A2A (Agent-to-Agent)

The @peac/mappings-a2a package carries verifiable evidence through Google's Agent-to-Agent (A2A) protocol. It uses the Evidence Carrier Contract to attach receipt references and JWS signatures to TaskStatus, Message, and Artifact objects.

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Install

Terminal

pnpm add @peac/mappings-a2a

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Quick start

a2a-evidence.ts

import { attachA2aCarrier, extractA2aCarrier } from '@peac/mappings-a2a';

// Attach evidence to an A2A artifact
const artifact = attachA2aCarrier(originalArtifact, {
  receipt_ref: 'sha256:abc123...',
  receipt_jws: 'eyJhbGciOiJFZERTQSIs...',
});

// Extract evidence from an A2A message
const carrier = extractA2aCarrier(incomingMessage);
if (carrier) {
  console.log(carrier.receipt_ref);  // 'sha256:abc123...'
  console.log(carrier.receipt_jws);  // 'eyJhbGciOiJFZERTQSIs...'
}

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Carrier placement

A2A evidence is placed in the metadata object using a reverse-DNS extension URI:

A2A metadata with PEAC carrier

{
  "metadata": {
    "https://www.peacprotocol.org/ext/traceability/v1": {
      "carriers": [
        {
          "receipt_ref": "sha256:abc123...",
          "receipt_jws": "eyJhbGciOiJFZERTQSIs..."
        }
      ]
    }
  }
}

The carrier array supports multiple receipts per A2A object. Each carrier includes at minimum receipt_ref (SHA-256 integrity anchor) and receipt_jws (signed compact JWS).

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Agent Card discovery

A2A agents advertise PEAC support via the Agent Card capabilities.extensions array:

Agent Card (excerpt)

{
  "name": "Evidence Agent",
  "capabilities": {
    "extensions": [
      "https://www.peacprotocol.org/ext/traceability/v1"
    ]
  }
}

3-step discovery flow

1. Agent Card -- check capabilities.extensions[] for the PEAC traceability extension URI
2. Well-known -- fetch /.well-known/peac-issuer.json from the agent's host for issuer configuration and JWKS URI
3. Header probe -- check for PEAC-Receipt header in HTTP responses from the agent

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Transport limits

Constraint	Value
Maximum embed size	64 KB
Carrier format	PeacEvidenceCarrier
Integrity check	sha256(receipt_jws) === receipt_ref

Integrity verification

When extracting a carrier from A2A metadata, always verify that sha256(receipt_jws) matches receipt_ref. This prevents tampering during transport.

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Supported A2A objects

Object	Carrier placement
TaskStatus	status.metadata[extensionURI].carriers[]
Message	message.metadata[extensionURI].carriers[]
Artifact	artifact.metadata[extensionURI].carriers[]

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Next steps

Evidence Carrier

The universal interface behind MCP, A2A, and HTTP evidence transport.

Agent Protocols

All supported protocol mappings: MCP, ACP, A2A, TAP, UCP, RSL.

MCP Server

Verify A2A-carried receipts using the PEAC MCP server tools.