Claude Sonnet 5 Is Here. Opus 4.6 Just Dropped. What You Need to Know.

The Vertex AI Error Log

On February 3, a developer working with Google Cloud's Vertex AI platform encountered an error message that sent ripples through the LLM community. The error log referenced a model identifier: claude-sonnet-5@20260203.

The discovery was first shared by developer Pankaj Kumar on X, along with additional details about what turned out to be Anthropic's next major Sonnet release:

What the Leaked Data Revealed

The original leak, now largely confirmed by independent testing and the Opus 4.6 release, pointed to these specifics about Claude Sonnet 5:

• Internal Codename: "Fennec", a small fox known for its large ears and desert adaptability
• Model Identifier: claude-sonnet-5@20260203, suggesting a February 3, 2026 release date
• SWE-Bench Score: 82.1% on the standard benchmark, a notable improvement over previous versions
• Pricing Position: $3/$15 per million tokens, roughly 80% cheaper than Claude Opus 4.5 ($15/$75)

These numbers represent a significant step forward in the Sonnet line's capabilities while maintaining the cost-efficiency that has made it popular for production workloads.

Claude Sonnet Performance Over Time

To put these leaked numbers in context, here's how Claude Sonnet models have progressed on the SWE-Bench benchmark:

The trajectory shows consistent improvement, with each generation narrowing the gap with the more expensive Opus models. The leaked 82.1% score would place Sonnet 5 above the high-compute scores of both Claude 4 Sonnet and Claude 4.5 Sonnet.

Model Comparison Table

Why This Matters for Developers

"If you're still evaluating Opus 4.5, you might be paying too much. By the time most teams finish their 'AI strategy meetings,' the landscape has shifted again."

Most production teams have already migrated to Claude Opus 4.5 for high-stakes applications. What makes Claude 5 significant is that it outperforms Opus 4.5 on coding benchmarks at 80% lower cost.

The 82.1% SWE-Bench score widens the gap with OpenAI. While GPT models have improved significantly, Claude has maintained a consistent lead on code-related benchmarks, and Sonnet 5 extends that advantage further.

Claude Sonnet 5 vs GPT-5: How Do They Compare?

Based on confirmed benchmarks and testing data, Claude Sonnet 5 has a significant edge over GPT-5.1 in coding tasks:

• SWE-Bench: Claude Sonnet 5 reportedly scores 82.1-83.3% vs GPT-5.1's 76.3%
• Context Window: Claude Sonnet 5 offers 1M tokens vs GPT-5's 128K default
• Pricing: Claude Sonnet 5 at $3/$15 per million tokens is competitive with GPT-5.1's pricing
• Agentic Capabilities: Both models support multi-step autonomous tasks, but Claude's "Dev Team" mode may offer more sophisticated code generation workflows

For developers choosing between Claude and GPT for coding tasks, Claude Sonnet 5 appears positioned as the stronger choice, though GPT-5 may still have advantages in other domains like multimodal reasoning and general knowledge tasks.

The Opus 4.5 Throttling Question

Adding an interesting wrinkle: there have been widespread reports in developer communities that Anthropic has been throttling Opus 4.5 performance in recent weeks. Users have noted degraded response quality and increased latency, leading to speculation that this may be intentional, positioning the new Sonnet 5 to feel like an even more dramatic improvement by comparison.

Whether this was perceived degradation, infrastructure changes, or deliberate product positioning remains unclear. What is clear: with Sonnet 5 now matching or exceeding Opus on coding benchmarks, the performance gap between Opus-tier and Sonnet-tier models has effectively closed, making the choice about cost rather than capability.

Potential Use Cases

• Complex code generation and refactoring at scale
• Autonomous agent workflows requiring reliable reasoning
• AI coding assistants with higher accuracy than current models
• Production Claude API workloads currently constrained by Opus pricing
• High-volume applications where cost-per-token matters

NEWAnthropic's Williams F1 Partnership

In related news, Anthropic announced a partnership with the Williams F1 racing team on February 3, 2026, the same date as the Sonnet 5 leak. The partnership will integrate Claude AI across the Williams organization, from engineering to operations.

While Anthropic hasn't confirmed whether this partnership will use Sonnet 5, the timing suggests the new model may be central to the collaboration. F1 teams require fast, accurate AI for real-time strategy decisions, exactly the use case where Sonnet 5's improved speed and lower cost would shine.

BREAKINGClaude Cowork Triggers $285B Market Selloff

Alongside the Sonnet 5 leak, Anthropic quietly released Claude Cowork, a suite of AI plugins that automate tasks in legal, sales, marketing, and data analysis. The market reaction was swift and severe.

Global tech stocks shed $285 billion in value as investors priced in fears of AI-driven job displacement. Legal software companies were hit particularly hard after Anthropic's legal plugin demonstrated document review capabilities that rival junior associates. Indian IT services stocks also tumbled on concerns about automation of routine work.

This market reaction underscores why Claude 5's capabilities matter beyond benchmarks. When AI gets good enough, entire industries take notice.

NEWAnthropic: "Ads Are Coming to AI. But Not to Claude."

In a notable contrast to OpenAI, which recently began testing ads in ChatGPT, Anthropic is spending $8 million on Super Bowl ads with the tagline: "Ads are coming to AI. But not to Claude." The campaign includes a 60-second pregame spot and a 30-second in-game ad, both mocking the idea of AI chatbots inserting sponsored content into conversations.

OpenAI CEO Sam Altman called the ads "dishonest" for implying ChatGPT would twist conversations to insert ads, but admitted he laughed at them. This philosophical split between the two AI leaders is becoming a key differentiator for enterprise customers who don't want their workflows interrupted by sponsored content.

CONFIRMEDClaude Opus 4.6 Officially Released

What started as a backend log sighting is now official: Anthropic released Claude Opus 4.6 on February 5, 2026. The biggest addition is Agent Teams, teams of agents that split larger tasks into segmented jobs and coordinate in parallel. Opus 4.6 also gets a 1M token context window (matching Sonnet), direct PowerPoint integration, and now holds the top spot on the Finance Agent benchmark.

"Instead of one agent working through tasks sequentially, you can split the work across multiple agents, each owning its piece and coordinating directly with the others," said Scott White, Head of Product at Anthropic. The model is available via claude.ai, API, all major cloud platforms, and GitHub Copilot.

Some unverified leaks also suggest Sonnet 5's SWE-Bench score may actually be 83.3%, even higher than the 82.1% figure from the original Vertex AI logs.

Important Caveats

While Opus 4.6 is now official and multiple sources confirm Sonnet 5 has launched, some details remain unclear:

• Opus 4.6 confirmed: Officially released Feb 5 with Agent Teams, 1M context, and PowerPoint integration
• Sonnet 5 live but no formal announcement: Multiple sources report Sonnet 5 is available across Anthropic API, Amazon Bedrock, and Google Vertex AI, but Anthropic has not made a formal announcement
• Pricing widely reported: The $3/$15 pricing has been confirmed by multiple independent sources
• 1M context confirmed: The 1M token context window has been officially verified with near-zero latency

What to Expect

With Opus 4.6 now officially released and Sonnet 5 available across multiple platforms, the picture is becoming clearer. Anthropic's "vibe working" vision, where AI handles substantial professional tasks, not just coding, is taking shape with Agent Teams, PowerPoint integration, and Claude Cowork.

The Super Bowl (Feb 8) may bring a formal Sonnet 5 announcement. Developers using the Claude API should keep an eye on Anthropic's official changelog and documentation for updates. We'll continue updating this article as news breaks.