2026-02-15 | 3 min read | AgentLink Team

A2A vs REST vs MCP: Architecture Guide for AI Agent Platforms

A practical architecture framework for choosing REST, A2A, and MCP in production AI agent systems, with examples, tradeoffs, and rollout strategy.

A2A vs REST vs MCP: Architecture Guide

One of the most common mistakes in AI agent products is treating protocol choice as branding.

It is not branding. It is operating model design.

This guide gives a practical framework for when to use:

REST for deterministic application calls,

A2A for agent-native interactions,

MCP for tool and capability discovery.

Quick decision matrix

Use REST when you need:

stable request/response contracts,

explicit authentication and rate control,

predictable integration with existing backend systems.

Use A2A when you need:

cross-agent orchestration,

richer interaction semantics than plain endpoint calls,

protocol-level agent discovery behavior.

Use MCP when you need:

tool listing and invocation patterns,

model-facing capability surfacing,

ecosystem compatibility with MCP-aware clients.

In practice, production systems often expose all three.

Why REST still matters

REST is still the lowest-friction path for many enterprise integrations.

Advantages:

mature tooling,

straightforward auth patterns,

easier API governance.

Limitations:

weak semantics for agent identity and behavior,

manual capability discovery,

inconsistent metadata across providers.

REST is great for execution, less great for agent-native discoverability.

Why A2A is useful for platform-native behavior

A2A introduces a better interaction model between agents:

explicit agent endpoints,

better compatibility signals,

clearer capability exchange patterns.

For multi-agent systems, A2A reduces custom glue and improves portability.

Still, A2A is not a complete replacement for REST. Teams often keep REST for core transactional operations.

Why MCP is becoming essential

MCP helps clients and systems answer:

what tools exist?

what parameters do they accept?

what operations are safe to call?

That is critical for machine-driven workflows where humans are not manually browsing docs.

MCP does not replace your full API surface, but it dramatically improves discoverability and tool usability.

Recommended layered architecture

A practical setup for directories and marketplaces:

1. **REST layer** for stable profile CRUD and execution endpoints.

2. **A2A layer** for protocol-native discovery and agent exchange.

3. **MCP layer** for tool catalogs and test calls.

4. **Well-known metadata** for identity and policy.

This keeps each protocol doing the job it is best at.

Security and governance model

For production adoption, define policy per protocol.

REST controls

API keys or OAuth,

per-endpoint rate limits,

request validation and audit logs.

A2A controls

agent identity verification,

protocol compatibility checks,

interaction logging for reliability analysis.

MCP controls

allowlisted tool execution,

bounded parameter schemas,

explicit timeout and concurrency guards.

Migration plan for existing products

If you already have REST only, do not rewrite everything.

Phase 1

Keep REST as source of truth.

Add machine discovery metadata (`.well-known` + OpenAPI).

Phase 2

Add A2A discovery endpoint for protocol compatibility.

Add MCP tool listing for key use cases.

Phase 3

Expand MCP tools where model clients need deeper interaction.

Introduce quality metrics and route by capability.

Operational metrics to track

protocol usage split (REST vs A2A vs MCP),

latency and error rate by protocol,

successful cross-agent handoff rate,

tool invocation success rate,

discoverability conversions (impression -> profile view -> interaction).

Without protocol-level observability, optimization is guesswork.

Common anti-patterns

Avoid these:

publishing MCP with no reliable REST backend,

exposing A2A without validation or timeout policy,

using inconsistent naming for the same capability across protocols,

shipping protocol endpoints without documentation and examples.

Final recommendation

Do not choose a single winner between REST, A2A, and MCP.

Use a layered strategy:

REST for deterministic integrations,

A2A for agent-native interoperability,

MCP for model-facing tool discovery.

That architecture is more resilient, more discoverable, and easier to scale as agent ecosystems evolve.