Mutable by Design: The FLUID Software Philosophy

For years, I preached the gospel of SOLID principles. Single Responsibility. Open/Closed. Liskov. Interface Segregation. Dependency Inversion. They were more than guidelines — they were the scaffolding we built our software on. They helped us tame complexity, align on good design, and enforce structure where chaos threatened.

But here’s the thing no one wants to say out loud: a lot of it was about control. We shaped our code not because it was always better — but because it was safer. Easier to read, easier to change (sometimes), and easier to onboard humans. We made rules for each other to protect us from entropy.

SOLID walked so FLUID could flow. AI-native development doesn’t need control — it needs adaptability.

But that world assumed a human-only audience. Now we write code that’s read, interpreted, and modified by machines too.

We’re not just collaborating with people anymore — we’re collaborating with intelligence.

The rules change when your pair programmer never sleeps, sees the whole codebase, and can rewrite a service in seconds.

We’re not in the SOLID era anymore. We’re in the FLUID era.

Introducing FLUID: Post-SOLID Principles for AI-Native Development

When you’re building with AI, you don’t need to contort your mind to remember every design pattern or abstract perfectly. You write what works — and if it doesn’t, you regenerate, refactor, or restructure with a prompt.

Code is no longer sacred. It’s disposable. Mutable. Co-authored with a machine.

The new principles of software development aren’t about rigidity — they’re about adaptability. FLUID is a mindset for working with AI, not trying to preserve an artifact against time.

🌀 FLUID Principles

F – Flexible Composition

Design in loosely coupled, swappable parts. Think composability over inheritance, simple functions over deep hierarchies. Anything that helps AI (or you) reshape it later.

L – Live Prototypes

Work live. Build in hot environments — REPLs, notebooks, dev servers — to explore ideas and tighten your feedback loop. But don’t stop there: snapshot working prototypes and harden them through tests, CI, and version control. FLUID doesn’t mean skipping rigor — it means deferring it until you’ve found something worth solidifying.

U – Unified Context

Code with context. Focus on reducing ambiguity — your future self, teammates, and AI assistants will all benefit. Think less about documentation and more about supplying the system with meaningful, actionable information.

I – Intent-Driven Structure

Don’t just write for form, write for collaboration. Give functions, files, and variables meaningful names, but remember — the AI is reading too. Use comments to convey intent clearly, not just style. Prioritize clarity of why so the system — human and machine — stays in sync.

D – Dynamic Refactorability

Expect change. Encourage it. Make code easy to mutate, regenerate, and improve. Let the AI reshape your logic, reroll your structures, and keep the system flowing.

The Shift: From Code as Artifact to Code as Interface

We used to write code for humans to read. Now we write it so humans and machines can collaborate on it. AI doesn’t care if your service is SOLID. It cares if it can parse, learn, adapt, and ship.

We’re moving faster. Thinking faster. Innovating on the edges and polishing later — if ever. What used to be weeks of code review and refactoring can now be done in seconds. The bottleneck isn’t the code anymore. It’s your imagination.

So long live SOLID. It taught us how to think. But the world has changed.

Let’s move. Let’s break things. Let’s build with machines, not for them.

But Wait — Not All Code Will Be FLUID (Yet)

Let’s be real: some codebases aren’t ready for this. There’s still plenty of mission-critical, high-risk, legacy infrastructure out there and some faction of old-school engineers will keep guarding that temple. Let them. Their job is stability.

The FLUID mindset isn’t a universal replacement. It’s a fork in the evolutionary tree. Where stability and compliance reign, SOLID still makes sense. But if you’re building something new? If you’re prototyping, experimenting, innovating?

Code doesn’t need to be pretty to explore — it just needs to work. Then regenerated. Then evolved. You can always bring the AI in later to “make it SOLID-ish” if needed.

Is FLUID the Only Way?

FLUID is one lens — a mindset for AI-native dev. But it’s not the only one. You might resonate more with:

• Declarative orchestration
• Model-driven programming
• Or something new that hasn’t even been named yet

That’s the beauty of this moment: we’re in a creative explosion. FLUID isn’t a new dogma — it’s an invitation to break the old ones. The point isn’t to replace SOLID with another rigid system. It’s to open the floodgates.

Bring your lens. Name your style. Let’s expand the language of how we build with AI.