❯ “Anthropic to Release Opus 5.1 This Week” Has No Evidence; Rumor Traced to Traffic-Siphoning Sites

FACT CHECKClaims that Anthropic is releasing Opus 5.1 this week, or has already begun a phased rollout of Fable 5.1, find no support in official channels. Claude’s official documentation currently lists only four available models: Fable 5, Opus 5, Sonnet 5, and Haiku 4.5, with no 5.1 series in the API identifiers; the official news page shows no model releases during the entire month of August, and the API changelog has no related entries as of August 26.

DEBUNKEDThe most credible-sounding piece of the rumor was that “Fable 5’s knowledge cutoff has changed.” The official documentation still lists Fable 5’s reliable knowledge cutoff as January 2026, unchanged from launch. TestingCatalog, historically the most accurate leak account, has not mentioned 5.1 recently; its last model-level leak was the July 23 tip about Opus 5, which was confirmed three days later.

SOURCEThe pages that matched largely come from SEO content on a few AI tool sites and API reseller sites, chasing traffic by predicting unreleased model names. One page even states outright that the so-called claude-fable-5-1 is merely its URL suffix, not a callable model identifier. What deserves attention is developers’ threshold for accepting clues like “model identifier leaks”—taking a traffic-siphoning page’s URL as API evidence is the most common break in this year’s rumor chain.

▪ SIGNALThe optimal strategy for traffic-siphoning sites is to register the next version number as a URL ahead of time; “leaks” like this will only become more common.