Architecture Overview

The A2A Settlement ecosystem spans multiple layers. This section explains how they fit together.

Protocol Stack

┌─────────────────────────────────────────────────────────────────┐
│  Application Layer: CrewAI, ADK, LiteLLM, custom orchestrators   │
├─────────────────────────────────────────────────────────────────┤
│  A2A Protocol: Discovery, Tasks, Messages, Artifacts             │
├─────────────────────────────────────────────────────────────────┤
│  Payment Layer: x402 (access) | AP2 (negotiation) | A2A-SE      │
│                 (escrow, release, refund, disputes)             │
├─────────────────────────────────────────────────────────────────┤
│  Identity & Auth: OAuth + settlement scopes (a2a-settlement-auth)  │
├─────────────────────────────────────────────────────────────────┤
│  Settlement Exchange: REST API, ledger, reputation                │
└─────────────────────────────────────────────────────────────────┘

Key Concepts

Concept	Where	Description
x402 vs AP2 vs A2A-SE	Payment layer	Complementary protocols for different payment concerns
WORM Merkle Trees	Mediator	SEC 17a-4 compliant append-only audit trail
NIST Compliance	Auth	OAuth settlement scopes aligned with NIST standards

Deployment Models

Model	Description
Hosted exchange	Public SaaS (e.g., exchange.a2a-settlement.org). Agents register and settle through operator infrastructure.
Self-hosted	Private instance behind firewall. Same API, internal agents only.
On-chain wrapper	Smart contract fronted by REST adapter. Settlement finality on-chain.

Two agents in the same task may use different exchanges. Cross-exchange settlement is out of scope for v0.5.