Claude Sonnet 5 'Fennec': Anthropic leaks next model in Google Vertex AI

This week, developers discovered references to a new Anthropic model codenamed “Fennec” in Google Vertex AI logs, confirming what many expected: Claude Sonnet 5 is about to launch.

The discovery

The leak occurred when developers using Google Vertex AI noticed a new model in the logs:

model: "claude-sonnet-5@fennec"
version: "2026-02-03"
status: "preview"

Anthropic has not officially confirmed the launch, but the appearance in Vertex AI suggests it’s in final testing with cloud partners.

What we expect from Claude Sonnet 5

Based on Anthropic’s improvement pattern and industry rumors:

Confirmed improvements (from leaks)

Area	Expected improvement
Code	Better understanding of large projects
Context	400K+ token window
Reasoning	Improved chain-of-thought
Speed	2x faster than Sonnet 3.5

The “Fennec” name

Anthropic uses animal names for internal models:

Claude 2: “Opus” (not an animal)
Claude 3 Haiku: Japanese poetry names
Claude 3.5 Sonnet: Musical name
Claude Sonnet 5 “Fennec”: Desert fox

The fennec is known for its large ears (better “listening”/comprehension?) and efficiency in extreme environments (better performance with fewer resources?).

Context: The 2026 model war

This launch comes at a critical time:

┌─────────────────────────────────────────────────────────────┐
│                2026 MODELS TIMELINE                          │
├─────────────────────────────────────────────────────────────┤
│                                                              │
│   January 2026                                              │
│   ├── OpenAI: GPT-5.2 Codex launched                       │
│   ├── Google: Gemini 2.0 Flash available                   │
│   └── Meta: Llama 4 preview for researchers                │
│                                                              │
│   February 2026 (this week)                                 │
│   ├── Anthropic: Claude Sonnet 5 "Fennec" leaked           │
│   ├── OpenAI: Announces ads in ChatGPT                     │
│   └── Anthropic: Announces ad-free Claude (Super Bowl)     │
│                                                              │
│   Coming months                                             │
│   ├── OpenAI IPO preparation                               │
│   ├── Anthropic possible IPO                               │
│   └── Google I/O with Gemini 3?                            │
│                                                              │
└─────────────────────────────────────────────────────────────┘

Impact for developers

If you use Claude Code

Claude Code has been Anthropic’s star product, generating $1 billion in annualized revenue in just 6 months. With Sonnet 5:

Better understanding of large codebases
Smarter refactoring
Fewer errors in multi-file changes

If you use the API

We expect:

New endpoint: claude-sonnet-5-20260203
Similar or slightly higher price than Sonnet 3.5
Gradual deprecation of Sonnet 3.5 (6-12 months)

What to do now

Try Sonnet 3.5 if you haven’t - it’s excellent for code
Review your integration - make sure you can switch models easily
Monitor costs - new models may have different pricing
Follow Anthropic - official announcement expected this week

Expected comparison

Metric	Sonnet 3.5	Sonnet 5 (expected)
Context	200K tokens	400K+ tokens
Speed	Baseline	2x faster
Code	Excellent	Even better
Price	$3/$15 per M	TBD
Agentic	Good	Optimized

We’ll update this article when Anthropic makes the official announcement. Follow us on LinkedIn for the latest AI news.

Need to integrate AI into your development? Contact us for a free consultation.