A new round of leak posts has tightened the launch-watch on Claude Sonnet 5.5, which Anthropic promised on September 22 would arrive "in the coming weeks" alongside Haiku 5.5. X account Pankaj Kumar claims the model is already in partner testing, with some partners getting temporary 24-hour access, and that feedback could push a ship as soon as September 30 or October 1.

On September 27, StartupFortune reported a leak claiming a last-minute upgrade ahead of a Monday (September 28) launch — and explainx.ai noted the string "claude-sonnet-5-5" has appeared in Factory Droid 0.228.0's model registry behind a feature flag. Users also report A/B-style tests where some accounts appear to be switched to the new version. One Monday launch bet failed to materialize, which is worth noting.

Treat the hype carefully: an earlier claim that Sonnet 5.5 was "already beating GPT-6 Sol" in secret testing turned out to rest on a spec sheet — 1M context, 128K output, $2/$10 pricing — that is simply Claude Sonnet 5's existing public spec. Anthropic still hasn't published a model card, pricing, or benchmarks for 5.5. The timing guesses keep lining up against OpenAI's DevDay on Tuesday, suggesting Anthropic may be aiming to blunt OpenAI's news cycle.

Resolved September 28: Anthropic officially released Claude Sonnet 5.5, confirming the rumor's core claim — it shipped a day or two earlier than the Sept 30 / Oct 1 window the leak pointed to.

Resolved 2026-09-30: confirmed — Anthropic shipped Claude Sonnet 5.5 on September 28, earlier than the rumored Sept 30 / Oct 1 window. See the Sonnet 5.5 release entry.