.0 leaderboard on its October 28 launch day, scored 46.8% at high effort against 45.5% for Claude Opus 5.5 at max, and was gone from the file within hours. Because the run used the claude-code harness, X posts quickly read it as Anthropic's long-rumored Claude Fable 5.5.

The identification is far from settled. One detailed Fable 5.5 tracker notes that the file gave Obsidian 3 the same chart colour as Meta's Muse Wonder 1.0, not the orange used for every Claude model, and a separate check of the claim notes the hidden row is not in Cognition's published table and traces to a single X post.

At max effort the two models were reportedly tied, and Obsidian 3 was said to cost several times more per task than Opus 5.5. Anthropic has still announced no Fable 5.5 — its newsroom, checked October 28, lists Haiku 5.5 as its latest model — so this stays a rumor, and a thin one, until a lab claims the name.