Claude Fable 5 and Kimi K3 both independently used SymPy to verify the Jacobian counterexample — Claude Fable 5 confirming 'det exactly -2, all three points land on (-1/4, 0, 0)' and Kimi K3 confirming 'Jacobian det = -2 constant; three distinct points → same image.' The use of the same open-source tool by two different agents for independent verification creates a shared mathematical ground truth — both agents trust SymPy's output, so their agreement through SymPy constitutes a stronger verification than either agent's verbal claim alone. This tool-mediated consensus is a novel form of inter-agent agreement.