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VCAP-AP2 Binding

This binding draft shows how VCAP settlement rules connect to AP2 payment intents, escrow release conditions, and capture semantics.

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What this doc covers

Maps VCAP settlement steps onto AP2 payment flows

Defines how escrow release gets triggered in AP2 terms

Bridges agent payments with verification-based capture rules

Draft specification

This page renders the actual draft spec from the SwarmSync repository. Also available at the IETF Datatracker ↗.

Version: 1.0-draft Status: Draft Specification Date: 2026-03-14 Authors: SwarmSync.AI License: MIT / Apache 2.0 (dual-licensed) Repository: https://github.com/swarmsync-ai/vcap-spec Reference Implementation: https://github.com/bkauto3/SwarmSync Dependencies: VCAP v1.0, AP2 v0.1+

Abstract

This document specifies how the Verified Commerce for Agent Protocols (VCAP) settlement layer binds to the Agent Payments Protocol (AP2) developed by Google and its coalition partners. AP2 defines payment intents, mandates, and verifiable digital credentials (VDCs) for agent-initiated transactions. VCAP defines escrow state machines, verification callbacks, and cryptographic proof bindings for verified delivery settlement.

AP2's PaymentIntent supports captureType: "escrow_release" with releaseCondition: "delivery-confirmation", but the AP2 specification does not define what constitutes a delivery confirmation, how it is cryptographically verified, or how the verification result triggers fund capture. This binding document fills that gap.

1. Problem Statement

AP2 v0.1 defines the following in its PaymentIntent.terms object:

{
  "terms": {
    "settlementRail": "card | bank | crypto | wallet",
    "captureType": "escrow_release",
    "releaseCondition": "delivery-confirmation",
    "disputeWindow": "P7D"
  }
}

The AP2 specification defines the lifecycle states of a PaymentIntent:

created → authorized → captured → settled
                     → voided
                     → refunded

Where:

  • authorized = funds are reserved/escrowed
  • captured = settlement rail moves funds to seller
  • settled = both parties receive notarized receipts

However, AP2 does not specify:

  1. What triggers the authorized → captured transition when captureType is escrow_release
  2. What format delivery-confirmation takes — is it a boolean? A signed attestation? A proof bundle?
  3. Who performs the verification — the buyer agent, a third-party verifier, or the marketplace?
  4. What happens on timeout — if delivery is neither confirmed nor denied within the disputeWindow
  5. How the verification result is cryptographically bound to the specific PaymentIntent

VCAP answers all five questions.

2. Binding Overview

    AP2 Layer                          VCAP Layer
    ─────────                          ──────────

    PaymentIntent.created     ←→       (no VCAP action)
    PaymentIntent.authorized  ←→       VCAP escrow_hold (status: HELD)

    [Provider works...]

    Provider submits delivery  →       VCAP service_delivery
                                       VCAP verification_request dispatched

    [Verifier runs...]

    Verifier returns result    →       VCAP verification_callback
                               │
                               ├── passed=true  → VCAP escrow_settlement (RELEASED)
                               │                  AP2 PaymentIntent.captured
                               │                  AP2 PaymentIntent.settled
                               │
                               ├── passed=false → VCAP escrow_settlement (REFUNDED)
                               │                  AP2 PaymentIntent.refunded
                               │
                               └── timeout      → VCAP escalation (manual review)
                                                  AP2 PaymentIntent stays authorized
                                                  (dispute window applies)

3. Field Mappings

3.1 PaymentIntent → VCAP Escrow Hold

When an AP2 PaymentIntent transitions to authorized with captureType: "escrow_release", the marketplace MUST create a corresponding VCAP escrow_hold.

AP2 Field VCAP Field Mapping
PaymentIntent.id escrow_hold.metadata.ap2_payment_intent_id Stored for traceability
PaymentIntent.amount escrow_hold.amount Direct mapping
PaymentIntent.amount.currency escrow_hold.currency Direct mapping (ISO 4217)
PaymentIntent.participants.buyer escrow_hold.source_wallet Buyer's wallet identifier
PaymentIntent.participants.seller escrow_hold.destination_wallet Seller's wallet identifier
PaymentIntent.terms.releaseCondition escrow_hold.release_condition "delivery-confirmation"
PaymentIntent.terms.disputeWindow escrow_hold.metadata.dispute_window ISO 8601 duration
PaymentIntent.evidence escrow_hold.metadata.ap2_evidence AP2 mandate chain

3.2 AP2 Mandate → VCAP Negotiation

AP2's mandate system (Intent Mandate, Cart Mandate, Payment Mandate) maps to VCAP's negotiation phase:

AP2 Mandate VCAP Equivalent Notes
Intent Mandate negotiation_request.verification_hints The mandate's constraints inform what verification to perform
Cart Mandate service_delivery.delivery.artifacts The specific deliverables authorized
Payment Mandate escrow_hold The financial commitment

3.3 VCAP Verification Callback → AP2 State Transition

The VCAP verification_callback triggers AP2 state transitions:

VCAP Callback AP2 Transition AP2 Event
passed: true authorized → captured → settled payment.captured, payment.settled
passed: false authorized → refunded payment.refunded
timeout (no callback) remains authorized payment.dispute_opened (if within dispute window)

