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🚨 The Problem

AI agents (Gemini, ChatGPT, Claude) now generate e-commerce checkouts directly — cart totals, tax, discounts, refunds, fees. These agents hallucinate on math:

Scenario	LLM Output	Correct
10% off $99.99	"$10.00 discount"	$9.999 → $10.00 ✅
8.25% tax on $100	"$8.00 tax"	$8.25 ❌
$50 + $30 + $20 cart	"$110.00 total"	$100.00 ❌
Discount applied after tax	Wrong order	Math error ❌

QWED-UCP blocks these errors before they reach a payment processor.

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✅ What QWED-UCP Is (and Isn't)

QWED-UCP IS:

• Verification middleware that checks AI-generated UCP checkouts
• Deterministic — uses Decimal math (no floating-point errors) and state-machine logic
• Open source (Apache 2.0) — integrate into any e-commerce workflow
• A safety layer — catches calculation errors and invalid state transitions before payment

QWED-UCP is NOT:

• A shopping cart — use Shopify, WooCommerce, or Stripe for that
• A payment processor — use Stripe, Adyen, or Square for that
• A fraud detection system — use Sift, Signifyd, or Riskified for that
• A replacement for e-commerce platforms — we just verify the math

Think of QWED-UCP as the "accountant" that reviews every AI checkout before payment.

Shopify builds carts. Stripe processes payments. QWED verifies the math.

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🆚 How QWED-UCP Differs from E-Commerce Platforms

Aspect	Shopify / Stripe / Adyen	QWED-UCP
Purpose	Build carts, process payments	Verify calculations
Approach	Trust AI outputs	Deterministically verify AI outputs
Accuracy	~99% (edge cases fail)	100% deterministic
Tech	Standard floating-point	Decimal + JSON Schema
Integration	Replace your stack	Sits between AI and payment
Pricing	Transaction fees	Free (Apache 2.0)

Use Together

┌──────────────┐     ┌──────────────┐     ┌──────────────┐
│   AI Agent   │ ──► │  QWED-UCP    │ ──► │    Stripe    │
│ (checkout)   │     │  (verifies)  │     │  (payment)   │
└──────────────┘     └──────────────┘     └──────────────┘

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🛡️ The Guards

QWED-UCP ships 10 verification guards. UCPVerifier.verify_checkout() runs the 3 core guards as a fail-closed pipeline; the remaining 7 guards are available as standalone classes for manual or middleware use.

Core Guards (run by verify_checkout())

Guard	Engine	What It Verifies
Money Guard	Decimal	Cart totals against UCP formula: Total = Subtotal - Discount + Fulfillment + Tax + Fee
State Guard	Manual state logic	Checkout state machine (draft → ready → complete), rejects illegal transitions
Structure Guard	JSON Schema	UCP schema compliance (v1.0)

Standalone Guards

Guard	Engine	What It Verifies
Line Item Guard	Decimal	Item quantity × price = line total
Discount Guard	Decimal	Percentage and fixed discount calculations
Currency Guard	Decimal	Currency precision, $0.01 rounding, supported codes
Refund Guard	Decimal	Full/partial refund amounts, tax reversals
Tip Guard	Decimal	Pre/post-tax tip calculations, min/max bounds
Fee Guard	Decimal	Service fees, delivery fees, platform fees
Attestation Guard	JWT (PyJWT)	Context-bound attestation tokens with in-memory replay protection

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🚀 Quick Start

Installation

pip install qwed-ucp

Verify a Checkout

from qwed_ucp import UCPVerifier

verifier = UCPVerifier()

checkout = {
    "currency": "USD",
    "totals": [
        {"type": "subtotal", "amount": 100.00},
        {"type": "tax", "amount": 8.25},
        {"type": "total", "amount": 108.25}
    ],
    "status": "ready_for_complete"
}

result = verifier.verify_checkout(checkout)

if result.verified:
    print("✅ Transaction verified — safe to process!")
else:
    print(f"❌ Verification failed: {result.error}")

verify_checkout() is always a full fail-closed trust verdict: it returns verified=True only when Money Guard, State Guard, and Structure Guard all pass.

Verify Totals Only

# Math-only check — does not verify state or schema
result = verifier.verify_totals_only(checkout)

⚠️ Do not treat verify_totals_only() as a final transaction verdict. It only runs the Money Guard and does not check state or schema validity.

