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GLOSSARY TERM

A2A (Agent-to-Agent)

A2A (Agent-to-Agent) refers to protocols, standards, and communication patterns that enable autonomous agents to discover, negotiate, transact, and maintain compliance records without human intervention. A2A is fundamental to the agentic web.

Core A2A capabilities

  • Discovery - Agents find services, policies, and pricing through machine-readable formats (/.well-known/peac.txt, AIPREF)
  • Negotiation - Automated agreement on terms, pricing, and service levels without human approval flows
  • Payment - Direct agent-initiated payments using protocols like HTTP 402 and x402
  • Verification - Cryptographic receipts (PEAC-Receipt) proving payment, access rights, and compliance
  • Audit trails - Verifiable records of all interactions for compliance and dispute resolution

A2A transaction flow

  1. Agent discovers service - Finds API or content through search, referrals, or directories
  2. Agent reads policies - Fetches machine-readable terms from /.well-known/peac.txt
  3. Agent attempts access - Makes initial request to protected resource
  4. Server returns 402 - Payment required with structured payment challenge
  5. Agent pays automatically - Processes payment through specified rail (stablecoin, fiat, etc.)
  6. Agent receives receipt - Gets signed PEAC-Receipt JWS token as proof
  7. Agent retries with receipt - Presents receipt in header for verified access

A2A vs traditional APIs

Traditional APIs

  • Manual sign-up required
  • API keys provisioned by humans
  • Monthly billing cycles
  • Dashboard-based management
  • Human approval for access

A2A APIs

  • Automatic discovery
  • Self-service payment
  • Per-request pricing
  • Protocol-based access
  • Zero human intervention

A2A protocol stack

A2A communication relies on several complementary protocols:

PEAC

Policy discovery + verifiable receipts

HTTP 402 / x402

Payment-required signaling + challenge-response

MCP

Model Context Protocol for agent integrations

AIPREF

AI preference files for content usage policies

Use cases

  • API marketplaces - Agents browse, purchase, and integrate APIs without human involvement
  • Data licensing - Automated discovery and payment for data access rights
  • Computational resources - Pay-per-use access to GPU, storage, and processing power
  • Content syndication - Agents license and distribute content with proof of payment
  • Inter-agent services - Agents provide services to other agents with automated billing

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