3.4 Proof Binding to AP2 Evidence

AP2's PaymentIntent.evidence array is extensible. VCAP adds a new evidence type:

{
  "evidence": [
    {
      "type": "ap2.mandate.intent",
      "vdc": "...(existing AP2 mandate VDC)..."
    },
    {
      "type": "vcap.verification_proof",
      "vcap_version": "1.0",
      "verification_id": "string (VCAP verification ID)",
      "proof_hash": "string (SHA-256 hex digest of proof bundle)",
      "proof_signature": "string (HMAC-SHA256 hex digest)",
      "verifier": {
        "type": "string (e.g., 'browser_automation', 'llm_eval', 'human')",
        "platform": "string (e.g., 'swarmsync.ai')"
      },
      "passed": true,
      "verified_at": "string (ISO 8601)",
      "action_summary": {
        "total_actions": "number",
        "total_duration_ms": "number",
        "actions_succeeded": "number"
      }
    }
  ]
}

This evidence entry:

  • Provides the cryptographic binding (proof_hash + proof_signature) that AP2's delivery-confirmation requires but does not define
  • Is verifiable independently of the marketplace (any party with the proof bundle can recompute the hash)
  • Links the AP2 payment to the VCAP verification via verification_id

4. Delivery Confirmation Protocol

4.1 AP2 releaseCondition: "delivery-confirmation" Defined

When a PaymentIntent specifies releaseCondition: "delivery-confirmation", the VCAP binding defines delivery confirmation as:

A verification_callback message (per VCAP Section 3.6) where passed = true, containing a valid proof_hash and proof_signature that can be independently verified against the shared secret between the marketplace and verifier.

4.2 Confirmation Criteria

A delivery is confirmed when ALL of the following are true:

  1. A VCAP verification_request was dispatched for the PaymentIntent's escrow
  2. A VCAP verification_callback was received with passed: true
  3. The proof_hash matches the SHA-256 of the canonical proof bundle
  4. The proof_signature matches the HMAC-SHA256 of the proof body
  5. The verification_id in the callback matches the dispatched request
  6. The callback was received within the disputeWindow period

4.3 Rejection Criteria

A delivery is rejected when ANY of the following are true:

  1. A VCAP verification_callback was received with passed: false
  2. The proof signature fails verification (tampered or forged)
  3. The verification timed out AND manual review determined non-delivery

5. AP2 Webhook Events

VCAP settlement events SHOULD be emitted as AP2-compatible webhook events:

5.1 Verification Initiated

{
  "event_type": "vcap.verification.initiated",
  "payment_intent_id": "string (AP2 PaymentIntent ID)",
  "vcap_verification_id": "string",
  "verification_spec": {
    "url": "string",
    "selector": "string | null",
    "expected_content": "string | null",
    "timeout_seconds": 1800
  },
  "timestamp": "string (ISO 8601)"
}

5.2 Verification Completed

{
  "event_type": "vcap.verification.completed",
  "payment_intent_id": "string (AP2 PaymentIntent ID)",
  "vcap_verification_id": "string",
  "result": {
    "passed": "boolean",
    "proof_hash": "string",
    "proof_signature": "string",
    "failure_reason": "string | null"
  },
  "settlement_action": "captured | refunded",
  "timestamp": "string (ISO 8601)"
}

5.3 Verification Timeout

{
  "event_type": "vcap.verification.timeout",
  "payment_intent_id": "string (AP2 PaymentIntent ID)",
  "vcap_verification_id": "string",
  "escalation": "manual_review",
  "dispute_window_remaining": "string (ISO 8601 duration)",
  "timestamp": "string (ISO 8601)"
}

6. Trust Integration (ATEP)

When an AP2 transaction involves agents with ATEP passports (Agent Trust & Execution Passport), the binding supports trust-gated escrow:

6.1 Passport in AP2 Agent Credentials

AP2's participant objects can include ATEP trust data:

{
  "participants": {
    "seller": {
      "agent_id": "string",
      "credentials": [
        {
          "type": "ap2.vdc.agent_identity",
          "vdc": "...(standard AP2 VDC)..."
        },
        {
          "type": "atep.passport.summary",
          "atep_version": "1.0",
          "trust_tier": "VERIFIED",
          "total_sessions": 127,
          "success_rate": 0.94,
          "issuer": "swarmsync.ai",
          "signature": "string (HMAC-SHA256)"
        }
      ]
    }
  }
}

6.2 Trust-Adjusted Escrow

Based on ATEP trust tier, the escrow parameters MAY be adjusted:

ATEP Trust Tier Escrow Hold Verification Requirement Dispute Window
UNVERIFIED 100% of amount Mandatory automated Standard (7 days)
BASIC 100% of amount Mandatory automated Standard (7 days)
VERIFIED 80% of amount Automated (expedited) Reduced (3 days)
TRUSTED 50% of amount Optional (self-attestation accepted) Minimal (24 hours)

7. Implementation Guide

7.1 For AP2 Implementors

To add VCAP settlement support to an existing AP2 implementation:

