Where MCP connects an agent to tools and data, A2A connects agents to other agents. An agent publishes a capability card; another agent can discover it and hand off a task. The two protocols are complementary layers of the agent stack.
MCP is agent-to-tool (an agent calls tools/data). A2A is agent-to-agent (agents delegate tasks to each other). They compose: an agent uses MCP for tools and A2A to collaborate.