Anthropic might be working on new Claude models called claude-marshmallow-eap and claude-melon-eap. The two identifiers surfaced in third-party developer apps and on Discord around August 24. Anthropic hasn’t officially announced either model yet. There’s no public model card, release date, or API listing for them right now.
The leaks have started all kinds of discussions about where these models could fit in Anthropic’s lineup. The most common reading is Marshmallow as Claude Opus 5.1 and Melon as Claude Sonnet 5.1, with a minority suggesting Melon could be a new Haiku-class model. Still, those guesses aren’t confirmed. What really stands out is the “-eap” ending, which means early access preview, a label Anthropic has used before for models that show up just ahead of a public release.
What the Marshmallow and Melon Leaks Reveal
There’s not a lot of information yet, mainly just the model names and some early rumours about how they perform. The identifiers are claude-marshmallow-eap and claude-melon-eap.
Leak posts describe Marshmallow as a likely Opus update and Melon as either a Sonnet or Haiku model. Testers said neither reaches Fable 5 level, but both perform solidly, closer to the Opus and Sonnet tiers.
Another report claims Marshmallow actually works better than Melon, and might already outdo the current Claude Opus 5. This has started rumors that Marshmallow could become Claude Opus 5.1, with Melon lined up as Claude Sonnet 5.1. However, none of these identities are confirmed so far.
Anthropic Opus 5.1 & Sonnet 5.1 Leaks: Marshmallow & Melon
— Pankaj Kumar (@pankajkumar_dev) August 24, 2026
– Both models are good at 3D generation and do it in one shot.
– They do a lot of thinking and use a lot of tokens.
– The model IDs are claude-marshmallow-eap and claude-melon-eap.
– Neither appears to have a first-party… pic.twitter.com/aHwU1UHky3
Looking at the current Claude lineup gives a bit more context. Claude Fable 5 costs $10 per 1 million input tokens and $50 per 1 million output tokens, comes with a 1 million-token context window and supports up to 128K output tokens. Claude Opus 5 is $5/$25 per 1 million tokens, also with a 1 million-token context window and 128K max output. Claude Sonnet 5 is $3/$15 per 1 million tokens, with a special price of $2/$10 through August 31. The most affordable, Claude Haiku 4.5, runs $1/$5 per 1 million tokens and has a 200K context window.
Haiku 4.5 is the oldest in this set. It launched back on October 15, 2025, so it’s pretty reasonable to expect a Haiku update soon. Sonnet 5 came out June 30, 2026, and Opus 5 launched July 24, 2026.
The EAP Clue and Anthropic’s Opus 5 Criticism
The clearest precedent comes from Claude Honeycomb. On July 8-9, 2026, a Claude Honeycomb EAP model briefly appeared in Cursor’s model picker before it was quickly removed. It supposedly offered a 1 million-token context window, an extra high effort mode, and was labeled early access.
Then, on July 24, the string claude-opus-5 showed up in Cursor error messages, and Opus 5 officially launched that day. That’s about 16 days between Honeycomb EAP showing up and Opus 5 going public. Reports figure the gap is right around two and a half weeks. But, as the Numbat codename shows, sometimes an internal model name sits unused for months. So, just because there’s an EAP label, doesn’t mean the model will launch publicly right away.
All this happens while Anthropic deals with criticism over Opus 5. Anthropic’s Thariq called Opus 5 really spiky and said they want Claude to feel more consistent and warm. Critics have described Opus 5 for being lazy, sloppy, and verbose, especially when its performance swings depending on settings or effort level.
That’s why this possible Marshmallow upgrade has caught so much attention. Early testers say it already gives good results, even on Medium reasoning, suggesting more consistent performance without always needing the highest settings.
No launch date is known as of yet. Leak posts suggest a release is near, and the Honeycomb precedent points to a window of a few weeks. Some posts suggest Anthropic is accelerating updates across its top, mid and budget tiers.
Opus 5.1 this week? Next week?
by u/Borat_2020 in Anthropic
For now, Anthropic hasn’t confirmed Marshmallow, Melon, Opus 5.1, or Sonnet 5.1. Google is also preparing Gemini 4, with some reports pointing to a possible December launch after Gemini 3.5 Pro was delayed. Still, it’s all speculation until we get official word from both these companies.
