Every Protocols (MCP & A2A) comparison and buyer's guide for building AI agents — 13 pieces and counting. Each is a head-to-head or a “best X for Y” roundup with a sources-backed verdict.
The Model Context Protocol defines three server primitives split by who's in control — the model, the app, the user. The ecosystem implemented one of them.
Most MCP servers only answer requests. Sampling and elicitation are the two features that let a server reach back through the client — one to the model, one to the human — and almost no one implements either.
Loading every tool definition into context and round-tripping every result is how MCP agents stall. Code execution flips the model into a programmer — and moves the hard part to your sandbox.
MCP standardized how an agent calls a tool. It said almost nothing about how the agent logs in as you — and that gap is the whole product these three are selling.
One agent, twenty MCP servers, and a context window drowning in tool definitions. The gateway is the layer that puts a single governed door in front of all of them.
They get pitched as competitors. They're not even the same kind of thing — and the difference that actually decides your architecture is what each one costs you in tokens.
They are not competing ways to give a model tools. One is the engine; the other is a distribution standard wrapped around it — and you pay for the wrapper in tokens and attack surface.
The Model Context Protocol replaced its HTTP+SSE transport with Streamable HTTP in 2025. Choosing it does not make your server serverless-friendly — and the reason is the part nobody reads.
The protocol everyone adopted in 2025 is simpler to build for than the hype suggests — but the part that decides whether your server works isn't the code.
The hard part of remote MCP auth was never the login. It's proving a token was minted for *your* server and no one else's — the audience claim that turns a friendly proxy back into a locked door.
There are two things called FastMCP, and one of them lives inside the official SDK. Picking the right way to build an MCP server starts with untangling that — and deciding how much you want the framework to do for you.
The model that emits a correctly-shaped tool call once is rarely the one that holds up across a multi-turn conversation and eight repeated trials. Pick by failure mode, not top-line score.
Stop reading "A2A vs MCP" as a fork in the road. One protocol points your agent down at tools; the other points it sideways at other agents. Here is how to use both without picking a loser.