AtsushiSakai/PythonRobotics · error · Exception

Agent {agent_index} tried to reserve a position already rese

Error message

Agent {agent_index} tried to reserve a position already reserved by another agent: {position} at time {t}, reserved by {current_reserver}

What it means

Raised by reserve_position when the target cell in reservation_matrix already holds a different agent's index during any timestep in the requested interval. This is a collision/conflict detection mechanism for multi-agent space-time reservation.

Source

Thrown at PathPlanning/TimeBasedPathPlanning/GridWithDynamicObstacles.py:319

        for i, node in enumerate(node_path.path):
            reservation_finish_time = node.time + 1
            if i < len(node_path.path) - 1:
                reservation_finish_time = node_path.path[i + 1].time

            self.reserve_position(node.position, agent_index, Interval(node.time, reservation_finish_time))

    """
    Reserve a position for the provided agent during the provided time interval.
    Raises an exception if the agent's index is 0, or if the position is already reserved by a different agent during the interval.
    """
    def reserve_position(self, position: Position, agent_index: int, interval: Interval):
        if agent_index == 0:
            raise Exception("Agent index cannot be 0")

        for t in range(interval.start_time, interval.end_time + 1):
            current_reserver = self.reservation_matrix[position.x, position.y, t]
            if current_reserver not in [0, agent_index]:
                raise Exception(
                    f"Agent {agent_index} tried to reserve a position already reserved by another agent: {position} at time {t}, reserved by {current_reserver}"
                )
            self.reservation_matrix[position.x, position.y, t] = agent_index

    """
    Clears the initial reservation for an agent by clearing reservations at its start position with its index for
    from time 0 to the time limit.
    """
    def clear_initial_reservation(self, position: Position, agent_index: int):
        for t in range(self.time_limit):
            if self.reservation_matrix[position.x, position.y, t] == agent_index:
                self.reservation_matrix[position.x, position.y, t] = 0

show_animation = True

def main():
    grid = Grid(
        np.array([11, 11]),

View on GitHub (pinned to 1fe4fb980f)

Solutions

1. Re-plan the conflicting agent with the reservation matrix as a constraint (treat reserved cells as blocked)
2. Adjust the path or departure time so it avoids the conflicting (position, time) pair named in the message
3. Clear stale reservations for an agent before replanning its path (clear_initial_reservation / reset matrix)

Example fix

# before
path = planner.plan(start, goal)
grid.reserve_path(path, agent_index)  # may collide

# after
# use a planner that respects reservations (SpaceTimeAStar/SafeInterval with the grid), then reserve
path = planner.plan(start, goal)
grid.reserve_path(path, agent_index)  # planner already avoided reserved cells

def is_free(grid, position, interval, agent_index) -> bool:
    return all(
        grid.reservation_matrix[position.x, position.y, t] in (0, agent_index)
        for t in range(interval.start_time, interval.end_time + 1)
    )

try:
    grid.reserve_path(path, agent_index)
except Exception as e:
    if 'already reserved by another agent' in str(e):
        # replan with reservations as constraints, then retry
        path = planner.plan(grid, start, goal)
        grid.reserve_path(path, agent_index)
    else:
        raise

When it happens

Trigger: Calling reserve_position/reserve_path for a path that passes through (or waits on) a cell-time already reserved by another agent; typically when planning paths sequentially without accounting for previously reserved intervals.

Common situations: Planning multi-agent paths where a later agent's plan crosses an earlier agent's reserved trajectory; wait-in-place actions that collide with another agent's reservation; replanning without clearing stale reservations.

Related errors

• Agent index cannot be 0
• Number of obstacles is greater than grid size!
• Path position not found for time {i}.
• No path found
• No path found

AI-assisted analysis of AtsushiSakai/PythonRobotics@1fe4fb980f (2026-08-28). Data as JSON: /api/errors/eb88ff8a258a73ce. Report an issue: GitHub.