Fidacy
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Concepts

UCP Protocol

Fidacy is a UCP-native trust layer. Send any assessment from a Universal Commerce Protocol flow, with a ucp block or the UCP-Agent header, and the engine returns the verdict the UCP way: as an independently-verifiable trust signal under com.fidacy.trust_verdict, embedding the signed verifiable credential anyone can check against our public JWKS.

Fidacy is the trust layer on UCP, not a UCP merchant. It does not implement checkout, catalog, or fulfillment, and is not UCP-official. It plugs the one thing UCP leaves open: the neutral risk verdict on the agent itself. UCP support is an additive overlay, a plain /v1/assess call behaves exactly as before, and the verdict core is protocol-blind (the same decision powers AP2, A2A, and UCP).

Engagement & version

UCP semantics engage when the request carries the UCP-Agent header (which advertises the platform profile) or a ucp block in the body. Unlike A2A, UCP has no version header, the version lives in the body. This engine speaks 2026-04-08. A version asserted in ucp.ucp_version is validated; an unsupported one is rejected before any work is done.

POST /v1/assess
UCP-Agent: profile="https://platform.example/profiles/shopping-agent.json"
Content-Type: application/json
Authorization: Bearer fky_live_…
→ 400
{ "errors": [
  { "type": "VersionNotSupportedError",
    "message": "UCP version 2099-01-01 not supported (this engine speaks 2026-04-08)." }
]}

The request

The AP2 payment mandate the UCP transaction carries is assessed by the core exactly as always, top-level mandate, with kind: ap2_payment. The ucp block only binds the assessment to the checkout. Every field is optional.

FieldMeaning
ucp.checkout_idThe UCP checkout this assessment belongs to (id is also accepted).
ucp.profileThe platform profile URL advertised via UCP-Agent.
ucp.ucp_versionProtocol version asserted by the caller (else the engine's pinned release).
idempotency_keyEchoed back; the mandate content_hash governs idempotency.
{
  "mandate": { "vct": "mandate.payment.1", … },
  "ucp": { "checkout_id": "checkout_123", "ucp_version": "2026-04-08" }
}

The verdict, as a UCP signal

With UCP engaged, the response carries a ucp block. The verdict is placed in UCP signals, the protocol's mechanism for independently-verifiable third-party attestations, under the reverse-domain namespace com.fidacy.trust_verdict. The signal embeds the signed credential (jws) and the public JWKS (provider_jwks) needed to verify it. A platform reads recommended_action and maps it to its own checkout state.

→ 200
{
  "decision": "review",
  "riskPayloadJws": "eyJ…",
  "ucp": {
    "version": "2026-04-08",
    "checkout_id": "checkout_123",
    "recommended_action": "step_up",
    "signals": {
      "com.fidacy.trust_verdict": {
        "format": "application/vc+jws",
        "jws": "eyJ…",
        "provider_jwks": "https://api.fidacy.com/.well-known/jwks.json",
        "kid": "1zoM57brjllufNTzCwI5-j5jFkiAslyzVDbPAcR_f-M",
        "payload": { /* the full signed outcome */ }
      }
    },
    "idempotency_key": null
  }
}

Decision → recommended action

Fidacy returns an advisory action, it does not own the checkout. The merchant or PSP sets the final UCP status; step_up corresponds to the UCP requires_escalation state.

DecisionRecommended actionMeaning
approveproceedTrust signal is green, safe to finalize the checkout.
reviewstep_upEscalate, require buyer input or stronger auth (≈ requires_escalation).
denydeclineDo not proceed with this agent on this transaction.
The signal value is a third-party attestation, verify it, don't trust it blind. Fetch the key at provider_jwks and verify the embedded jws (EdDSA) before acting on the verdict.

Verify the signal

The embedded credential is the same EdDSA-signed risk payload Fidacy issues everywhere, a self-contained verifiable credential. Verify it against the public JWKS with one call; no Fidacy account or secret needed.

import { verifyRiskPayload } from "@fidacy/verify";

const sig = res.ucp.signals["com.fidacy.trust_verdict"];
const verdict = await verifyRiskPayload(sig.jws); // checks EdDSA vs public JWKS
// → throws if tampered; returns the trusted claims otherwise

Binding spec

The full, dereferenceable Fidacy Trust-Verdict Binding (UCP) documents the com.fidacy.trust_verdict signal, its schema, the advisory action, and the verification flow. The signed jws is the source of truth, and Fidacy does not publish a /.well-known/ucp profile (that is for businesses and platforms); key discovery is the provider_jwks inline.

Pluggable by design

UCP, A2A, and AP2 differ only at the protocol edge, engagement and verdict packaging. Internally they dispatch through a single adapter registry over a protocol-blind core. A new protocol is one adapter and one line, the verdict engine never changes.

Next →Verify a Verdict