Three protocols define how AI agents interact with the outside world in 2026: Anthropic's Model Context Protocol (MCP) connects agents to tools and data, Google's Agent-to-Agent Protocol (A2A) enables inter-agent collaboration, and CopilotKit's Agent-User Interface Protocol (AG-UI) handles agent-to-user communication. These protocols are complementary, forming a layered stack rather than competing standards.¹⁾

These three protocols answer different fundamental questions:²⁾

• MCP: What tools and data can the agent access?
• A2A: How do agents discover each other and delegate tasks?
• AG-UI: How does the agent communicate with the user interface?

MCP is the oldest and most mature protocol, launched by Anthropic in November 2024. It standardizes how AI models and agents access external tools, resources, and data sources.³⁾

Key features:

• Three distinct primitives: Tools (model-controlled), Resources (app-controlled), Prompts (user-controlled)
• Transport via stdio, SSE, or Streamable HTTP (added November 2025)
• Donated to the Linux Foundation in December 2025
• 97 million downloads and 10,000+ MCP servers as of early 2026⁴⁾

Adoption: OpenAI added MCP support in March 2025, followed by Google DeepMind in April 2025 and Microsoft. MCP has become the de facto standard for agent-tool integration.

A2A was launched by Google in April 2025 with 50+ launch partners including Salesforce, PayPal, and MongoDB. It enables independent agents to discover each other's capabilities and collaborate on tasks.⁵⁾

Key features:

• Agent Cards published at /.well-known/agent.json for capability discovery
• Stateful task lifecycle with multi-turn tracking for long-running workflows (hours/days)
• OAuth 2.0 built-in authentication with signed Agent Cards
• gRPC support added in v0.3 (July 2025) for high-performance scenarios
• Now under the Linux Foundation

When to use: Multi-agent systems where an orchestrator delegates tasks to specialized sub-agents. Example: a travel booking agent delegates flight search to one agent, hotel search to another, and payment to a third.

AG-UI, developed by CopilotKit, standardizes real-time communication between AI agents and frontend user interfaces.⁶⁾

Key features:

• Generative UI specification for rendering interactive interfaces in AI-powered apps
• Real-time streaming of agent state to frontends
• Integrates with both MCP and A2A as a UI layer on top

Related protocols: Google has introduced A2UI (Agent-to-UI), a similar protocol for declarative UI generation. MCP has added MCP-UI/MCP Apps (SEP-1865, January 2026) for interactive UIs within MCP clients.⁷⁾

The protocols form a complementary stack:⁸⁾

• Bottom layer (MCP): Each agent connects to its tools and data via MCP
• Middle layer (A2A): Agents discover and communicate with each other via A2A
• Top layer (AG-UI): Results are presented to users via AG-UI

Google's reference architecture uses A2A orchestrators where each sub-agent has its own MCP tool connections. A typical production stack in 2026 uses MCP + A2A together, with AG-UI for consumer-facing applications.⁹⁾

MCP is production-ready with 18+ months of usage. A2A is earlier but gaining adoption in enterprise orchestration. AG-UI is the most nascent, still converging with competing UI protocols (A2UI, MCP-UI).¹⁰⁾

• Agent Marketplace
• Coding Agents Comparison 2026
• Computer Use Agents Comparison