The Modern AI Stack

When you use ChatGPT, Copilot, or any AI-powered tool, you're interacting with the application layer — the tip of a larger technological stack. Understanding the three layers helps you make better decisions about which tools to use and why.

Layer 1: Foundation Models

At the base are foundation models — massive AI systems trained on enormous datasets of text, code, images, and more. Examples include GPT-5 (OpenAI), Claude Opus (Anthropic), Gemini Ultra (Google), and Llama (Meta). These are general-purpose reasoning engines.

Key traits: - Trained once at enormous expense (millions to hundreds of millions of dollars) - Can perform many tasks without task-specific retraining - Accessed via APIs (application programming interfaces) — essentially a standardized plug

Layer 2: APIs and Platforms

Between the raw model and end-user products sit platforms and APIs. OpenAI's API, Anthropic's API, and Google's Vertex AI let developers call a foundation model from their own software. Companies like Microsoft integrate these APIs into Office 365 (Copilot). Google integrates Gemini into Workspace.

This layer also includes: - Retrieval-Augmented Generation (RAG): connecting models to your private documents - Fine-tuning services: adjusting a base model for a specific domain - Orchestration tools: LangChain, LlamaIndex, and others that chain AI calls together

Layer 3: Applications

At the top are the products you actually use: ChatGPT, Microsoft Copilot, Google Gemini for Workspace, Claude.ai, Perplexity, GitHub Copilot, Cursor, and thousands of others. These are user interfaces and workflows wrapped around foundation models.

Why This Matters Practically

When a product shows AI capability, it may be licensing access to someone else's model (e.g., many SaaS tools use OpenAI or Anthropic APIs under the hood)
Data flows to the model provider regardless of which application you use, unless you're using a private or self-hosted deployment
Evaluating an AI product means evaluating both the application experience and the underlying model
A competitor can build a similar product by switching foundation model providers — the model choice is often a key differentiator

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