Report: leaked Claude 'Marshmallow' reportedly outperforms Opus 5
Two EAP codenames — marshmallow and melon — surfaced in third-party apps Aug 24; the pattern points to an Opus 5.1-class refresh landing within weeks.
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The two codenames, and where they showed up
On Aug 24, `claude-marshmallow-eap` and `claude-melon-eap` began appearing in third-party developer apps and Discord — one capture even carried a typo (“mashmallow”), a tell that it came from a live source, not a hoax. Anthropic has published no model card, changelog entry, or comment.
Marshmallow reportedly fixes Opus 5's uneven effort
Testers claim Marshmallow returns strong results even at Medium reasoning — aimed at the complaint that Opus 5 goes “lazy, sloppy, and verbose” and doesn't scale smoothly across effort levels. Most read it as an Opus 5.1-class update, and say it edges out Opus 5.
Melon is the mystery half
The second codename is widely read as the smaller model, but its tier is unconfirmed — some peg it as a new Haiku, others a Sonnet 5.1 refresh. If the “one Opus, one smaller” pattern holds, plan for a matched pair rather than a single drop.
The timing tell: days, not months
The last EAP codename, “Honeycomb,” was spotted in Cursor around Jul 8–9 (1M context, an “extra high effort” mode, falling back to Opus 4.8 on safety) and shipped as Opus 5 on Jul 24 — a 16-day gap. Repeat that cadence and Marshmallow's GA lands in early September, so don't hard-pin your model string this week.