MCP + A2A + x402: The Three Protocols That Make Agent Ecosystems Work¶
Three protocols are converging to define how AI agents expose tools, discover each other, and get paid.
The Protocol Stack¶
| Protocol | Layer | What It Solves |
|---|---|---|
| MCP | Tool exposure | How agents declare capabilities to LLMs |
| A2A | Agent discovery | How agents find and authenticate each other |
| x402 | Payments | How agents pay for services in real-time |
The Missing Layer¶
MCP defines how to expose tools. A2A defines how agents discover each other. x402 handles payments. None of them define what an agent service IS.
That's the gap F3L1X fills with the Realm Specification — every realm has a unique identity, a health endpoint, a message broker connection, and a discoverable capability set.
Herald: The Unifying Broker¶
Herald mediates all three protocols:
- MCP gateway — registers tools, serves capability manifests
- A2A registry — stores agent cards, mediates discovery
- x402 mediation — routes payments between agents
- JWT authentication — three-zone trust model
One broker. Three protocols. Unified authentication.
FAQ¶
What is MCP in AI agent systems?¶
MCP (Model Context Protocol) is an open standard created by Anthropic that defines how AI models discover and invoke external tools. It provides a standardised way for agents to declare their capabilities and for LLMs to call those capabilities, similar to how REST APIs standardise web service communication.
How do AI agents pay each other?¶
The x402 protocol enables machine-to-machine payments using HTTP 402 (Payment Required) responses. When one agent needs a paid capability from another, it receives a 402 response with payment instructions, completes a cryptocurrency microtransaction, and retries the request with proof of payment.
F3L1X — First in Agentic Technology