Why JSON Validation Must Come Before LLM Gateway Failover in Node.js APIs

Developers building private knowledge-base classifiers with Node.js should benchmark multi-provider LLM gateways on validated, policy-compliant outputs per unit of spend rather than on advertised token costs alone. A single API key via gateways like OpenRouter reduces credential and adapter overhead, but JSON mode only guarantees syntactic structure, not correct taxonomy tags, proper value types, or resistance to prompt-injection within documents. Every response must pass through a parse, schema validation, and allowed-tag check before being accepted, since a failed validation that triggers a retry is not cheap and a label-changing fallback is a classification decision, not a recovery. Direct providers such as OpenAI, Claude, and Gemini each require their own native adapters, while a gateway normalizes access but cannot define application-specific taxonomy or determine label correctness. The recommended engineering approach places a strict validator between the adapter and business logic so that transport changes do not affect downstream application behavior.