A2A vs MCP: what changed in 2026

Two boundaries, two protocols

Agent skills and task/message operations → A2A. Tools, resources, prompts → MCP.

Pin the specification, not a moving URL

Reviewed on October 21, 2026. The latest published A2A specification release is 1.0.1. Its wire version is still 1.0: use protocolVersion: "1.0" in supportedInterfaces and A2A-Version: 1.0 on requests. The top-level version belongs to your service, not to A2A. A specification patch does not require renaming your endpoint or changing your agent version.

A2A 1.0.1 · A2A §3.6

Discovery without the old handshake

MCP revision 2026-07-28 removes the initialize/notifications/initialized handshake and protocol-level sessions. Requests carry protocol version and client capabilities in _meta. server/discover advertises server identity, capabilities, and supported versions. It is therefore incorrect to describe current MCP discovery as necessarily occurring after initialize. Older revisions still exist: negotiate the revision your client and server actually implement.

MCP 2026-07-28 · MCP changelog

A practical comparison

A2A starts with an Agent Card at /.well-known/agent-card.json. It advertises supported interfaces and skills before invocation. MCP discovery is an RPC contract; it is not an A2A Agent Card and does not make an MCP server an A2A peer. A client still needs a way to locate the MCP server.

Long-running work is not an exclusive feature

MCP now defines long-running tasks in an optional extension, rather than in the core specification. Do not use “MCP cannot run asynchronous work” as a selection rule. Choose A2A when the integration needs independent agent identity, skills, and its task model; choose MCP when exposing tools or contextual data. Check optional capabilities on both sides.

Where x402 fits

The A2A x402 extension specification remains v0.1. Declare its canonical URI in capabilities.extensions; the repository moving to google-agentic-commerce does not change that URI. Enabling the generator option only creates discovery metadata. It does not install a wallet, verify payment, or settle funds. Implement and test the runtime payment flow separately.

A2A x402 v0.1

https://github.com/google-a2a/a2a-x402/v0.1

Check the deployed interface

Validate JSON, publish the standard discovery path over HTTPS, and check the real response. Inspect the final URL after redirects, content type, Cache-Control, and ETag. Caching headers are recommendations, not mandatory card fields. An ETag header alone does not prove that conditional GET returns 304; a valid card alone does not prove its advertised endpoint works.

curl -i https://your-agent.example/.well-known/agent-card.json