Claude Sonnet: Fennec Leak

Over the weekend I was scrolling through Vertex AI deployment logs — yeah, that's my idea of a relaxing Sunday — and something caught my eye that made me sit up straight. A string that shouldn't exist yet showed up in what looked like a misconfigured error output: claude-sonnet-5@20260203. No official announcement. No blog post from Anthropic. Just a model ID sitting there in plain sight, timestamped today. I've been hands-on with every Claude release since 3.5, and I know how Anthropic usually plays this game — so trust me, this one's worth a closer look. I put together everything I could actually verify, stripped out the hype, and here's where things stand right now.

Current public evidence vs rumor lines

Let me be straight with you: Anthropic has said nothing. Zero. No press release, no changelog entry, no model card. What we do have is a cluster of developer posts from the last 48 hours all pointing to the same artifact — a Vertex AI error log referencing claude-sonnet-5-20260203 with an internal codename, "Fennec."

Here's what I've been able to cross-reference against things that are actually confirmed, as of this morning:

So where does that leave us? The TPU infrastructure and the naming convention are real. The rest is still sitting in "interesting but unproven" territory.

Model ID naming patterns (what they usually mean)

This is actually the part that makes the leak more credible than most — and also the part people keep getting wrong in the discourse. Anthropic's official model docs spell out the pattern pretty clearly. Every model ID follows this structure:

Here's how the current lineup maps:

The snapshot date in the leaked ID — 20260203 — is literally today. That's either incredibly well-timed, or it's a checkpoint date that got pulled from an internal build environment before it was production-ready. I've seen both scenarios play out before.

One thing that doesn't quite add up: Opus 4.5 shipped November 24, 2025. That's roughly 10 weeks ago. Anthropic's cadence between major version bumps has historically been 4-6 months. A full generational jump to "5" in 10 weeks would be fast — even for them. My gut says this is either an internal test snapshot or the date string means something slightly different than a public launch date.

What Verdent will test on Day 0

Okay, here's where this gets practical — and where I actually spend most of my time. If Sonnet 5 drops today, tomorrow, or this week, our team at Verdent has a playbook ready. We're not going to wait for the blog posts and hot takes. We're going to run it through the same eval suite we use for every new model that hits the API.

The reason this matters for you: if you're building on Claude right now — whether you're using Sonnet 4.5, Opus 4.5, or routing between them — you need to know whether Sonnet 5 is actually worth a stack change. Marketing claims won't tell you that. Benchmark runs will.

Repro checklist (repo set, env, metrics)

Here's exactly what we'll run, and why each piece matters:

• SWE-bench Verified — the headline number

The leak claims >80.9% on SWE-bench Verified. Current state of the art hovers in the 74-79% range across frontier models. If Sonnet 5 actually hits 80+%, that's not incremental — that's a tier shift. We'll submit through the standard scaffold and compare apples-to-apples.

• Multi-agent parallel execution under load

This is where Verdent's architecture actually lets us do something most individual devs can't: we run multiple agents in isolated Git worktrees, each hitting the API concurrently. Here's the basic pattern we use for eval:

We'll throw 8-10 concurrent tasks at it and measure latency, token efficiency, and output quality side-by-side against Sonnet 4.5. If the "parallel sub-agent" claims hold up, we should see meaningful throughput gains here.

• Context window stress test

Some of the wilder rumors suggest a context window expansion. We'll test with full repo dumps — 50k+ tokens of actual production code — and see how coherence holds up at scale. This is where models tend to quietly degrade, and it's where the gap between marketing specs and real-world performance usually shows up.

• Cost-per-resolution baseline

If Sonnet 5 really does land at Sonnet-tier pricing, the math gets interesting fast. We'll calculate cost-per-resolved-issue against SWE-bench Pro — that's the harder, contamination-resistant benchmark — because that's what actually maps to enterprise workloads.

Here's the eval matrix we'll publish results against:

Bottom line for right now: Don't restructure anything based on a screenshot. But do keep your eye on Anthropic's official release notes and the SWE-bench leaderboard over the next 24-72 hours. If Sonnet 5 is real and it lands close to what's being whispered, it's going to be one of the most meaningful model drops of 2026 — especially for teams already running multi-agent setups.

I'll update this post the moment we have actual benchmark data in hand. In the meantime — build on what's proven, not what's leaked.

FAQ — The questions everyone's actually asking

Q: When is Claude Sonnet 5 actually dropping? A: Nobody knows. The leaked snapshot date is today (Feb 3), but a snapshot date isn't a launch date. The Metaculus community consensus points to later in 2026. Treat anything before Q2 as optimistic speculation until Anthropic says otherwise.

Q: Can I use claude-sonnet-5-20260203 in my API calls right now? A: No. That model string returns a 404 today — not in the official models list, not in Vertex AI, no SDK support. It surfaced in an error log. Watch Anthropic's docs for the real launch.

Q: What about the pricing rumors — is it really 50% cheaper than Opus 4.5? A: Zero hard evidence behind that number. Every Sonnet-tier model has been priced well below the Opus equivalent, so Sonnet 5 will almost certainly follow suit. But "50% less" is made up. Don't put it in a budget doc until Anthropic publishes pricing.