Every successful platform has them. They’re worth billions. Yet we don’t even have a name for them.

I’m talking about developers who build tools for OTHER developers on your platform. Not your internal team. Not external app developers. The fourth party that makes platforms explode.

I’ve been calling them “4p developers” throughout my career building developer platforms at Microsoft, Parse, Firebase, and Google Cloud. I wanted to share this framework because if you’re building a platform without thinking about them, you’re leaving massive ecosystem value on the table.

The Invisible Force Multiplier

Walk into any enterprise using .NET and you’ll find something interesting. Beyond the Microsoft tools and the custom applications, there’s a third category: components from DevExpress, Telerik, ComponentOne, Syncfusion. These companies built a billion-dollar market not by creating applications, but by making other developers more productive.

They’re not ISVs in the traditional sense. They’re not building vertical solutions or SaaS products. They’re building horizontal tools that multiply the power of every other developer on the platform.

These 4p developers are the difference between a platform and an ecosystem.

The Framework

Here’s how I break down the developer audiences one might build for:

  • 0p (Zero Party): The platform team itself. For Firebase, this was the Firebase team. The distinction between 0p and 1p matters because 1p are using your platform, not just building it.
  • 1p (First Party): Your broader internal organization. For Firebase, this was other Google teams using our platform.
  • 3p (Third Party): External developers building applications on your platform.
  • 4p (Fourth Party): Developers building tools, libraries, and extensions for OTHER developers on your platform.

Most platform teams obsess over the 3p journey (and may find time for 0p and 1p so they can remain productive and eat their own dogfood). Documentation, evangelism, developer experience - it’s all optimized for application developers. But the platforms that win big? They optimize for 4p.

Why 4p Developers Matter More Than You Think

They Create Exponential Value

When a 3p developer builds an app, you get one success story. When a 4p developer builds a tool, you enable hundreds or thousands of success stories.

At Microsoft, the .NET component vendor ecosystem wasn’t just big - it was essential. These vendors:

  • Made complex UI patterns accessible to average developers
  • Standardized enterprise patterns across thousands of companies
  • Reduced development time from months to weeks for common scenarios
  • Created a competitive marketplace that drove innovation

The .NET component vendor ecosystem produced a market worth billions. But their real value? The massive amount of enterprise value they enabled by making every other developer more productive.

They’re Your Innovation Lab

4p developers push your platform in ways you never imagined. They find the gaps, build the bridges, and show you what your platform could become.

Every successful language ecosystem demonstrates this:

  • npm didn’t just host packages - it enabled an explosion of developer tools
  • pip turned Python from a language into an ecosystem
  • RubyGems made Rails possible

These aren’t just distribution mechanisms. They’re 4p enablement platforms.

They Create Network Effects

When developers depend on tools built for your platform, your ecosystem becomes exponentially more valuable. It’s not just about the direct value - it’s about the interconnected web of dependencies, integrations, and workflows that emerge.

VS Code understood this. Extensions aren’t just features - they’re 4p developers creating network effects. GitHub Actions? Same story. Every action in the marketplace is a 4p developer making the platform more valuable for everyone else.

How Platforms Accidentally Kill Their 4p Ecosystem (And How I Know)

The Firebase Shared State Problem

At Firebase, we used singleton patterns extensively. One active user, one database connection, one configuration. Super simple for app developers.

What we didn’t fully consider: What happens when someone wants to build a developer tool that uses Firebase? Maybe they want to store their tool’s data in Firebase Realtime Database while their users are also using Firebase for their own apps.

With singletons everywhere? The tool’s database connection would conflict with the app’s database connection. The tool’s auth state would overwrite the app’s auth state. We made it nearly impossible for 4p developers to dogfood their own tools.

The fix (moving to instance-based initialization where each tool could maintain its own Firebase app instance) wasn’t technically complex once we recognized the problem. But the opportunity cost was high.

Android’s Resource Revolution

Android provides an even clearer example. For years, you couldn’t include resources or Android Manifest entries in libraries. Want to build a UI component library with layouts and drawables? Too bad. Want to ship a library that registered its own services? Impossible.

4p developers had to resort to terrible workarounds:

  • Asking developers to manually copy resources
  • Providing error-prone instructions for manifest modifications
  • Building code-generation tools to work around platform limitations

The introduction of Android Archive (AAR) files changed everything. Suddenly, 4p developers could build real, professional libraries. The explosion in Android library quality wasn’t coincidental - it was 4p developers finally being able to build the tools they’d always wanted to create.

The 4p Litmus Test

Want to know if your platform is 4p-friendly? Ask these questions:

Can tool developers use their own tools?
If a developer builds a monitoring library for your platform, can they monitor their own library? This sounds obvious, but singleton patterns, global state, and certain authentication models make this impossible.

