Semantic Routing
Semantic routing chooses a processing path from the meaning of an incoming request. A router may compare query embeddings with example routes or use a classifier to select a tool, workflow or model; the key skill is making routing decisions measurable and handling uncertain or overlapping cases explicitly.
What it is
Unlike keyword routing, semantic routing attempts to recognize related intentions expressed with different wording. A route can be represented by example utterances embedded into a vector space, with a similarity score used to choose a destination. Other systems use learned classifiers or a model-based decision. These are implementation choices, not a single standardized algorithm. Routing differs from retrieval because its main output is a control decision. It can occur before generation to choose a cheaper model, or inside a workflow to select an operation appropriate to the request.
What the work involves
A practitioner defines routes that correspond to distinct application behaviors, labels representative requests and establishes a fallback for weak or conflicting matches. Thresholds are calibrated on held-out examples, including requests that belong to no route. The evaluation reports confusion between routes and the consequences of wrong routing, not just aggregate accuracy. Versioned examples, route definitions and routing traces provide a reviewable artifact. When routing selects a model, end-to-end output quality and savings need measurement after the choice is applied.
Illustrative example
A workplace assistant has separate workflows for finding policies, opening an IT ticket and requesting an expense explanation. A request to replace a damaged laptop should reach IT even without the word ticket. A request comparing laptop reimbursement policies should reach policy search. The router tests these near neighbors, and uncertain requests ask for clarification or use a general handler. Route similarity is exposed to debugging tools so a misclassification can be traced to the examples used.
Limits and common mistakes
Embedding similarity is not a calibrated probability of intent. Broad example sets can absorb neighboring routes, and language or domain changes can invalidate a threshold. Requests containing several goals may need decomposition instead of one destination. A wrong route can invoke an inappropriate action, so authorization remains downstream of routing. Semantic routing is useful when explicit routes improve the application; it should be compared with a simpler rule or direct model call.
Prerequisites
- mediumContext Engineering
Routing decisions are context-dependent — understanding what context each model handles best requires context engineering
Related skills
- → is part of: Context Engineering
- → is part of: LLM API Gateway
Sources and further reading
- Semantic Router
Official implementation documentation for semantic route definitions, utterances and routing behavior.
Last updated: 2026-10-10