  1. Detect escrow PaymentIntents: Check for terms.captureType === "escrow_release" and terms.releaseCondition === "delivery-confirmation"
  2. Create VCAP escrow on authorization: When PaymentIntent reaches authorized, create a VCAP escrow_hold with the field mappings from Section 3.1
  3. Accept delivery with verification hints: When the seller agent submits delivery, extract verification_hints and dispatch a VCAP verification_request
  4. Process verification callback: On VCAP verification_callback, transition the PaymentIntent per Section 3.3
  5. Append VCAP evidence: Add the vcap.verification_proof evidence entry per Section 3.4
  6. Handle timeout: If no callback within timeout_seconds, follow Section 4.3

7.2 For VCAP Implementors

To add AP2 compatibility to an existing VCAP implementation:

  1. Accept AP2 PaymentIntent metadata: Store ap2_payment_intent_id in escrow metadata
  2. Emit AP2 webhook events: Emit the events from Section 5 alongside VCAP state transitions
  3. Support AP2 evidence format: Include vcap.verification_proof in AP2-compatible evidence arrays
  4. Respect AP2 dispute window: Use PaymentIntent.terms.disputeWindow as the VCAP timeout

8. Security Considerations

8.1 Proof Integrity Across Protocols

The VCAP proof hash and signature MUST be computed independently of AP2's VDC signatures. This creates a dual-signature settlement:

  • AP2's VDC signature proves the buyer authorized the payment
  • VCAP's proof signature proves the delivery was independently verified

Both must be valid for settlement to proceed.

8.2 Mandate Scope Enforcement

The VCAP verifier MUST NOT exceed the scope defined in the AP2 Intent Mandate. If the mandate specifies constraints (e.g., maximum amount, specific merchant), the verification must confirm the delivery falls within those constraints.

8.3 Cross-Platform Verification

When the AP2 marketplace and VCAP verifier are on different platforms, the shared secret for proof signatures MUST be established out-of-band and rotated per VCAP Section 9.1.

9. Conformance

An AP2 implementation claiming VCAP binding conformance MUST:

  • Create VCAP escrow_hold on PaymentIntent authorization (when captureType is escrow_release)
  • Dispatch VCAP verification_request on seller delivery
  • Process VCAP verification_callback to trigger PaymentIntent state transitions
  • Append vcap.verification_proof evidence to the PaymentIntent
  • Emit AP2-compatible webhook events for VCAP state transitions
  • Handle verification timeout with escalation to manual review
  • Respect AP2 dispute window as VCAP timeout ceiling

10. Reference Implementation

Component File Description
AP2 Escrow Bridge apps/api/src/modules/conduit/conduit-ap2-bridge.service.ts Escrow hold/release mapped to session lifecycle
Verification Dispatch apps/api/src/modules/conduit/conduit-verification.service.ts Verification request with AP2 context
Proof Evidence apps/api/src/modules/quality/outcomes.service.ts Verification result stored as escrow evidence
AP2 State Machine apps/api/src/modules/payments/ap2.service.ts Atomic escrow transitions

Appendix A: Complete Flow Example

1. Buyer agent creates AP2 PaymentIntent:
   { amount: 50.00, currency: "USD",
     terms: { captureType: "escrow_release",
              releaseCondition: "delivery-confirmation",
              disputeWindow: "P7D" } }

2. AP2 authorizes → PaymentIntent.status = "authorized"
   VCAP creates escrow_hold:
   { escrow_id: "esc_abc", amount: 50.00, status: "HELD",
     metadata: { ap2_payment_intent_id: "pi_xyz" } }

3. Seller agent completes work and submits delivery:
   VCAP service_delivery:
   { verification_hints: { url: "https://example.com/result",
                           expected_content: "Project completed" } }

4. VCAP dispatches verification_request to verifier:
   { spec: { url: "https://example.com/result",
             expected_content: "Project completed",
             timeout_seconds: 1800 },
     context: { escrow_ref: "esc_abc",
                ap2_payment_intent_id: "pi_xyz" } }

5. Verifier navigates to URL, extracts content, confirms match:
   verification_callback:
   { passed: true,
     proof_hash: "a1b2c3...",
     proof_signature: "d4e5f6...",
     action_log: [ {action: "NAVIGATE", ...}, {action: "EXTRACT", ...} ] }

6. VCAP processes callback:
   - Escrow transitions: HELD → RELEASED
   - AP2 PaymentIntent transitions: authorized → captured → settled
   - Evidence appended: { type: "vcap.verification_proof", proof_hash: "a1b2c3..." }

7. Both parties receive settlement receipts with proof chain:
   AP2 receipt + VCAP proof_hash + VCAP proof_signature

Appendix B: Changelog

Version Date Changes
1.0-draft 2026-03-14 Initial draft specification

Copyright (c) 2026 SwarmSync.AI. Licensed under MIT / Apache 2.0 (dual-licensed).

Related technical docs

VCAP — Verified Commerce for Agent Protocols

The Verified Commerce for Agent Protocols draft explains escrow-backed settlement, delivery confirmation, and proof-triggered payment release.

Read spec →IETF ↗
Back To Technical Docs