Claude Opus 5.1 Release Date Rumors Heat Up — Is Anthropic About to Drop Its Opus 5 Fix?

Claude Opus 5.1 could be waiting in the wings—and AI fans think Anthropic may unleash it sooner rather than later.

The rumor mill is working overtime after two mysterious model identifiers—“claude-marshmallow-eap” and “claude-melon-eap”—reportedly appeared in third-party developer tools and online testing circles in late August.

Some Claude watchers believe Marshmallow could become Claude Opus 5.1, while Melon may represent a future Sonnet or Haiku update. But before everybody starts popping the digital champagne, Anthropic has not confirmed what either codename means.

So, When Could Claude Opus 5.1 Be Released?

Our best estimate is sometime in September 2026, although an October release remains possible.

That prediction is based on Anthropic’s aggressive release schedule and its previous early-access testing pattern—not an official announcement.

A model reportedly called Claude Honeycomb EAP appeared in limited testing around July 8 and 9. Claude Opus 5 was then officially released on July 24, roughly two weeks later. If Marshmallow follows a similar schedule, Opus 5.1 could theoretically arrive at almost any moment.

However, that previous two-week window has already passed, suggesting Anthropic may still be testing the model, preparing additional capacity or waiting for the right launch date.

Bottom line: September looks like the strongest rumored window, but there is currently no verified release date—and Anthropic could delay it, rename it or decide that Marshmallow is something else entirely.

The Marshmallow Mystery

The “EAP” attached to the leaked model identifier is believed to mean Early Access Preview. That usually suggests a model is being tested privately before a wider rollout.

Early testers and online leakers have claimed that Marshmallow:

• May outperform the existing Claude Opus 5 on certain tasks.
• Appears particularly capable at coding, visual design and one-shot 3D projects.
• Uses extensive internal reasoning and may consume a large number of tokens.
• Could deliver more consistent results than the sometimes unpredictable Opus 5.

Those claims remain unverified. There are no official Opus 5.1 benchmarks, pricing details, model card or public API identifier available as of September 7, 2026.

Why Users Are Begging for Opus 5.1

Claude Opus 5 arrived on July 24 with some seriously impressive numbers. Anthropic says the model more than doubled Opus 4.8’s performance on Frontier-Bench v0.1 while costing less per completed task.

It also reportedly scored three times higher than the next-best model on ARC-AGI 3 and delivered approximately 1.5 times the pass rate of its closest competitor on Zapier AutomationBench at the same cost per task.

But benchmarks haven’t stopped some Claude users from complaining about uneven real-world performance. Community criticism has focused on Opus 5 occasionally being verbose, inconsistent or less dependable than the popular Opus 4.8.

That has fueled speculation that Opus 5.1 could be designed as the cleanup job—a tighter, steadier update that keeps Opus 5’s intelligence while ironing out its rough edges.

Fable 5.1 May Be the Biggest Clue

Anthropic released Claude Fable 5.1 on September 1, proving the company is already rolling out “5.1” updates across its newest model family.

Fable 5.1 targets demanding reasoning and long-running agentic work, while the official Claude documentation still recommends Opus 5 for most general workloads. The current Opus model costs $5 per million input tokens and $25 per million output tokens, with a one-million-token context window.

Anthropic could keep those prices unchanged for Opus 5.1, just as Opus 5 retained its predecessor’s pricing—but that is only an educated guess until the company speaks.

The Verdict

Claude Opus 5.1 appears plausible, but it is not official.

The Marshmallow sighting, Anthropic’s rapid release schedule and the arrival of Fable 5.1 all point toward another Claude update being prepared. Still, calling Marshmallow “Opus 5.1” remains a community theory rather than a confirmed leak.

If Anthropic follows its previous playbook, a September surprise would make sense. If additional testing or infrastructure work is required, the launch could slip into October—or Marshmallow could emerge under a completely different name.

For now, Claude fans are watching Anthropic’s model selector like it’s the final rose ceremony. One unexpected refresh, and Opus 5.1 could finally step into the spotlight.