# Solid Principles > Apply the SOLID principles as design diagnostics when writing, reviewing, or refactoring code with classes, modules, or service boundaries - detecting god classes, fragile hierarchies, fat interfaces, and hard-wired dependencies, and prescribing the smallest structural fix. Treats SOLID as a smell detector, not a ceremony to impose. Use when designing a new module or class, reviewing object-oriented code, untangling a class that keeps changing for unrelated reasons, or deciding where an abstraction belongs. - Skill: `khaledsaeed18/solid-principles` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add khaledsaeed18/solid-principles` - Raw SKILL.md: https://api.skillmd.com/api/skills/khaledsaeed18/solid-principles/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: KhaledSaeed18 (https://skillmd.com/u/khaledsaeed18) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/khaledsaeed18/solid-principles --- Use SOLID to diagnose, not to decorate. Each principle names a specific way designs rot under change; the skill is recognizing the rot early and making the smallest cut that stops it. The failure mode of SOLID is applying it as ceremony - an interface for every class, a factory for every constructor - which produces the same unmaintainability it was meant to prevent, with more files. Every abstraction this skill recommends must be justified by a change pressure that actually exists in this codebase, not one imagined. ## Step 1: Find the change pressure Before judging any structure, learn what actually changes here: - `git log --oneline --follow` on the files in question: what has forced edits, and do unrelated reasons keep touching the same file? - Where have bugs clustered? A class that breaks every time a neighbour changes is telling you where the coupling is. - What does the team add regularly (new payment providers, new report types, new integrations)? That axis deserves an extension point; axes that never vary do not. SOLID violations only matter along axes that change. A "god class" that has been stable for two years is a lower priority than a small class edited weekly by three features. ## Step 2: Diagnose against each principle For each, the smell, the test, and the minimal fix - in the codebase's own idiom (modules and functions count; none of this requires classes). **S - Single responsibility.** A unit should have one reason to change. - *Smell:* commits touch the same file for unrelated features; the class name contains "Manager", "Handler", "Util"; you describe it with "and". - *Test:* list the actors/features that force edits to it. More than one - it is doing more than one job. - *Fix:* split along the reasons-to-change, not along method count. Ten cohesive methods are fine; three methods serving three features are not. **O - Open/closed.** Add behaviour by adding code, not by editing a growing conditional. - *Smell:* a switch/if-else chain over a type tag that grows a branch with every new variant, duplicated in several places. - *Test:* "when the next variant arrives, how many existing files change?" More than one is the smell; zero or one (plus the new variant file) is the goal. - *Fix:* one extension point - a strategy map, a handler registry, polymorphism - introduced at the second or third variant, not speculatively at the first. **L - Liskov substitution.** A subtype must be usable wherever its base is expected, without surprises. - *Smell:* overrides that throw `NotImplemented`, no-op overrides, `instanceof` checks on the subtype after receiving the base, subclasses that tighten what inputs they accept. - *Test:* can every caller of the base run unmodified against each subtype and stay correct? - *Fix:* usually the hierarchy is wrong - replace inheritance with composition, or split the base into what all subtypes truly share. (A `Square extends Rectangle` problem is never fixed inside `Square`.) **I - Interface segregation.** Depend on the methods you use, not on a fat contract. - *Smell:* implementations stubbing methods they do not need; a change to one client's method recompiling/breaking every other client; mocks that stub fifteen methods to test one. - *Test:* group the interface's methods by which clients call them. More than one group - the interface is a bundle. - *Fix:* split by client need (`Reader`/`Writer`, not `Storage`), or in structurally-typed languages, accept the narrow shape the function actually uses. **D - Dependency inversion.** Policy should not import mechanism. - *Smell:* business logic constructing its own database client or HTTP client inline; tests that cannot run without real infrastructure; a domain module importing a vendor SDK. - *Test:* can the core logic be exercised in a test with in-memory fakes, without patching module internals? - *Fix:* pass the dependency in (constructor, parameter, factory) behind the narrowest interface the logic needs. Plain function parameters count as injection; a DI framework is not the point. ## Step 3: Prescribe proportionately - Order findings by cost of leaving them: violations on hot, frequently-edited paths first; stable code last or not at all. - Each finding names the principle, the concrete evidence (file, the commits or branches that show the pressure), and the smallest refactor - shown as the target structure in the project's own style, not a pattern name. - Say what you would *not* change. Explicitly bless the simple code that hypothetically "violates" a principle but sits on an axis with no change pressure. Recommending restraint is part of the review. - When writing new code rather than reviewing: start concrete, and let the second variant or second consumer trigger the abstraction. Note in the design where that extension point will go when it is earned.