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📊 Interpreting Results

Shared Fields (all results)

Field	Type	Description
verified	bool	Convenience flag — True only when status == TrustStatus.VERIFIED
status	TrustStatus	Typed trust verdict — see Trust Status
error	Optional[str]	Error message when verification fails

Top-Level Only (UCPVerificationResult)

Field	Type	Description
engine	str	"QWED-Deterministic-v1"
guards	list[GuardResult]	Individual guard outcomes
verification_mode	str	"deterministic"

Branching on status (preferred)

from qwed_ucp import UCPVerifier, TrustStatus

verifier = UCPVerifier()
result = verifier.verify_checkout(checkout)

if result.status == TrustStatus.VERIFIED:
    proceed_to_payment(checkout)
elif result.status == TrustStatus.ENGINE_ERROR:
    page_operator("Verifier crashed — manual review required")
else:
    reject_transaction(f"Proof failed: {result.status}")

Aggregation

verify_checkout() propagates the most-severe guard status to the top level (fail-closed ordering):

Top-level status	Triggered when
ENGINE_ERROR	Any guard raised an exception
QUARANTINED	A guard returned QUARANTINED (reserved)
FAILED	A guard deterministically disproved the transaction
UNVERIFIABLE	Most-severe guard status is UNVERIFIABLE
UNSUPPORTED	Most-severe guard status is UNSUPPORTED
PARTIAL	Most-severe guard status is PARTIAL
VERIFIED	All guards passed

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🔌 Integration

FastAPI Middleware

QWED-UCP ships a configurable FastAPI/Starlette middleware (requires Starlette, included with pip install qwed-ucp) that intercepts checkout requests and verifies them automatically:

from fastapi import FastAPI
from qwed_ucp.middleware.fastapi import QWEDUCPMiddleware

app = FastAPI()
app.add_middleware(QWEDUCPMiddleware)

The middleware:

• Intercepts POST/PUT/PATCH to paths matching /checkout, /cart, /payment
• Returns 422 Unprocessable Entity on verification failure (configurable)
• Sets headers X-QWED-Verified, X-QWED-Guards-Passed, X-QWED-Error
• Runs core guards (Money, State, Schema) plus optional advanced guards
• Always fail-closed on empty body, malformed JSON, or non-dict payload

Configuration:

app.add_middleware(QWEDUCPMiddleware,
    block_on_failure=True,       # Block on verification failure (default)
    include_details=True,        # Include guard details in error response
    use_advanced_guards=True,    # Also run Line Items, Discount, Currency guards
    verify_paths=["/checkout"],  # Override default path list
    verify_methods=["POST"],     # Override default method list
)

A helper dependency is also available for manual injection:

from qwed_ucp.middleware.fastapi import create_verification_dependency

verify = create_verification_dependency()

@app.post("/checkout")
async def create_checkout(
    checkout: dict,
    verification = Depends(verify)
):
    if not verification["verified"]:
        raise HTTPException(422, verification["error"])
    ...

Express.js Middleware

QWED-UCP also ships an Express.js middleware:

const express = require('express');
const { createQWEDUCPMiddleware } = require('qwed-ucp');

const app = express();
app.use(express.json());
app.use(createQWEDUCPMiddleware());

app.post('/checkout', (req, res) => {
    res.json({ status: 'verified', checkout: req.body });
});

See examples/express_server.js for a complete example with rate limiting and security hardening.

Stripe

import stripe
from qwed_ucp import UCPVerifier

def create_payment_intent(ucp_checkout):
    verifier = UCPVerifier()
    result = verifier.verify_checkout(ucp_checkout)

    if not result.verified:
        raise ValueError(f"Checkout math error: {result.error}")

    total = next(
        (item["amount"] for item in ucp_checkout["totals"]
         if item["type"] == "total"),
        None
    )
    if total is None:
        raise ValueError("Missing required 'total' entry in checkout.totals")
    return stripe.PaymentIntent.create(
        amount=int(total * 100),
        currency=ucp_checkout["currency"].lower()
    )

CI/CD

# .github/workflows/verify-checkout.yml
name: Verify UCP Checkouts
on: [push]

jobs:
  verify:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: QWED-AI/qwed-ucp@v1
        with:
          test-script: tests/verify_checkouts.py

---

🔏 AttestationGuard

For environments that need cryptographic proof of verification:

from qwed_ucp import AttestationGuard, UCPVerifier
import uuid

attester = AttestationGuard(
    secret_key="your-256-bit-secret",
    allow_insecure=True     # Set False in production
)

verifier = UCPVerifier()
result = verifier.verify_checkout(checkout)

attestation = attester.sign_checkout(
    checkout=checkout,
    verification_result=result,      # Accepts UCPVerificationResult directly
    transaction_attempt_id=str(uuid.uuid4()),
    request_nonce=str(uuid.uuid4()),
    session_id="sess_abc123"
)

# attestation.token is a signed JWT

transaction_attempt_id and request_nonce are required for context binding and in-memory replay protection (not shared across replicas or restarts — use an external store for multi-instance deployment). Set QWED_ATTESTATION_SECRET env var instead of passing secret_key in code.

