12.5Standard Protocols: MCP and A2A (as of July 2026)

The custom message bus above is still the right choice inside a single testing system you control. But across system boundaries, the industry has standardized on two protocols, and you should design against them rather than inventing your own:

• MCP (Model Context Protocol) standardizes agent-to-tool communication: an MCP server exposes tools with JSON schemas, and any MCP-capable agent can discover and call them. MCP is mature infrastructure now -- there is an official MCP Registry, and testing tools like Playwright MCP publish to it on every release.
• A2A (Agent-to-Agent) standardizes agent-to-agent communication across vendors and runtimes: capability discovery via agent cards, task lifecycles, and structured message exchange -- the cross-organization version of the AgentMessage protocol in this file.

The QA consequence is significant enough to get its own chapter: once your agents talk MCP and A2A, those integration points become testable surfaces with schemas you can contract-test. See Testing Agentic Systems.