Claude Sonnet 5 Leak: I Found Fennec on Vertex AI

🚨 UPDATE: Claude Opus 4.6 Officially Released

The leaks were real. Opus 4.6 has landed with massive upgrades: Agent Teams, 1M Context, and 500+ Zero-Day findings.

While we are still tracking "Fennec" (Sonnet 5), Opus 4.6 is the new king of reasoning. I have written a complete breakdown of the architecture, the new "Computer Use" capabilities, and benchmarks.

👉 Read the Deep Dive: Claude Opus 4.6: Everything New & Architecture Breakdown

🚨 BREAKING UPDATE: Perplexity API Leaks Opus 4.6 & Thinking Models

Status: CONFIRMED via API Config Source: Perplexity (rest/models/config)

Claude Opus 4.6 and Claude Opus 4.6 Thinking are now visible in Perplexity's public API configuration. This suggests an imminent release (potentially today), with Sonnet 5 likely following. The config also contains placeholders for GPT-5 and Gemini 3.

Evidence (JSON Response):

{
  "claude46opus": {
    "label": "Claude Opus 4.6",
    "description": "Anthropics fortschrittlichstes Modell",
    "mode": "search"
  },
  "claude46opusthinking": {
    "label": "Claude Opus 4.6 Thinking",
    "description": "Anthropics Opus-Reasoning-Modell mit Denken",
    "mode": "search"
  },
  "gpt5": {
    "label": "GPT-5",
    "description": "OpenAIs neuestes Modell",
    "mode": "search"
  }
}

Full API Endpoint: https://perplexity.ai/rest/models/config?config_schema=v1&version=2.18&source=default

🔴 Breaking Update: The HTTP 403 Proof (Feb 4, Late Afternoon)

While speculation runs rampant, Ben Taleb Jr. (@macintoch) has delivered the most significant technical evidence to date—directly from the Google Cloud Vertex infrastructure.

His monitoring script identified a critical anomaly in the API responses that effectively confirms the existence of the models:

"breaking : claude sonnet -5 and claude opus4.6 exist in google cloud vertex , but not yet availbe to public. they can be unlocked any time from today to next week [...]" — Ben Taleb Jr. via X

The Technical Analysis (Status Code Forensics)

The script performed a port scan against the Model Garden endpoints. The results are definitive:

1. Control Group (Fake ID claude-sonnet-99): Returns 404 Not Found.
   • Meaning: The resource does not exist.
2. Target Group (Leaked ID claude-sonnet-5): Returns 403 Forbidden.
   • Meaning: The resource exists, but access is denied (Permission Gated).

The Architect's Take: In RESTful API design, a 403 is a specific signal. The server understands the request and locates the resource, but refuses authorization. If claude-sonnet-5 were a hallucination or typo, the Vertex gateway would default to 404, just as it did for the fake ID.

Verdict: The artifacts are likely in "Stage Zero" deployment within Google's infrastructure, waiting for the public visibility toggle.

The Analysis: What is Real?

⚠️ Disclaimer: This article analyzes unverified leaks. Anthropic has not confirmed any of this. Treat all claims with appropriate skepticism.

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. Today.

The AI community is losing its collective mind. But before you rewrite your entire tech stack, let's separate signal from noise.

What Actually Leaked

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", a release window pointing to February 3, 2026 (inferred from the version string), and TPU optimization.

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

But here's the problem: the source is a Twitter screenshot with no verifiable access to the original logs.

What We Can Actually Verify

VERIFIED: Anthropic Has Massive TPU Access

In October 2025, Anthropic announced access to up to 1 million Google TPUs—worth tens of billions of dollars. This isn't speculation; it's a press release.

"Anthropic will have access to well over a gigawatt of capacity coming online in 2026."

If you're training the next generation of Claude, you'd want TPU optimization. This part of the leak is architecturally consistent.

VERIFIED: Claude Code Already Has Sub-Agents

The leak mentions "Dev Team Mode" where Claude spawns specialized sub-agents (backend, QA, researcher) working in parallel.

Here's the thing: this already exists. Claude Code introduced sub-agents in July 2025. You can run up to 10 simultaneously, each with specialized expertise. Developers are already using this to build "Product Trinity" patterns—PM, Designer, and Implementation agents working in parallel.

