DRY — Don't Repeat Yourself

Every piece of knowledge has one authoritative representation. Business rules, validation logic, constants, schemas, config.

Rules for agents

• Before writing a helper, search the codebase for an existing one.
• Extract on the third occurrence, not the second. Two similar blocks may be coincidence.
• Duplicated shape is not duplicated knowledge. Two functions that look alike but change for different reasons must stay separate.
• Prefer deriving over restating: generate types from schemas, constants from one source, docs from code.

Counterweight

Wrong abstraction costs more than duplication. If unifying two callers needs a boolean flag parameter to switch behaviour, do not unify. Inline the duplicate back and move on.

Smells

Copy-pasted blocks, the same magic number in several files, parallel lists that must be updated together, logic mirrored in frontend and backend.