When adding a feature broke a dozen classes

Short summary

When adding features requires editing dozens of unrelated classes, your architecture is brittle. SOLID principles fix this by splitting concerns, enabling extension without modification, maintaining contracts, avoiding unnecessary interfaces, and inverting dependencies. Applied correctly, these patterns reduce test time from 18m to 4m and make changes touch only 1-2 classes instead of causing shotgun surgery across the repo.

• •SOLID principles prevent 'shotgun surgery' when adding features to large codebases
• •Real example: PaymentProcessor refactor cut test suite runtime from 18 minutes to 4 minutes
• •Pragmatic application: use these principles when they reduce complexity, not for every script

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