Agent architecture: A2A vs MCP — tools inside your agent vs agents across the org chart, and where the line falls now that MCP went stateless and gained Tasks
MCP connects an agent to tools, data sources and context: vertical, inside one agent. A2A connects agents to other agents: horizontal, across teams, departments and organisations. They are complementary, not competing.
That definition is the easy part. The decision it implies is where teams get it wrong, and the 2026 revisions removed the evidence they used to defend it.
A long-running MCP tool call and an A2A task are now the same HTTP request. Both encode calls as JSON-RPC 2.0 over stateless HTTP — MCP as its base encoding, A2A as one of three bindings (MCP specification, A2A specification). Both return a server-generated ID. Both can sit waiting for input, both offer polling plus a push path, and both run behind an ordinary load balancer with no session affinity.