Over the weekend, a Vertex AI error log surfaced showing claude-sonnet-5@20260203—a model version that doesn't officially exist. The timestamp points to February 3, 2026.
The AI community is losing its collective mind. But before you rewrite your entire tech stack, let's separate signal from noise.
The Leak
The information comes from what appears to be a misconfigured Vertex AI deployment log. The key claims: a model ID of claude-sonnet-5@20260203, an internal codename "Fennec", and TPU optimization.
The versioning follows Anthropic's established pattern—Opus 4.5 is claude-opus-4-5@20251101. A February 3rd checkpoint would be @20260203. Plausible.
The problem: the source is a Twitter screenshot with no verifiable access to the original logs.
What I Could Verify
TPU Access: Anthropic announced access to 1 million Google TPUs in October 2025. Training next-gen Claude on TPUs is architecturally consistent.
Sub-Agents: The leak mentions "Dev Team Mode" with parallel sub-agents. This already exists in Claude Code since July 2025—not a Sonnet 5 exclusive.
Benchmarks: The leak claims 80.9%+ on SWE-Bench. Current Opus 4.5 sits at 80.9%, Sonnet 4.5 at 77.2%. If Sonnet 5 matches Opus performance at Sonnet pricing, that's significant. But plausible is not verified.
What Remains Speculation
Pricing: "50% cheaper than Opus" has no source.
"One generation ahead of Snow Bunny": We're comparing two unverified leaks. Rumor squared.
February 3rd release: Anthropic shipped Opus 4.5 only 10 weeks ago. Surprise-dropping a flagship model with zero marketing? Not their style.
The Full Analysis
I've written a complete breakdown covering:
- Detailed verification of each claim with sources
- What this means for developers if true
- The competitive context (Snow Bunny, GPT-5.2 rumors)
- Why you should be skeptical
- FAQ section for quick reference
Read the full article with sources and FAQ on my blog:
Claude Sonnet 5 "Fennec" Leak: What the Vertex AI Logs Actually Show
What's your take—real leak or fabricated hype?
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