This isn't a Sonnet 5 feature. It's a Claude Code feature that works with current models.

Benchmarks & Implications

VERIFIED: The Benchmark Numbers Are Plausible

The leak claims Sonnet 5 "surpasses 80.9% on SWE-Bench."

Current standings:

• Claude Opus 4.5: 80.9% (first to break 80%)
• GPT-5.1: 76.3%
• Gemini 3 Pro: 76.2%
• Claude Sonnet 4.5: 77.2%

If Sonnet 5 matches or exceeds Opus 4.5's coding performance at Sonnet pricing, that's massive. But this is exactly what Anthropic's model strategy has always been: Sonnet catches up to previous Opus, new Opus pushes the frontier.

Plausible ≠ Verified.

UNVERIFIED: 50% Cheaper Than Opus 4.5

No pricing information has leaked from any credible source. This is pure speculation.

Sonnet has always been the value play. A 50% discount versus Opus would be consistent with historical positioning, but there's no evidence.

UNVERIFIED: "One Generation Ahead of Snow Bunny"

The leak claims Fennec is "one full generation ahead" of Google's internal Gemini 3.5 codename "Snow Bunny."

Snow Bunny itself is an unverified leak from last week claiming Gemini 3.5 can generate 3,000 lines of code in a single prompt. We're now comparing two unverified leaks against each other.

This is rumor squared.

What Would This Mean for Developers?

If the leak is accurate:

1. Sonnet becomes the default for most workloads. If it matches Opus 4.5 coding performance at Sonnet pricing, there's no reason to use anything else for 90% of tasks.
2. Multi-agent architectures go mainstream. The "Dev Team Mode" pattern—where you brief an AI and it builds the full feature autonomously—would become standard workflow.
3. Pricing pressure across the industry. If Anthropic ships Opus-level performance at mid-tier prices, OpenAI and Google have to respond.

If the leak is fabricated:

You've lost nothing by waiting for official confirmation. Opus 4.5 and Sonnet 4.5 are already best-in-class for coding.

Context & Verdict

The Timing Problem

Here's what makes me skeptical: the leak points to today (February 3, 2026) as the release date.

Anthropic's release pattern:

• Claude 3: March 2024
• Claude 3.5 Sonnet: June 2024
• Claude 4: May 2025
• Claude Opus 4.5: November 2025

That's roughly 4-6 months between major releases. We're only 10 weeks out from Opus 4.5.

Could they ship Sonnet 5 this fast? Theoretically yes—they have the compute. But surprise-dropping a flagship model with zero marketing buildup? That's not Anthropic's style.

More likely scenarios:

• Internal test build that leaked accidentally
• Scheduled checkpoint that isn't production-ready
• Fabricated screenshot riding the AI hype cycle

The Competitive Context

This leak doesn't exist in a vacuum. Last week, Gemini 3.5 "Snow Bunny" leaked. Google I/O is coming. OpenAI is rumored to be preparing GPT-5.2 upgrades.

Every major AI lab is in a sprint. Leaks—real or fabricated—create market pressure. They force competitors to respond, accelerate roadmaps, and generate media coverage.

Cui bono? Who benefits from a Claude Sonnet 5 leak right now?

• Anthropic: Yes, if it's real and they want buzz
• Competitors: Maybe, if they want to set unrealistic expectations
• Crypto grifters: Absolutely (remember $CLAWD?)

Be skeptical.

What To Do Right Now

For developers:

1. Don't rewrite anything based on unverified leaks
2. Keep monitoring Anthropic's release notes
3. If it drops, test before migrating production workloads

For businesses:

1. Budget flexibility is smart—a Sonnet 5 at current pricing would reduce inference costs
2. Don't make procurement decisions based on rumors

For everyone:

1. Wait for official confirmation
2. Ignore any crypto tokens that appear (they're scams)
3. Check back here—I'll update if/when Anthropic announces anything

The Verdict

I'll update this post if Anthropic makes an official announcement. For now, treat this as what it is: an interesting rumor that may or may not pan out.

What's your take—real leak or fabricated hype? Drop a comment.