Examples of the present disclosure describe processing by an input understanding system/service. A received input is processed to generate a set of alternatives for recognizing the received input. The set of alternatives is filtered. Filtering comprises ranking the set of alternatives and propagating a plurality of the ranked alternatives for additional processing. The propagated alternatives are processed to generate an expanded set of alternatives for potential hypotheses based on the received input. The expanded set of alternatives is filtered. Filtering comprises ranking alternatives of the expanded set and propagating a plurality of the ranked alternatives of the expanded set for additional processing. The propagated alternatives of the expanded set are evaluated based on application of knowledge data fetched from external resources. A response to the received input is generated. Generating of the response comprises ranking the evaluated alternatives and selecting the response based on the ranked and evaluated alternatives.