Can developers monetize tools?
Your license, terms of service, and marketplace policies either enable or prevent 4p developers from building businesses. The platforms that thrive make it easy for 4p developers to make money.

Can tools compose with each other?
If two developers build complementary tools, can they work together? Or do your architectural decisions force them to conflict?

Is tool discovery built into your platform?
npm’s registry, VS Code’s extension marketplace, GitHub’s Actions marketplace - these aren’t just nice-to-haves. They’re essential 4p infrastructure.

What 4p-First Architecture Looks Like

Shared State: Isolation Over Singletons

The difference between 4p-hostile and 4p-friendly often comes down to state management. It’s not just configuration - it’s the active user, the cache, offline storage, any shared state. In an AI agent library, one conversation chain had better not unintentionally pollute another. Every piece of shared state is a potential 4p blocker.

Authentication: Delegation and Scoping

Your auth system needs to support scenarios where tools act on behalf of users. OAuth got this right with scopes and delegation. API keys that can only represent one identity? That’s 4p-hostile because it forces developers to build their own layers over your systems.

APIs: Middleware and Extensibility

Build your APIs with extension in mind. Let tools add functionality, modify behavior, and compose with each other. The platforms that treat their APIs as immutable, locked-down interfaces that are the only entry points customers should ever interact with are the ones that stay small.

Resources: Full Platform Access

Android’s evolution from JARs to AARs shows this perfectly. 4p developers need access to the same platform capabilities as 0p and 1p developers. Second-class access means second-class tools. If it would be useful to you when building the platform, it’s probably useful to 4p developers to extend it.

Thinking in 4p Journeys

The key lesson isn’t about any specific technical pattern. It’s about thinking through scenarios for 4p developers. What are their user journeys? What kinds of things might they want to build? Will they be locked out by decisions you’ve made, or enabled by them?

Some questions to ask:

  • Can a logging library use your platform’s logging without interfering with app logging?
  • Can apps and tools test their code built atop your platform without completely removing it?
  • Can a development tool maintain its own user session alongside the app’s session?
  • Can complementary tools from different vendors work together seamlessly?

Every architectural decision can be evaluated through the 4p lens.

Developer Ecosystems Are Social Networks

Here’s something crucial that took me years to fully appreciate: Developer ecosystems are social networks, not just technologies. You need it to be possible for all the right players to engage.

The technology enables the connections, but the value comes from the network itself. 4p developers aren’t just building on your platform - they’re building relationships with 3p developers, creating communities, establishing trust, and forming an economy.

When you make 4p development hard, you’re not just limiting tools. You’re preventing connections, blocking relationships, and stunting the social graph of your ecosystem.

The AI Platform Gold Rush

Today’s AI ecosystem provides a real-time case study in 4p dynamics.

Some examples of who’s getting it right:

  • Anthropic’s MCP specification: Built specifically for tools to extend Claude’s capabilities
  • Google’s Agent Developer Kit: Designed for developers building agent frameworks
  • Claude, GPT, and Gemini in dev tools: Cursor, Windsurf, Aider, and countless other developer tools are building on the capabilities of these models to make them do even more. The models are adding capabilities like tool calling – something that explicitly enables 4p developers.

The winners in AI won’t just be the best models or the best interfaces. They’ll be the platforms that enable the richest 4p ecosystem.

The Strategic Reality of Platform Success

Here’s what building platforms at Microsoft, Parse, Firebase, and Google has taught me: Your platform’s success isn’t determined by your features, your performance, or even your developer experience.

It’s determined by whether you enable 4p developers to build businesses on top of your platform.

.NET didn’t win because it was better than Java. It won because component vendors could build million-dollar businesses on it.

npm didn’t succeed because it was a better package manager. It succeeded because it made tool publishing frictionless.

The platforms winning today understand this. They’re not just building for developers - they’re building for developers who build for developers.

A Call to Action for Platform Builders

If you’re building a platform, start asking different questions:

Instead of just “How can we make this easier for developers?” also ask “How can we make this easier for tool builders?”

Instead of just “What features do developers need?” also ask “What would enable developers to build the features they need?”

Instead of just “How do we grow our developer community?” also ask “How do we enable our community to grow itself?”

Every architectural decision you make either invites or excludes 4p developers. A state management pattern, an authentication model, a licensing term - these seemingly minor decisions determine whether you’ll build a product or spawn an ecosystem.

The Future Belongs to 4p

As software development becomes increasingly democratized, the role of 4p developers becomes even more critical. They’re the ones who make complex capabilities accessible. They’re the bridge between platform power and developer productivity.

The age of AI makes this even more apparent. The winners won’t be the platforms with the best models - they’ll be the platforms with the richest ecosystem of tools making those models accessible to every developer.

So tend to your 4p developers. Design for them. Enable them. Celebrate them.

They’re not just using your platform. They’re multiplying it.

And that multiplication - that’s the difference between building a tool and building an ecosystem.