or-tools: Problem with negative capacities

Hi! While prototyping a VRP problem using this awesome library (kudos to the team) I’ve faced a problem in a kind of pickup & delivery problem.

For each node, the transit values for the capacity dimension are mixed (deliveries are positive and pickups are negative), so they can be, for example:

• 1 = 10 deliveries & 9 pickups or only 1 delivery
• -2 = 4 deliveries & 6 pickups or 3 deliveries & 5 pickups
• 0 = the same number deliveries and pickups

I create the capacity dimension with fix_start_cumul_to_zero = False and slack_max = <The max capacity of a vehicle>. For checking that the capacity constraint of a vehicle is respected after optimizing, I obtain the total capacity used with something like: used_capacity = max(cumul_capacities) - min(cumul_capacities), where cumul_capacities is an array of the accumulated capacity of a vehicle while traversing its solution. But sometimes, used_capacity is greater than the max capacity of the vehicle.

How can I ensure the capacity constraint is respected?