Why Flexible Interfaces Destroy Modularity

The software industry has built impressive tools for making interfaces technically flexible. They solve the wire-level problem but ignore the human problem. When interfaces are flexible, teams stop negotiating contracts. They stop taking ownership of their boundaries. The system drifts from modular to entangled without anyone deciding to make it so.

Clear, stable contracts that teams negotiate explicitly. Instead of hoping consumers will tolerate new fields, define what the interface guarantees and what it doesn't. If both sides understand the contract, the system stays modular. If you can change the interface without talking to the other team, you have a trust-based arrangement, not a contract.