The leaked internal document details "Fennec," reportedly the codename for Claude Sonnet 5, positioned as a full generation advance beyond Gemini's "Snow Bunny." An alleged Vertex AI error log entry, claude-sonnet-5@20260203, indicates an imminent release window of February 3, 2026.

Key characteristics and rumored capabilities include:

• Cost-Efficiency: The model is rumored to be 50% cheaper than Claude Opus 4.5 while simultaneously outperforming it across various metrics.
• Context Window and Performance: It retains a 1-million token context window, a substantial capacity, while exhibiting significantly faster operational speeds.
• Hardware Optimization: Allegedly trained and optimized on Google TPUs, this foundational hardware integration is posited to enable higher throughput and lower latency.
• Advanced Agentic Architecture (Claude Code Evolution): The model demonstrates an advanced capacity for spawning specialized sub-agents. These sub-agents can include roles such as backend developers, quality assurance (QA) specialists, and researchers. They operate in parallel directly from the terminal.
• Autonomous Development Mode ("Dev Team" Mode): Building upon its agentic capabilities, the model incorporates a "Dev Team" mode. In this mode, the specialized agents run autonomously in the background. A user provides a high-level brief, and the agents are designed to autonomously build a full feature, mimicking the collaborative workflow of human development teams.
• Benchmarking Performance: Insider leaks suggest exceptional performance on coding benchmarks, specifically claiming it surpasses 80.9% on SWE-Bench, indicating it effectively outscores contemporary coding models.
• Infrastructure Presence: A 404 error encountered on the specific Sonnet 5 ID within Vertex AI is interpreted as confirmation of the model's existence within Google’s infrastructure, awaiting activation.