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The 3-Actor Model: Designing Systems for Humans, APIs, and AI Agents

·5 min read·Sentrix Labs

AI Agents are a new Actor that will reshape how software is designed.

A fundamental shift in system architecture is already underway. According to Gartner, over 80% of enterprises will have used or deployed GenAI by 2026. What this means is that we're experiencing the emergence of a 3rd Actor in enterprise applications:

  • Humans who use UIs, such as graphical (GUIs) and voice user interfaces (VUIs)
  • Systems which user APIs to interact with other systems
  • AI Agents which will use ???

AI agents aren't just another user type to accommodate. They represent an entirely new category of Actor. This shift demands that we rethink how we design, build, and scale enterprise systems.

The traditional two-actor model that has governed system design for decades is being rapidly expanded into a three-actor model.

The End of the Two-Actor Model

For as long as we've been building enterprise systems, we've recognized two fundamental types of actors:

Humans interact through user interfaces. Whether it's a graphical interface on your laptop, a voice interface with Siri, or a mobile app on your phone, humans need presentation layers designed for their cognitive and sensory capabilities.

Systems communicate through APIs. When your CRM needs to talk to your payment processor, or your inventory system needs to update your e-commerce platform, they use structured protocols designed for machine-to-machine communication.

This binary model has served us well. It's clean, logical, and maps perfectly to how we've traditionally thought about system interactions. But AI agents don't fit neatly into either category.

Consider what happens when an AI agent needs to interact with your system. It has the automation capabilities of a system actor, operating 24/7 without fatigue, processing vast amounts of data, and executing complex workflows. Yet it communicates using natural language, understanding context and nuance like a human actor.

Current solutions force AI agents into one of these existing categories. Computer Use technologies have agents clicking through GUIs designed for humans. Protocol layers like MCP (Model Context Protocol) try to standardize agent-to-system communication. Our bet is that these are temporary solutions to bridge AI with existing interaction modes.

Why AI Agents Demand Their Own Architecture

The unique nature of AI agents becomes clear when you examine their operational characteristics:

Unlike humans, agents can:

  • Process thousands of requests simultaneously
  • Operate continuously without breaks
  • Maintain perfect recall of all interactions (not exactly true due to context rot, but this will improve with time)
  • Execute complex calculations instantly
  • Scale horizontally across unlimited instances

Unlike traditional systems, agents can:

  • Understand and generate natural language
  • Adapt to new scenarios without reprogramming
  • Handle ambiguous or incomplete requests
  • Learn from interactions and improve over time
  • Engage in creative problem-solving

Not all of the properties above are present in AI today, but these properties are expected to emerge with time.

This combination creates requirements that neither human-centric UIs nor system-centric APIs can efficiently address.

Transitional Technologies

Today's agent communication methods reveal our struggle to adapt:

Computer Use allows agents to interact with existing GUIs by simulating mouse clicks, keyboard input, and screen reading. It's a hard problem that requires advanced engineering, but it's also inefficient. Every interaction is resource-intensive, error-prone, and scales poorly.

Model Context Protocol (MCP) attempts to create a standardized way for agents to access context and tools. It's a step in the right direction, acknowledging that agents need something different from traditional APIs.

These transitional technologies serve a purpose. They allow us to integrate agents into existing architectures without massive rewrites. But they're not optimal long-term solutions.

As agent usage grows relative to human usage, the inefficiencies become untenable. Imagine a future where 70% of your system interactions come from agents, but they're all clicking through GUIs designed for humans. The computational overhead alone would cripple your infrastructure.

The Future Is Already Being Written

The question isn't whether AI agents will become a dominant force in system interactions. The data already shows explosive growth in agent adoption across industries.

Organizations that recognize agents as a distinct third actor are building competitive advantages today. They're creating systems that leverage the unique capabilities of agents rather than forcing them into ill-fitting interfaces. They're preparing for a future where the majority of system interactions might come from non-human actors.

The traditional two-actor model served us well in an era of human users and deterministic systems. But that era is ending, rapidly. The three-actor model is a fundamental shift in how we think about system design and automation.

Key Takeaways

  • AI agents represent a third category of system actor, distinct from both humans and traditional systems, requiring unique architectural considerations
  • Current approaches like Computer Use and MCP are transitional technologies that will bridge the gap between existing technologies and AI agents
  • The shift from human-majority to agent-majority system usage will happen faster than most organizations expect, making proactive architecture changes critical

Next Steps

The organizations that thrive in the agent era will be those that fundamentally rethink their system design around three distinct actors: humans, systems, and agents. If you're ready to explore how to transform your business, the team at Sentrix Labs specializes in designing and implementing agents that drive measurable business value.

The 3-Actor Model: Designing Systems for Humans, APIs, and AI Agents | Sentrix Labs Blog | Sentrix Labs