vLLM Semantic Router v0.3 Themis: Stateful Production Routing with Session-Aware Agentic Routing
vLLM Semantic Router v0.3 (codename Themis), released June 5, 2026, transforms routing from a classification tool into a stateful, observable production system. Key additions: a unified v0.3 configuration format eliminating dialect fragmentation; signal enrichment extracting evidence from 15+ signal families (auth, safety, conversation shape, tool-loop detection); Session-Aware Agentic Routing (SAAR) combining router-owned session memory, safety locks during tool loops, provider-state portability checks, and replayable diagnostics; a revamped operator dashboard; and an Intel OpenVINO binding for C++/Go integration. The release represents 350+ commits since v0.2.0. The router ranked #1 on RouterArena with a 75.4 weighted Arena Score and adds native Anthropic `/v1/messages` protocol support alongside OpenAI compatibility.
Why it matters
SAAR directly addresses a practical agentic deployment problem — multi-turn agents switching models mid-session and destabilizing behavior. The Anthropic protocol support broadens applicability beyond pure OpenAI-compatible stacks, and the #1 RouterArena ranking validates production readiness.
Importance: 3/5
Major version release of production routing infrastructure with novel SAAR capability; 350+ commits; top RouterArena benchmark score.