Problem: Conflict Between FixedCell and FixedAgent

The issue arises from a contradiction between how FixedCell enforces immobility and how FixedAgent performs cleanup.

Conflict in cell_agent.py

In FixedCell, the cell setter prevents movement:

@cell.setter
def cell(self, cell: Cell | None) -> None:
    if self._mesa_cell is not None:
        raise ValueError("Cannot move agent in FixedCell")
    cell.add_agent(self)  # Fails if cell is None
    self._mesa_cell = cell

Because this setter blocks reassignment, FixedAgent.remove() bypasses the public API and modifies internal state directly:

def remove(self):
    super().remove()
    self.cell.remove_agent(self)  # Manual cleanup
    self._mesa_cell = None        # Direct private state change

Why This Is a Problem

1. API Inconsistency
   For a CellAgent, removing the agent is simple:

agent.cell = None

For a FixedAgent, the same operation raises:

ValueError("Cannot move agent in FixedCell")

2. Fragility

The setter has an additional edge case:

agent.cell = None

If the agent is already None, the setter attempts:

None.add_agent(self)

which raises an error because None has no such method.

3. Encapsulation Violation

FixedAgent.remove() modifies the private attribute:

self._mesa_cell = None

If a user performs the same action in their model code, they would be breaking encapsulation, yet the framework itself relies on this workaround.

Result

• Inconsistent agent removal behavior
• Fragile setter logic
• Internal reliance on private state manipulation