Fennec Leak: Claude Sonnet 5 Discovered in Vertex AI Logs
Developer discovers Claude Sonnet 5 'Fennec' in Google Vertex AI with 80.9% SWE-bench score. Analysis of the leak and what it means for release timeline.
Breaking: Claude Sonnet 5 "Fennec" Discovered
A developer scanning Google Cloud's Vertex AI endpoints has discovered evidence of Claude Sonnet 5, internally codenamed "Fennec."
The Discovery
On February 3, 2026, a developer discovered the following:
Model Identifier: claude-sonnet-5@20260203 Key Evidence:- Querying this model ID returns HTTP 403 (Forbidden)
- Fake model IDs return HTTP 404 (Not Found)
- The 403 response suggests the model EXISTS but isn't publicly accessible
Performance Indicators
Early sources suggest Fennec achieves:
- SWE-bench Verified: 80.9% (matching Opus 4.5)
- Pricing: Potentially 50% cheaper than Opus 4.5
- Speed: Improved latency over current Sonnet
What This Means
1. Claude Sonnet 5 is real - The 403 response confirms existence
2. Release is imminent - Model is in deployment infrastructure
3. Sonnet-first launch - Following Claude 3.5 pattern
4. Vertex AI partnership - Google Cloud distribution confirmed
Timeline Implications
If the date string (20260203) indicates training completion:
- Model training finished February 3, 2026
- Safety testing: 2-4 weeks
- Public release: Mid-to-late February 2026
Community Reaction
The AI community is buzzing:
- Prediction markets jumped to 65% for pre-April release
- Developer forums speculating on capabilities
- Enterprise customers requesting beta access
Conclusion
The Fennec leak provides strong evidence that Claude Sonnet 5 is production-ready and deployment is imminent. Watch for official Anthropic announcements in the coming weeks.