Agentic Overlays’ Technical Mechanism and Implementation Strategy

Agentic overlays provide a “thin wrapper layer” to make traditional REST-based services compatible with A2A (Agent-to-Agent) communication. This technology, as a tool compliant with the Model Context Protocol (MCP), exposes REST APIs and reduces agent sprawl while reusing existing business logic. (Source: Retrofit, don’t rebuild: Agentic overlays for transforming legacy enterprise services)

Difference in Paradigms between REST and A2A

REST APIs are designed for deterministic integration between clients and servers, characterized by stateless request-response flows based on HTTP semantics. In contrast, A2A emphasizes interoperability between autonomous agents, adopting agent discovery through metadata and structured message exchange (such as JSON-RPC). This design difference has posed challenges when integrating traditional REST services into A2A frameworks, requiring agent rebuilding or parallel infrastructure operation. (Source: Retrofit, don’t rebuild: Agentic overlays for transforming legacy enterprise services)

Implementation Patterns of Agentic Overlays

Agentic overlays consist of two main components:

  1. Generating A2A Interaction-Capable Agents
    Adding an agent layer to wrap REST endpoints and converting them into A2A protocol-compliant message formats. This process realizes capability negotiation based on agent cards and task orchestration.
  2. Exposing MCP-Compliant Tools
    Publishing REST APIs as “tools” compliant with the Model Context Protocol (MCP), enabling collaboration with LLMs or agent frameworks. This makes traditional REST services usable as “external commands” for agents.

For example, wrapping the /api/v1/order/create endpoint as an agent would involve the following transformation:

# REST API Endpoint
POST /api/v1/order/create
{
  "customer_id": "123",
  "items": [{"product_id": "456", "quantity": 2}]
}

# Agentic Overlay's A2A Message Transformation
{
  "agent_id": "order_service_agent",
  "action": "create_order",
  "parameters": {
    "customer_id": "123",
    "items": [{"product_id": "456", "quantity": 2}]
  }
}

(Source: Retrofit, don’t rebuild: Agentic overlays for transforming legacy enterprise services)

Migration Strategy and Limitations

The introduction of Agentic overlays is recommended to follow these steps:

  1. Evaluating Existing Services for Agentization
    Assessing the necessity and cost of MCP compliance for each service, considering aspects like request-response asynchronicity and state management needs.
  2. Utilizing Reference Architectures
    Designing the agent layer based on the reference architecture provided by AWS.
  3. Gradual Introduction
    Implementing agentization sequentially starting from critical services and testing their integration with the A2A framework.

However, the following limitations should be noted:

  • The complexity of the business logic in REST services can increase the implementation cost of the agent layer.
  • Implementing A2A protocols requires new tasks such as defining agent cards and managing metadata.
  • Publishing as MCP-compliant tools may necessitate restructuring APIs, potentially altering existing REST endpoints.

(Source: Retrofit, don’t rebuild: Agentic overlays for transforming legacy enterprise services)

Summary

  • Converting REST Services to A2A Agents
    Utilizing Agentic overlays to reuse existing REST endpoints as A2A protocol-compliant agents, with implementation possible based on the reference architecture in the AWS blog.
  • Publishing as MCP-Compliant Tools
    Exposing REST APIs as “tools” that can collaborate with LLMs or agent frameworks, enabling API design compliant with the Model Context Protocol specifications.
  • Reducing Agent Sprawl
    By reusing existing services, the development and operation costs of new agents can be reduced. The implementation example in the AWS blog can be referenced for designing agents that leverage existing infrastructure.