Sonnet 5.5 Leak: What We Know About Anthropic's New Model

The Sonnet 5.5 leak covers unverified claims about Anthropic's new mid-tier model that have spread quickly over the past few days. The topic is drawing attention because Claude Opus 5.5 has already shipped, and the Sonnet family is widely assumed to be next in line. But the company has not yet given an official date.

In short:

• Anthropic said Sonnet 5.5 and Haiku 5.5 are coming "in the coming weeks," without sharing a specific date.
• Leaks claim a 1 million token context window and $2/$10 pricing; these numbers already exist in Sonnet 5.
• Some reports suggest an 872,000 token context, which contradicts the earlier claim.
• Most sources rely on unverified X accounts and anonymous leakers.

Where did these claims come from?

These leak claims picked up speed after Opus 5.5's announcement on September 22, 2026. In that announcement, Anthropic stated that Sonnet 5.5 and Haiku 5.5 were coming soon. However, the company did not disclose a firm date.

That gap has fueled a wave of speculation on social media. A pseudonymous X account and a second unverified leaker have become the main sources for these claims. To date, Anthropic has not officially used the name "Sonnet 5.5."

Still, some technical traces are intriguing. One developer shared that the model identifier "claude-sonnet-5-5" appeared in the backend of a third-party coding client. This could point to the existence of a real test endpoint.

Are the leaked features actually new?

No, most of the leaked features aren't new. The claims circulating attribute a 1 million token context window, a maximum output of 128,000 tokens, and $2/$10 pricing to the model. But these numbers are already published for the existing Sonnet 5 model.

Even more confusing, a report dated September 28 cites a different context window figure. That report claims Sonnet 5.5 has an 872,000 token context window. This number directly contradicts the earlier 1 million token claim.

This contradiction raises questions about the reliability of the leak sources. Most of the claims circulating don't corroborate one another. For now, they should be treated as speculative.

When might it launch?

There's no confirmed date yet, but there are predictions. Pankaj Kumar, one of the leak sources, suggested the model could launch on Tuesday, September 30. That date coincides with OpenAI's DevDay event.

This timing doesn't look like a coincidence. Companies often time their own announcements to the same week as a rival's major event. But this, too, is an unverified prediction, not an official statement from Anthropic.

In the previous .5 generation, the entire model family was completed within 56 days. This time the order is different; Opus came first. That strengthens the likelihood that the new Sonnet model will arrive soon.

What does Opus 5.5's performance tell us?

Opus 5.5's results set a reference point for Sonnet 5.5. According to Anthropic's announcement, Opus 5.5 is 20 percent cheaper than its predecessor in input and output token costs. In typical workloads, that gap can widen to as much as 40 percent.

Output generation speed also increased by more than 30 percent. This aligns with the performance gains we detailed in our Claude Opus 5.5 article. Anthropic hinted that the next Sonnet release would carry similar efficiency and safety improvements.

The chart below shows the price difference simply.

Opus 5.5 (tipik iş yükü)	40
Opus 5.5 (token başına)	20

Kaynak: Startup Fortune

These figures belong to Opus 5.5; there's no verified data yet for its smaller sibling. Still, it seems likely that the pricing trend will continue.

Why is pricing such a hot topic?

The pricing debate stems from an expected price hike claim. Anthropic's pricing page currently lists the $2/$10 rate as the "standard price." Some sources had suggested a jump to $3/$15 was planned for September 2026.

That increase hasn't happened, at least not yet. That's also why the $2/$10 figure mentioned in the leaks isn't considered new information. It's identical to the existing Sonnet 5 pricing.

This matters for businesses. If a price increase has been postponed, budget planning becomes easier. But it's worth staying cautious until an official announcement arrives.

How should you approach these claims?

The safest approach is to treat the leaks with caution. The Sonnet 5.5 leak claims contain technically interesting clues, but most remain unverified. Following companies' official channels is always more reliable.

For developers and product managers, the practical advice is this: keep working with the current Claude Sonnet 5. When the new model arrives, the transition is usually smooth. Anthropic has a consistent track record on this front.

Meanwhile, our Claude speed optimization article explains the technical details behind model speed-ups. Similar improvements are expected when the new version arrives.

Frequently asked questions

Is Sonnet 5.5's release date confirmed?

No. Anthropic has only used the phrase "in the coming weeks" without sharing a specific date. Dates like September 30 are leaker predictions, not official statements.

Is the leaked 1 million token context window accurate?

Unclear. This figure already exists in Sonnet 5, so it isn't a new feature. Also, some reports claim a different number, 872,000 tokens, which creates a contradiction.

Will prices change?

For now, it looks like no. Anthropic's official pricing page still shows the $2/$10 rate, and claims of an expected increase have not been confirmed.

What can Opus 5.5 tell us about Sonnet 5.5?

Somewhat informative. Anthropic has hinted that Sonnet 5.5 will carry similar performance and efficiency gains, but a full benchmark comparison hasn't been shared yet.

This leak story is exciting but unproven. Until an official announcement arrives, it's best to approach the claims with some distance. At EngerekTech, we're ready to support businesses through the transition once the new model launches.

Sources