Users on X report that Anthropic is quietly testing unreleased models, with claims that Claude Opus 5.5 requests are being routed to a new version dubbed Fable 5.5. Although Anthropic has not officially announced this model and its public Mythos page still lists version 5.1, technical signals suggest ongoing development. On November 1, a "Mythos 5.5" label appeared in the hidden HTML code of Anthropic’s official website before being removed. Additionally, an entry for "claude-fable-5-5" was added to the LiteLLM catalog on October 13, copying specifications from Fable 5.1, but this entry was also later deleted.
The speculation intensified after pseudonymous user Chetaslua posted on October 11 that "Claude Opus 5.5 is routing to a new model," citing outputs that appeared stronger than standard Opus 5.5 results. Other developers, such as Shikhar, shared animations created using Opus that exhibited superior quality, leading some to suspect an A/B test where a new model is served under the old name. Mythos is described as a restricted variant of Fable, offering lighter safety filters for vetted cybersecurity and biology researchers.
Parallel rumors emerged regarding OpenAI, with users noting that "gpt-6.2-astra" appeared in Codex usage analytics for some accounts on November 1. While the model picker still displays GPT-6 Astra, usage breakdowns listed the 6.2 variant separately, suggesting another potential A/B test. OpenAI released GPT-6 Astra on September 3 and has not announced a successor. These developments coincide with Anthropic’s recent IPO filing on October 26, which reported $16.29 billion in revenue for the first half of 2026. SEC rules require the S-1 paperwork to be public for 15 days before the roadshow can begin, setting the earliest start date for November 10.
The simultaneous emergence of unconfirmed model versions at both Anthropic and OpenAI highlights the opaque nature of current AI development cycles, where technical artifacts often leak into public view before formal announcements. The presence of "Fable 5.5" and "GPT-6.2 Astra" in third-party catalogs and internal analytics suggests these companies are actively iterating on their flagship commercial tiers. For Anthropic, the timing is particularly sensitive given its recent IPO filing; any confirmation of superior model performance could significantly impact investor sentiment and market valuation during the pre-roadshow period.
From a Market Structure perspective, the reliance on community-driven leaks rather than official channels creates information asymmetry among stakeholders. Developers and enterprises integrating these APIs may face uncertainty regarding stability and pricing if they base decisions on rumored capabilities. The removal of the leaked entries indicates a controlled environment where companies monitor and correct public-facing data, yet the persistence of user-generated evidence underscores the difficulty of maintaining secrecy in large-scale distributed systems. This dynamic pressures competitors to accelerate release timelines to maintain perceived technological leadership.
