Which LLM should power a MERN chatbot in 2026?

Two-model routing is the default 2026 pattern for a MERN OpenAI chatbot or Claude chatbot. Claude Sonnet 4.5 for reasoning-heavy or long-context questions (complex support tickets, agentic workflows requiring multi-step planning), GPT-4o mini for high-volume cheap tasks (intent classification, quick greetings, factual lookups). A MERN OpenAI chatbot deployment routing to GPT-4o mini can cut LLM costs by 60% vs Claude-only routing. Route by request classification, roughly 60% to 80% of traffic goes to the cheap model. Add automatic fallback to the cheaper model when premium quotas trip. Every MERN AI chatbot we ship supports both providers with a single config line, so LLM lock-in is never a concern.