Working Effectively with Legacy Code

Big Class Responsibility Extraction

Identify and extract responsibilities from an oversized class using Feathers' 7 heuristics + feature sketches. Use whenever a developer faces 'this class is too big', 'god class', 'monster class', '500-line class', 'class with 50+ methods', 'class with too many responsibilities', 'SRP violation', 'hard to test because class does too much', 'team keeps editing same class', 'merge conflicts on one class'. Activates for 'single responsibility', 'extract class', 'class decomposition', 'responsibility split', 'interface segregation', 'class bloat', 'swamp class', 'god object', 'feature sketch', 'class too big'.

+67%

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'.

+71%

Characterization Test Writing

Write tests that pin down the actual current behavior of untested legacy code as a safety net for change. Use whenever a developer needs to create a regression safety net before modifying code — 'I don't know what tests to write', 'what should I test in legacy code', 'how do I write tests for code I didn't write', 'tests to preserve behavior', 'golden tests', 'snapshot tests for old code', 'characterization test', 'cover before modify'. Activates for 'legacy code testing', 'untested code', 'write tests before changing', 'regression safety net', 'behavior-preservation tests', 'pin down behavior'.

+72%

Dependency Breaking Technique Executor

Select and execute the right dependency-breaking technique from Michael Feathers' catalog of 24 named techniques (Part III of Working Effectively with Legacy Code) for a specific testability obstacle. Use when a class or method cannot be placed under test due to a hard-coded dependency, and you need concrete step-by-step mechanics for breaking it safely — without existing tests protecting you. Activates for 'can't test this class', 'constructor dependency', 'global variable blocking tests', 'static call in method', 'inject a fake', 'break dependency for testing', 'extract interface legacy', 'subclass and override', 'parameterize constructor', 'encapsulate global', 'dependency injection without framework', 'extract and override', 'how to fake this in test', 'legacy code testability refactoring'.

+63%

Duplication Removal Via Extraction

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'.

Legacy Code Addition Techniques

Add new functionality to untested legacy code using Sprout Method, Sprout Class, Wrap Method, or Wrap Class — whichever best fits the dependency profile. Use whenever a developer needs to add a feature, log statement, validation, or any new behavior to legacy code that they can't easily test — 'I have to add this feature fast', 'no time for a big refactor', 'just need to log this', 'add a check to existing method', 'need to add behavior without breaking legacy', 'sprout method', 'sprout class', 'wrap method', 'wrap class', 'decorator for legacy'. Activates for 'quick change to legacy', 'under time pressure', 'can't test this class but need to add a feature', 'extend without editing'.

+67%

Legacy Code Change Algorithm

Guide safe modification of legacy code (untested production code) using Feathers' 5-step Legacy Code Change Algorithm. Use this skill whenever a developer needs to change code that lacks test coverage — adding a feature, fixing a bug, refactoring, or optimizing — and wants to avoid regressions. Activates for 'I need to change this code but there are no tests', 'how do I safely modify legacy code', 'inherited codebase', 'untested code', 'legacy system change', 'code without tests', 'refactor without regressions', 'make this code testable', 'cover and modify', 'don't break anything', 'risky change to old code', 'no test coverage', 'adding feature to old code', 'fixing bug in untested code', 'legacy codebase change safely'.

+86%

Legacy Code Symptom Router

Diagnose any legacy-code situation in plain language and route to the right technique. Use as the FIRST skill when a developer has a vague or specific complaint about a codebase — 'I have to change this but there are no tests', 'this method is huge', 'can't test this class', 'library is killing us', 'changes take forever', 'don't know where to start', 'overwhelmed', 'inherited this mess'. Activates for 'legacy code', 'untested code', 'where do I start', 'what should I do with this code', 'help me plan a refactor', 'I'm stuck with legacy', 'how do I change X safely', 'symptom triage'. Dispatches to technique-specific skills.

