Agentic Overlays -- Retrofit Legacy REST Services into A2A Agents¶

Ch04.503 Agentic Overlays -- Retrofit Legacy REST Services into A2A Agents¶

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Agentic Overlays -- Retrofit Legacy REST Services into A2A Agents¶

AWS + Cisco joint proposal: "agentic overlay" pattern wraps legacy REST services into A2A-compatible agents using a thin wrapper layer, without rewriting business logic.

Core Problem¶

Enterprise REST APIs are stable, well-tested, deeply embedded in production, but not designed for A2A communication. Most existing agents are REST-based or proprietary, outside the A2A framework.

Three Approaches Comparison¶

Agentic Overlay Architecture¶

Legacy REST Service
    |
    +-- Agentic Overlay (thin wrapper)
        +-- A2A Agent Card (discovery metadata)
        +-- A2A Protocol Handler (JSON-RPC)
        +-- MCP Tool Interface (tool exposure)
        +-- Auth/Validation Bridge

Key properties: - No code changes: overlay is independent layer, calls original REST API via HTTP - Simultaneous A2A + MCP: one overlay exposes as both A2A agent and MCP tool - Eliminates agent sprawl: reuses existing services as agents, no new infrastructure

REST vs A2A Paradigm Difference¶

• REST: deterministic, client-server, stateless request-response, optimized for stable interfaces
• A2A: autonomous agent interop, agent card discovery, JSON-RPC coordination, optimized for reasoning-driven coordination

Practical Significance¶

For enterprises with large REST microservice estates (finance, telecom, government), agentic overlay is the lowest-cost agent-ification path: 1. No rewrite or refactor of existing services 2. No parallel infrastructure 3. Gradual migration: overlay critical services first, then expand

Reference¶

• Source: Original Article