APIs, MCP and A2A get discussed as competing standards. They're not. They solve different layers of the same problem.
Here's the clean separation.
Three layers, three jobs
| Connects | Consumer | Solves | |
|---|---|---|---|
| APIs | Applications to services | Code you wrote | Deterministic, contract-based access |
| MCP | Agents to tools and data | An LLM-driven agent | Discovery and invocation without bespoke glue |
| A2A | Agents to other agents | Another agent | Delegation and multi-agent coordination |
An API assumes the caller already knows what it wants and how to ask. MCP assumes the caller must discover what's available and figure out how to use it. A2A assumes the caller wants to hand off a goal rather than make a call.
Why you end up with all three
They compose. A production agent stack typically has A2A between agents, MCP from agents to capabilities, and plain APIs underneath MCP servers doing the actual work.
Choosing between them is usually a sign the question was framed wrong.
The layer none of them provides
All three move requests. None of them governs meaning.
Whichever combination you land on, something still has to answer:
- Which entity does
customerrefer to, at what grain? - Does a valid join path exist between these two concepts?
- Is this caller entitled to these specific rows?
- Can this answer be reproduced six months from now?
Those aren't transport concerns and no protocol will grow into them. They belong to a semantic execution layer sitting behind whichever protocol carries the request.
The protocols move the request. Something else has to govern the truth.
The full breakdown — the full architectural comparison, when to reach for each, and how they compose in production — is here:
👉 APIs vs MCP vs A2A: What Enterprise AI Teams Need to Know
Originally published at colrows.com/blogs/apis-vs-mcp-vs-a2a
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