Change Effect Analysis

Trace the blast radius of a legacy code change and produce a test placement plan with pinch points. Use whenever a developer needs to decide WHERE to write tests for a pending change — 'what should I test?', 'where to test?', 'how far do the effects propagate?', 'what else could this break?', 'how to find test points', 'pinch point', 'effect sketch', 'impact analysis', 'blast radius', 'interception point', 'high-leverage test'. Triggers for 'I need to make many changes', 'many classes affected', 'cluster of related changes', 'cross-class refactoring'.

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Install

Claude CodeClawhHubnpxCursorCodexGemini CLI

1. Add marketplace

› /plugin marketplace add bookforge-ai/bookforge-skills

2. Install plugin

› /plugin install working-effectively-with-legacy-code@bookforge-skills

3. Use the skill

› /change-effect-analysis

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Source Book

Working Effectively with Legacy Code

Michael C. Feathers

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Remove duplicated code across methods and classes by extracting small shared utilities first and letting larger structure (superclass, Template Method) emerge. Use whenever a developer says 'I'm changing the same code in multiple places', 'I'm fixing the same bug in 3 files', 'copy-pasted code everywhere', 'duplicate logic', 'DRY violation', 'same code in multiple classes', 'repeated patterns', 'shotgun surgery'. Activates for 'extract method', 'extract class', 'pull up method', 'Template Method pattern', 'superclass for duplication', 'deduplicate', 'shared utility', 'parallel classes'.

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