What problem does A2A solve that MCP does not?
MCP connects one agent down to tools and data (vertical); A2A connects agents to peer agents across org boundaries (horizontal). Different layers, complementary, not competitors.
Think of a general contractor renovating a house. MCP is the contractor's toolbox: the drills, saws, and tape they personally reach for. A2A is the contractor phoning a licensed electrician from another company to wire the kitchen. The toolbox helps the contractor do their own work. The phone call lets them delegate a whole sub-job to a specialist they do not employ and cannot micromanage. Both happen on the same renovation. One is about reaching tools, the other about coordinating with independent professionals. An AI agent works the same way: it uses MCP to grab data and run functions itself, and uses A2A to hand a task to another company's agent it cannot see inside.
Concept explanation~2 min read
Everything you need to truly understand this topic: intuition, mechanics, step by step explanation, code, formulas, and worked example. Click to expand.
Concept explanation~2 min read
Everything you need to truly understand this topic: intuition, mechanics, step by step explanation, code, formulas, and worked example. Click to expand.
The MCP versus A2A question is a 2026 favorite because both arrived in the same agent-protocol wave and both have the word agent attached, so candidates assume they compete. They do not. The clean answer is that they live on two different axes of the same agent architecture and solve genuinely different problems. An interviewer asking this is really probing whether you can locate a protocol on the stack rather than memorize a feature list.
MCP answers the question, how does one agent reach down to the tools, files, and data it needs to do its own work? A2A answers a different question, how does one agent reach across to another autonomous agent, often owned by a different organization, and delegate a whole task to it? The first is a connectivity problem against passive infrastructure. The second is a coordination problem against an independent reasoner.
Think of it as vertical versus horizontal. MCP is vertical: agent down to capabilities. A2A is horizontal: agent across to peer agents. Get that one image right and every downstream detail, discovery, trust, lifecycle, falls out of it. This deep dive walks through both axes, shows how they compose inside a single real agent, explains what MCP structurally cannot express, and gives you the framing to defend the boundary cleanly in an interview.
MCP is the vertical axis: agent to tool
MCP, the Model Context Protocol, standardizes how one LLM application discovers and invokes capabilities it controls. The host connects to one or more servers over JSON-RPC and pulls a catalog of three primitive kinds: tools (callable functions), resources (read-only data references), and prompts (parameterized templates).
The defining property is that the other side is passive. An MCP server does not reason. It does not plan. It exposes a typed schema and waits to be called. You, the host, decide when to call tools/call, with what arguments, and what to do with the result. The control loop lives entirely on your side.
The integration problem MCP solves is the M times N explosion. Without it, every app hand-wrote a bespoke wrapper per tool per vendor. With it, one filesystem or Postgres server plugs into any compliant host, collapsing M times N integrations to M plus N. That is the whole value proposition, and notice it says nothing about other agents.
One more property worth naming in an interview: MCP interactions are essentially stateless from the host's view. Each tools/call is a discrete request with a result. The server may hold session state, but the host drives every step and owns the orchestration loop. There is no concept of the tool taking initiative, pausing to think, or coming back later. That assumption, callee as obedient infrastructure, is precisely what A2A relaxes.
Situations where this technique stops working.
2–4 min · Everything important, quickly.
| Concern | MCP | A2A |
|---|---|---|
| Axis | Vertical, agent to tool | Horizontal, agent to agent |
| Callee | Passive tool or data server | Autonomous opaque peer agent |
| Unit of work | A single tool or resource call | A delegated task with a lifecycle |
| Trust boundary | Inside what you orchestrate | Across organizations, black box |
| Discovery | tools/list on a server | Agent Card advertising skills |
| Origin | Anthropic, November 2024 | Google, April 2025, then Linux Foundation |
Real products, models, and research that use this idea.
- Google open-sourced A2A in April 2025 with 50-plus launch partners including Salesforce, SAP, and MongoDB, then donated it to the Linux Foundation in mid-2025.
- An A2A travel agent delegates hotel search to a specialist agent via an Agent Card, while each agent uses MCP servers internally for its own databases and APIs.
What an interviewer would ask next. Try answering before peeking at the approach.
QHow does an A2A client discover what a remote agent can do before delegating?
Agent Card, a JSON document at a well-known URL listing skills, endpoints, auth schemes, and streaming support; the client reads it, then opens a task.
Red flags & common mistakes
The phrases that signal junior thinking. Click to expand.
Red flags & common mistakes
The phrases that signal junior thinking. Click to expand.
Calling A2A a competitor that replaces MCP. They sit on different axes, vertical agent to tool versus horizontal agent to agent, and a real system runs both.
60 second bullets to scan on the way to the call.
The vertical versus horizontal axis framing
What MCP connects and what A2A connects
Primary sources. Browse if you want the original framing.
Same topic, related formats. Practice these next.