MCP vs A2A: Choose the Right Agent Protocol

MCP vs A2A is a boundary decision, not a contest for one universal agent protocol. Use MCP when an application exposes tools, prompts, or data to a model-facing client; use A2A when one agent delegates a durable outcome to another agent that owns task progress.

This guide implements the same supplier-check workflow both ways. The comparison keeps discovery, authority, task state, streaming, and failure ownership visible so agent interoperability does not become a vague architecture slogan.

Start MCP vs A2A at the ownership boundary

The supplier example begins with a buyer asking whether a component is available, then reserving it if approved. In the Model Context Protocol version, the host agent calls catalog and reservation tools while retaining plan, user conversation, and completion state. In the Agent2Agent protocol version, the host delegates a procurement outcome to a supplier agent and follows a task whose remote authority may request input, stream an artifact, or terminate.

Both can move JSON across a network, but their promises attach to different owners. The question is who interprets the goal and who can truthfully say the work is complete.

Draw the protocol boundary before listing features

A useful boundary diagram starts outside the transports. The user authorizes a purchasing purpose; the host decides whether it needs a capability or a delegated outcome; the supplier owns either a narrow operation or a task. MCP discovery describes available server capabilities and schemas, while A2A discovery describes an agent and its skills.

That difference shapes error handling: a failed tool call returns control to the host's loop, whereas a delegated task can remain working or input-required independently. Agent protocols become easier to compare when the surrounding product state is drawn first instead of inferred from method names.

Use MCP for capabilities the host composes

Choose MCP when the host should decide the sequence and meaning of operations. A catalog search, inventory lookup, price calculator, or reservation command is naturally a tool because the host can validate each response and continue its own plan. The server publishes capability metadata, input schema, and result shape; it does not need to inherit the entire procurement objective.

Model Context Protocol can also expose resources and prompts, which makes it broader than a function-call wire format. The authority still stays narrow: discovering a reservation tool is not permission to spend, and returning content is not a durable business receipt.

Runnable artifact: The dependency-free classifier exercises capability access, delegated task ownership, a split boundary, and an ordinary API escape hatch. Its agent-boundary-classifier.test.mjs receipt keeps the article's simplified boundary executable and reviewable.

Save the inspectable proof as agent-boundary-classifier.test.mjs and run node agent-boundary-classifier.test.mjs. Expected final line: PASS: protocol boundary classified.

import assert from "node:assert/strict";
const choose=x=>{if(x.exposesCapability&&x.delegatesOutcome)return "split-boundaries";if(x.exposesCapability&&!x.delegatesOutcome)return "MCP";if(x.delegatesOutcome&&x.remoteOwnsTaskState)return "A2A";return "ordinary-API"};
const fixtures=[[{exposesCapability:true,delegatesOutcome:false,remoteOwnsTaskState:false},"MCP"],[{exposesCapability:false,delegatesOutcome:true,remoteOwnsTaskState:true},"A2A"],[{exposesCapability:true,delegatesOutcome:true,remoteOwnsTaskState:true},"split-boundaries"],[{exposesCapability:false,delegatesOutcome:false,remoteOwnsTaskState:false},"ordinary-API"]];
for(const [input,want] of fixtures)assert.equal(choose(input),want);console.log("PASS: protocol boundary classified");

Use A2A for delegated outcomes with remote progress

Choose A2A when the remote agent can own a meaningful subgoal and report progress using task semantics. The supplier agent might compare warehouses, ask which substitute is acceptable, wait for authorization, and deliver a reservation artifact. The host should not reproduce that internal plan as a chain of brittle tool calls.

A2A does not erase application policy: the parties still define authentication, artifact meaning, retention, and idempotent business effects. Its value is the explicit delegation boundary and lifecycle vocabulary, especially when the remote worker can continue after the initiating connection has disappeared.

Score discovery, authority, state, and streaming

The decision matrix uses five axes rather than a winner column. Discovery asks whether consumers need callable capabilities or a delegable skill. Authority asks who chooses steps and who validates consequential action. State asks whether progress must survive a connection and remain queryable as one task.

Streaming asks whether the consumer receives call results or task updates plus artifact parts. Failure ownership asks whether the host retries a failed capability or the remote agent reconciles its own work. Weight these axes for the workload; do not sum generic feature checkmarks from product pages.

Split mixed workflows instead of forcing one protocol

Real systems can use both protocols without tunneling one fiction through the other. The host may delegate procurement through A2A, while the supplier agent uses MCP servers for inventory and policy tools inside its own boundary. The important rule is that task identity does not masquerade as a tool invocation ID and a tool result does not masquerade as task completion.

Trace context can connect the layers while preserving separate contracts. This composition also creates an honest ordinary-API option for stable service-to-service operations that need neither model-facing discovery nor agent delegation.

Test failure ownership with the supplier fixture

The matched fixture injects a catalog timeout, a request for substitute approval, duplicate delivery, and a reservation that succeeds after the client disconnects. In the MCP path, the host owns timeout policy, conversation state, and whether to call again. In the A2A path, the supplier task owns its accepted state transitions and exposes an input boundary; the host reconnects rather than replaying the whole objective.

Neither path permits an ambiguous reservation to be blindly repeated. The classifier is intentionally smaller than the protocols, but its fixtures reveal which owner must carry recovery evidence.

Make the MCP vs A2A choice reversible

A final architecture note should say why this boundary is selected now and which changes would reopen it. Moving from a host-composed lookup to a remote managed investigation may justify A2A; collapsing an expensive delegation into one stable operation may justify MCP or a conventional API. Measure operational outcomes such as unresolved tasks, schema failures, authorization rejections, reconnect success, and receipt coverage rather than protocol popularity.

The best agent interoperability story is not maximal protocol surface. It is a boundary that another team can explain, test, and replace without losing the user's work.

Protocol texts worth keeping beside the diagram

Google's agent protocol field guide is useful orientation, while the normative details live in the A2A specification and MCP specification. For implementation follow-through, compare the site's deeper notes on A2A task lifecycle, long-running MCP tasks, MCP OAuth audiences, and tool-schema evolution; together they turn the boundary sketch into concrete lifecycle and authorization work.

Run the supplier-check thought experiment

Put one ordinary supplier check on a whiteboard and assign every verb to a party: discover supplier records, read evidence, request a fresh investigation, observe progress, approve a recommendation, and recover after interruption. MCP vs A2A usually stops feeling abstract when the remote party must own the investigation rather than merely expose the records and actions used by the host. Now change only that assumption and redraw the boundary; the exercise reveals whether the protocol choice follows actual authority or merely mirrors whichever SDK the team opened first.

Keep the resulting authority map beside the integration tests and label which actor may cancel, retry, or disclose intermediate state. That turns MCP vs A2A into a reviewable contract: a later product change can update one ownership claim, rerun the supplier fixture, and reveal whether the protocol boundary should move without reopening a feature-by-feature argument.

MCP vs A2A becomes a clear choice when plan ownership, durable state, and recovery sit beside the supplier fixture. Reopen the protocol decision when the remote party begins or stops owning a real outcome.