Claude Sonnet 5 Leaked — Here's Why Devs Are Losing Their Minds

Claude Sonnet 5 leaked. Here's why devs are losing their minds.

This weekend, a Vertex AI error log revealed something that doesn't officially exist:

claude-sonnet-5@20260203

Internal codename: "Fennec" 🦊

Current Benchmarks (SWE-Bench)

• Opus 4.5 → 80.9% (the best, but expensive)
• Sonnet 4.5 → 77.2% (good value)
• GPT-4o → ~70%

What the leak suggests for Sonnet 5: → 80.9%+ (Opus-level… at Sonnet prices)

What the Vertex AI Log Reveals

1. The @20260203 format = February 3, 2026
2. Google TPU optimization (Anthropic has access to 1M TPUs since October)
3. "Dev Team" mode with parallel sub-agents

My Take

Anthropic released Opus 4.5 ten weeks ago. Dropping a new model without communication? Not their style.

But the leak is technically coherent.

If true → game changer for devs who want top-tier without blowing the API budget.

Stay tuned. 👀