+36%

Library Seam Wrapper

Isolate third-party library dependencies behind thin wrapper interfaces to break vendor lock-in and enable testing. Use whenever a developer has direct calls to library classes scattered through production code and can't test or swap the library — 'library is killing me', 'vendor lock-in', 'can't mock this library', 'integration tests only for this SDK', 'AWS SDK everywhere', 'Stripe calls in 50 files', 'all API calls', 'wrapping a library', 'adapter for third-party'. Triggers for 'third party', 'SDK', 'library coupling', 'external service', 'API client'.

Monster Method Decomposition

Decompose a very large method (100+ lines, deeply nested) safely using automated refactoring and Feathers' Bulleted/Snarled classification. Use whenever a developer faces 'a huge method', 'I have a 500-line function', 'deeply nested conditionals', 'monster method', 'god method', 'need to break up this giant method', 'can't test this method it's too big', 'where do I even start with this method'. Activates for 'method extraction', 'IDE refactoring', 'automated extract method', 'introduce sensing variable', 'find sequences', 'skeletonize', 'coupling count', 'bulleted method', 'snarled method', 'break out method object'.

Safe Legacy Editing Discipline

Apply 4 editing disciplines when modifying legacy code: Hyperaware Editing, Single-Goal Editing, Preserve Signatures, Lean on the Compiler. Use whenever a developer is about to edit untested code, refactor without a safety net, or is caught in avoidance anti-patterns (Edit-and-Pray, Minimization Freeze, Legacy Code Dilemma paralysis). Activates for 'how do I refactor safely', 'edit legacy code without breaking it', 'preserve behavior during refactor', 'lean on the compiler', 'preserve signatures', 'single-goal editing', 'make safe changes', 'edit and pray', 'cover and modify', 'careful editing', 'refactor without tests', 'break dependencies safely', 'avoid regressions in legacy code'.

+54%

Scratch Refactoring For Code Understanding

Guide a developer through throwaway refactoring — restructure code freely without tests to understand it, then DISCARD. Use whenever a developer says 'I don't understand this code', 'this code is too complex to change safely', 'need to read legacy code', 'can't figure out what this does', 'overwhelmed by legacy', 'code archaeology', 'understand before change'. Also activates for 'scratch refactoring', 'throwaway refactoring', 'code comprehension', 'code reading technique', 'feature sketch', 'effect sketch', 'notes on legacy code'.

Seam Type Selector

Select the right seam type (Preprocessor / Link / Object) for breaking a dependency in legacy code. Use whenever a developer needs to substitute behavior for testing without editing in place, is choosing between dependency-injection strategies in an existing codebase, or asks 'how do I intercept this call in a test' / 'how do I fake this library' / 'how do I test around this hard-coded dependency'. Activates for 'seam', 'test seam', 'substitution point', 'dependency injection for legacy code', 'mock this without DI framework', 'C++ testing', 'linker-level fake', 'preprocessor substitution', 'polymorphic substitution', 'enabling point'.

+40%

Tdd And Programming By Difference

Add features to tested code using TDD or Programming by Difference. Use whenever a developer is adding a feature to a class that already has tests (or can be brought under test) — 'how do I add this feature', 'test-driven development for legacy', 'red green refactor', 'TDD cycle', 'add behavior with tests', 'inheritance to add feature', 'programming by difference', 'subclass to add behavior', 'Liskov substitution', 'LSP violation'. Triggers for 'I just got this class under test, now I need to add X'.

Test Harness Entry Diagnostics

Diagnose exactly why a class or method cannot be placed under test and route to the right dependency-breaking technique. Use whenever a developer says 'I can't instantiate this class in tests', 'the test harness won't compile', 'this class has too many constructor dependencies', 'the constructor connects to the database', 'can't test this private method', 'I need to sense what this method does', 'hidden singleton dependency'. Activates for 'test harness', 'class under test', 'constructor dependencies', 'irritating parameter', 'hidden dependency', 'construction blob', 'pass null', 'construction test', 'method not accessible', 'method side effects', 'can't get this class in a test harness', 'can't run this method in a test harness', 'singleton in constructor', 'include dependencies', 'onion parameter', 'aliased parameter'.

+50%