Claude Sonnet 5.5 is said to be near release — but nobody can source its 1.5M context window

The claim: Claude Sonnet 5.5 is in the final stages before release and could launch as early as next month. Faster responses and lower latency than previous Sonnet models. A 1.5M-token context window. Designed to bring near-Fable 5 capability at Sonnet pricing.

We need to be straight about this one, because it is the weakest of the batch and the number in it is the reason.

We could not source the 1.5M-token context window anywhere. Not on Anthropic's own surfaces, not in its platform release notes, not in any leak reporting we could reach. The closest thing to a corroborating leak describes a next Sonnet — carrying the codename Fennec in one report — at a 1M-token context window, which is the same figure Opus 5 already ships with. A 1.5M window would be a new high for the Claude line, and a new high is exactly the kind of claim that should have more than one card behind it.

We also could not find the name "Claude Sonnet 5.5" in any first-party Anthropic material. That does not make it wrong. Anthropic does not pre-announce, and the model that eventually ships as the next Sonnet will have had a period exactly like this one where it existed and nobody outside had a name for it. But "we could not find it" and "it is confirmed to exist" are very different states, and the leak card does not distinguish them.

The rest of the claim is at least consistent with the shape of the line. Sonnet's whole positioning is capability-per-dollar under the flagship, so "near-Fable 5 at Sonnet pricing" is what a Sonnet refresh is supposed to be. Prediction markets currently price a next Claude Sonnet by December 31 at around 92% — high, but December is five months away and says nothing about next month.

Our board watches this under a deliberately generic pattern rather than a specific version number, because the version it ships under is precisely the thing nobody outside Anthropic knows yet.

Grade: rumor, and the softest one in this batch. If you are planning against a 1.5M context window, plan against 1M until somebody produces a source.