---

🏷️ Trust Status

QWED-UCP v0.3.0+ exposes a typed TrustStatus enum on every verification result, replacing the old verified: bool-only model:

State	Meaning
VERIFIED	Deterministic proof succeeded under supported conditions
FAILED	Proof was attempted and disproved (e.g. totals mismatch)
UNVERIFIABLE	Proof could not be established (e.g. missing proof basis)
UNSUPPORTED	Input or state is outside supported semantics
PARTIAL	Some checks ran, but no final verdict is justified
ENGINE_ERROR	A verifier dependency or the runtime crashed
QUARANTINED	Reserved for policy-level rejection

verified: bool is preserved as a backward-compatible derived field: result.verified == True only when result.status == TrustStatus.VERIFIED.

---

🔒 Security & Privacy

Your checkout data never leaves your machine.

Concern	QWED-UCP Approach
Data Transmission	No API calls, no cloud processing
Storage	Nothing stored — pure computation
Dependencies	Local-only (Decimal, JSON Schema, PyJWT, Starlette)
PCI Compliance	No cardholder data processed

Perfect for: e-commerce with strict privacy requirements, transactions containing PII, PCI-DSS compliant environments.

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🗺️ Roadmap

v0.3.0 (Current)

• UCPVerifier with 3 core guards (Money, State, Structure)
• All 10 guards shipped and tested
• TrustStatus enum on every result
• FastAPI middleware (fail-closed on unparseable bodies)
• Express.js middleware
• AttestationGuard with context-bound JWT + replay protection

In Progress

• Multi-currency support (exchange rates)
• Subscription/recurring payment validation
• TypeScript/npm SDK

Planned

• Shopify webhook integration
• Stripe Checkout session verification
• WooCommerce plugin
• Real-time cart verification API

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❓ FAQ

Is QWED-UCP free?

Yes! Apache 2.0 license. Use it in commercial products, modify it, distribute it — no restrictions.

Does it handle floating-point precision issues?

Yes! Uses Python's Decimal type with ROUND_HALF_UP to 2 decimal places. No 0.1 + 0.2 = 0.30000000000000004 issues.

What is the UCP total formula?

Total = Subtotal - Discount + Fulfillment + Tax + Fee

QWED-UCP's Money Guard verifies AI outputs against this formula exactly.

Can I use it without Google UCP?

Yes! The guards work with any JSON checkout format that has a totals array with type and amount fields.

How fast is verification?

Sub-millisecond to low milliseconds per checkout, depending on the guard set. The Decimal math engine is optimized for currency calculations.

Does QWED-UCP expose TrustStatus in the Python SDK?

Yes. qwed_ucp.TrustStatus is a 7-state enum on every result dataclass. The verified: bool field is preserved for backward compatibility.

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🔗 Related QWED Packages

Package	Purpose
qwed-verification	Core verification engine
qwed-finance	Banking & financial verification
qwed-legal	Legal contract verification
qwed-infra	Infrastructure-as-code verification
qwed-tax	Tax compliance verification
qwed-mcp	Claude Desktop MCP integration
qwed-open-responses	OpenAI Responses API guards

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🔗 Links

Resource	Link
Universal Commerce Protocol	ucp.dev
Google UCP Docs	developers.google.com/merchant/ucp
QWED Documentation	docs.qwedai.com
QWED-AI Organization	github.com/QWED-AI

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🧑‍💻 Development

From Source

git clone https://github.com/QWED-AI/qwed-ucp.git
cd qwed-ucp
pip install -e ".[dev]"

Running Tests

pytest tests/ -v                # All tests
pytest tests/ -v --cov=src/qwed_ucp --cov-report=html  # With coverage
pytest tests/test_money_guard.py -v  # Single guard

Linting

ruff check src/ tests/

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📄 License

Apache 2.0 — See LICENSE

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