Debugging OR-Tools VRP: How to Track Why Solver Skips/Excludes Locations from Solution?

[Vivek Paladiya]

Debugging Skip Decisions in OR-Tools VRP Optimization

Problem:
Using OR-Tools for VRP optimization (package delivery system), where:
- Multiple delivery locations
- Fleet of vehicles
- Time window constraints
- Various real-world constraints

Implementation:
- OR-Tools with PARALLEL_CHEAPEST_INSERTION
- GUIDED_LOCAL_SEARCH
- Using Constraint Programming

Key Constraints:
Hard:
- Time windows (delivery hours)
- Vehicle capacity
- Maximum route duration
- Paired deliveries (pickup → delivery)

Soft (Penalties):
- Distance between stops
- Late deliveries
- Extra vehicle usage
- Non-preferred routes

Challenge:
Optimizer skips some deliveries, but we can't determine why. Example:
```python
skip_penalty = base_cost
if delivery.is_priority:
    skip_penalty += priority_penalty
routing.AddDisjunction([delivery_index], skip_penalty, True)
```

Need to understand:
1. Why deliveries are skipped
2. Which constraints caused the skip
3. Was it due to:
   - Time window violations?
   - Distance/cost inefficiency?
   - Capacity constraints?
   - Accumulated penalties?

Looking for: An approach to track and analyze solver's skip decisions without significant performance impact.

Question: How can we implement skip reason tracking in OR-Tools VRP to understand why certain deliveries are excluded from the solution?