Claude Opus 5.1 Rumors Surface Checkpoints

The leak signals are interesting, but the evidence is nowhere near strong enough to call this a model release.

• –Community interpretations conflict: Marshmallow is often described as stronger or Fable-like, while Melon is sometimes described as an Opus-like checkpoint; neither mapping is confirmed
• –Early tester impressions are subjective, contradictory, and unsupported by reproducible coding or reasoning evaluations
• –The EAP identifiers suggest prerelease infrastructure testing, not guaranteed public availability
• –Developers should keep production model IDs configurable and wait for Anthropic’s documentation before testing migration
• –Anthropic’s rapid 2026 release cadence makes frequent checkpoint testing plausible, but codenames can be renamed, merged, or abandoned