Open/Closed Principle

Software should be open for extension, closed for modification. Add behaviour by adding code, not by editing working code.

Rules for agents

• When a new case arrives, prefer registering a new implementation over editing a switch statement.
• Use polymorphism, strategy objects, or a registry/plugin table for variation points that genuinely vary.
• Keep the extension point narrow: one interface, clearly documented.
• Existing tested code should stay untouched when a variant is added.

Counterweight (important)

Do not build extension points speculatively — that violates yagni.md. Apply Open/Closed at the second or third variation, once the axis of change is proven. Until then, edit directly.

Smells

Long if type == / switch chains that grow with every feature, needing to edit five files to add one case, core files with heavy churn.