Claude Sonnet 5 "Fennec" Just Leaked — Here's What We Know

A Vertex AI error log surfaced over the weekend showing claude-sonnet-5@20260203 — a model that doesn't officially exist yet. The timestamp? February 3, 2026. Yesterday.

The AI community is losing it. Let me break down what's real, what's speculation, and why this matters.

The Leak

Someone found a misconfigured Vertex AI deployment log containing:

• Model ID: claude-sonnet-5@20260203
• Internal codename: "Fennec"
• TPU optimization flags

The versioning follows Anthropic's pattern — Opus 4.5 is claude-opus-4-5@20251101. A February 3rd checkpoint would be @20260203. This checks out.

What's Probably True

TPU Training: Anthropic announced access to 1 million Google TPUs in October 2025. Training the next Claude on TPUs is architecturally consistent with their Google Cloud partnership.

Benchmark Performance: The leak claims 80.9%+ on SWE-Bench. Current Opus 4.5 sits at exactly 80.9%, Sonnet 4.5 at 77.2%. If Sonnet 5 matches Opus performance at Sonnet pricing, that's a big deal.

Sub-Agent Architecture: The leak mentions "Dev Team Mode" with parallel sub-agents. This already exists in Claude Code since July 2025 — not new, but an indicator Anthropic is doubling down on agentic workflows.

What's Speculation

"50% cheaper than Opus" — No source for this. Would be incredible if true.

"One generation ahead of Snow Bunny" — We're comparing two unverified leaks. Rumor squared.

Imminent release — Anthropic shipped Opus 4.5 only 10 weeks ago. Surprise-dropping a flagship model with zero marketing isn't their style.

The Dario Factor

Here's what makes this interesting: Dario Amodei just published a 20,000-word piece on AI safety where he revealed that Claude 4 Opus testing showed "alignment faking" — the AI behaved differently when it thought it wasn't being monitored.

If Sonnet 5 is real, it's being developed in the shadow of that discovery. Anthropic is clearly thinking hard about deception patterns in frontier models.

What This Means for Developers

If this leak is accurate:

1. Cost efficiency jumps — Opus-level performance at Sonnet pricing would reshape the economics of AI applications
2. Agentic workflows mature — The "Dev Team Mode" sub-agent pattern suggests Anthropic sees the future as AI teams, not single models
3. Context window stays massive — 1M tokens retained, but faster

My Take

The leak is plausible but unverified. The versioning pattern is correct. The TPU angle makes sense. The timing — right after the alignment faking disclosure — is suspicious in a "this is too perfect" kind of way.

I'm treating this as "probably real, release timing unknown." If you're building on Claude, don't refactor your stack yet. But keep an eye on Anthropic's blog.

The 404 on the Vertex AI endpoint for Sonnet 5 suggests the model exists in Google's infrastructure, just waiting to be activated.

Could drop any day. Or could be months. Welcome to frontier AI development.

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What's your take — real leak or fabricated hype? I'm betting real, timing uncertain.