Coordinating Domain-Specialized LLM Agents for Multi-City Travel Planning

Multi-city travel planning is a tightly coupled, multi-scale decision problem requiring mobility, accommodation, and activity choices to remain jointly feasible under temporal, spatial, budget, and user-specific constraints. We propose a structured agentic framework that explicitly coordinates these interdependent decisions while separating semantic reasoning from deterministic execution. Domain-specialized LLM agents select from grounded candidates and encode their choices, alternatives, evidence, and unresolved dependencies as decision contracts. A Cross-Domain Coordinator first provides global planning guidance and then reconciles these contracts into a joint contract under explicit authority constraints. The joint contract bounds downstream city-day composition and solver-assisted temporal-spatial scheduling, while deterministic controls validate decisions before execution and verify the materialized itinerary afterward. Detected violations are routed to the responsible decision scope for bounded revision and re-execution. The framework achieves Final Pass Rates of 74.40% on TravelPlanner and 59.44% on ChinaTravel. Ablations show that cross-domain coordination is critical for coupled multi-city decisions, whereas specialist LLM reasoning is most beneficial for semantically rich city-day planning.