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The Browser Harness is the second half of the agentic-commerce pipeline: it starts from a UCP quote and turns that estimate into a placed, verified order. Most merchants don’t expose a direct payment API — so to actually place an order, someone has to go through the merchant’s own checkout. The Browser Harness does exactly that: it drives the real Shopify checkout, works out the true final total, and pays with the one-time tokenized card — so an AI application completes the purchase without ever touching the raw card or the merchant’s checkout code. 
  UCP: search → product → quote (estimate)   →   Browser Harness: confirm true total → checkout → order
The Browser Harness is the checkout step after a UCP quote. It powers prava shop checkout and hosted agentic purchases.

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What it does

Completes the real checkout

Fills the merchant’s checkout — contact, shipping address, delivery option — and submits payment with the one-time token. No merchant integration required.

Confirms the true total first

Reconciles subtotal, shipping, and tax against the live checkout and waits for the amount to settle before charging — so there are no surprise totals.

Returns a verifiable result

On success you get an order id and a payment status — a checkable confirmation the order was placed.

Reliable, repeatable

It learns a merchant’s checkout on the first run and completes later runs consistently, healing automatically if the page changes.

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Why it matters

1

No surprise charges

The amount is reconciled and stable before payment is attempted. If the total shifts mid-checkout (late shipping, tax, or add-ons), reconciliation restarts rather than charging the wrong amount.
2

One clean attempt

A checkout runs as a single, tracked flow — not a blind retry loop — so a purchase either completes and returns an order, or fails cleanly for you to re-quote.
3

Card safety preserved

It pays with the single-use, merchant-scoped credential from the session. Even inside the checkout, the credential can’t be reused or redirected to another merchant.

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How it fits the flow

  UCP quote            →     Browser Harness         →     Result
 ───────────               ───────────────────           ───────────
  price the item            confirm true total,           order id + status
                            pay with one-time token
The UCP quote gives an estimate; the Browser Harness turns it into a completed, verified order. Today it targets Shopify checkouts.

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Next

UCP

How products are discovered and quoted before checkout.

Agentic Commerce

The full discover → quote → checkout → pay pipeline.