Costco Same-Day — Build a Cart from a Grocery List

Purpose

Given a list of grocery items, find the best-matching Costco Same-Day (Instacart-powered) product for each item and add it to the cart, biasing toward previously purchased products when a logged-in "Buy It Again" history is available. Returns, per item, the product chosen (name + price) and whether it came from purchase history, plus the running cart count. This skill is write-to-cart but read-only past that — it stops at the cart and never proceeds to checkout, payment, or order placement.

When to Use

"Add organic milk, large eggs, and bananas to my Costco Same-Day cart."
Reorder/restock flows where a user wants their usual groceries queued for delivery.
Converting a meal-plan or shopping-list into a ready-to-review Costco cart.
Any flow that needs products selected and added — but not purchased.

Workflow

The recommended method is browser driving the public storefront. Costco Same-Day is an Instacart white-label storefront; its internal product/cart APIs are session-token-gated and not usable cookieless, so the rendered storefront is the reliable surface. A bare remote session is sufficient — no residential proxy or stealth/verified browser was needed (homepage anti-bot probe: none detected; a plain --remote run completed the full add-to-cart flow cleanly).

Two distinct capability tiers — know which one you're in:

Guest (no credentials): search + add-to-cart fully work. The previously-purchased bias is NOT available — the "Buy It Again" page is empty for guests. Fall back to relevance search for every item.
Logged in (Costco.com SSO): the "Buy It Again" page is populated with order history, enabling the previously-purchased bias. Sign-in is delegated to Costco.com and requires real member credentials; an automated agent without them cannot complete it — treat it as an auth wall and proceed as a guest.

Steps

Open the entry page. browse open https://sameday.costco.com/ --remote, then browse snapshot. The page offers two buttons: Sign in via Costco.com and Browse as a guest.
- If you have valid Costco member credentials and need the purchase-history bias, click Sign in via Costco.com and complete the Costco.com SSO. Otherwise click Browse as a guest.
- browse wait load (the transition takes ~10s).
Land on the storefront. You arrive at https://sameday.costco.com/store/costco/storefront. The delivery ZIP is auto-set from the session's IP (e.g. 97818) and the cart starts at 0. The header shows a View Cart. Items in cart: N button — this N is your source of truth for confirming adds.
Check previously-purchased (the bias step). Navigate to https://sameday.costco.com/store/costco/buy_it_again.
- Logged in: this lists products from the member's order history. For any grocery-list item that matches an item here, prefer that product (set from_previously_purchased: true).
- Guest: the page shows "Reordering is a breeze / Items you order from this store will show up here" — i.e. empty. Record previously_purchased_available: false and fall back to search for every item.
Search each item. For each list entry, navigate directly to the search URL (no need to type in the box):
```
https://sameday.costco.com/store/costco/s?k=<URL-encoded query>
```
Then browse wait timeout 3000 (results render 1–3s after load), then browse snapshot.
Pick the best match. Each result is a group: Product node containing:
- a heading with the full product name,
- a StaticText: Current price: $X.XX,
- a sibling button: Add 1 ct <product name>.
Choose the first result that genuinely matches the requested item. Prefer Kirkland Signature and the size/variant closest to the request (e.g. "organic milk" → an organic whole/2% milk multi-pack, not almond/soy unless asked). Costco sells in bulk multi-packs — there is rarely a single-unit option.
Add to cart. Click the item's Add 1 ct ... button. It morphs into Decrement/Increment controls, and the header View Cart. Items in cart: N increments by 1. browse wait timeout 2000, snapshot, and verify N increased before moving to the next item.
Confirm and emit. After all items, read the final cart count and confirm it equals the number of items successfully added. Emit the JSON in Expected Output. Stop here — do not open the cart's checkout flow.

Site-Specific Gotchas

WRITE-TO-CART, NOT CHECKOUT. Add items and stop. Never click through to checkout, payment, scheduling, or "Place order."
Previously-purchased bias requires login. The "Buy It Again" page (/store/costco/buy_it_again) is the only previously-purchased surface, and it is empty for guests. Without Costco.com credentials the bias is impossible — be honest in the output (previously_purchased_available: false) rather than pretending. This is the core limitation of the task for an unauthenticated agent.
Sign-in is delegated to Costco.com. The "Sign in via Costco.com" button leaves the Instacart storefront for Costco's own SSO. There is no in-storefront login form; an agent without real member credentials cannot pass it — treat as an auth wall, not a bug.
Storefront/Buy-It-Again URLs 302-redirect to /?next=... until context is set. Hitting /store/costco/storefront or /store/costco/buy_it_again cold (fresh session, no guest/login choice made) bounces you to the landing page. Always click "Browse as a guest" (or sign in) first in the same session, then navigate.
Delivery ZIP is IP-geolocated. The storefront picks a delivery ZIP from the session's outbound IP (97818 on a bare US session). Product availability and prices are ZIP-specific. If you need a specific delivery area, set it via the Delivery <ZIP> button in the header before searching; otherwise document the ZIP you actually got (it's in the header and in the output).
No proxy/stealth needed. A bare --remote session completed the entire flow; no Akamai block, captcha, or login wall on the storefront/search/add surfaces. --verified/--proxies are unnecessary (and only change which ZIP you geolocate to).
Search is URL-driven — skip the typeahead. https://sameday.costco.com/store/costco/s?k=<q> renders results directly; you don't need to interact with the search textbox. Wait ~3s after navigation before snapshotting (results render progressively).
Snapshots are large (500–700 a11y refs). Filter for Add 1 ct, Current price, and Items in cart rather than reading the whole tree.
Add button vs. product button. Each product card has TWO buttons: the product tile itself (opens the detail page) and the separate Add 1 ct <name> button. Click the Add button to add to cart without leaving the results page.
Internal APIs are session-gated. The Instacart backend that powers search/cart is not usable cookieless (requires session bundle tokens). Don't waste time trying to hit a JSON endpoint directly — drive the rendered storefront.
Bulk-only sizing. Costco results are warehouse multi-packs (e.g. "Kirkland Signature Eggs, Large, 5 dozen-count"). Match on product identity, not pack size, when the list item is generic ("eggs").

Expected Output

{
  "success": true,
  "store": "costco-same-day",
  "delivery_zip": "97818",
  "logged_in": false,
  "previously_purchased_available": false,
  "previously_purchased_note": "Buy It Again is empty for guests; requires Costco.com login",
  "items_added": [
    {
      "query": "organic milk",
      "product_name": "Kirkland Signature Organic A2 Whole Milk, Half Gallon, 3-count",
      "price": "$16.33",
      "from_previously_purchased": false
    },
    {
      "query": "large eggs",
      "product_name": "Kirkland Signature Eggs, Large, 5 dozen-count",
      "price": "$13.61",
      "from_previously_purchased": false
    },
    {
      "query": "bananas",
      "product_name": "Bananas, 3 lbs",
      "price": "$2.26",
      "from_previously_purchased": false
    }
  ],
  "cart_count": 3,
  "error_reasoning": null
}

Variant outcomes:

// An item had no usable match — product fields null, continue with the rest
{
  "query": "saffron threads",
  "product_name": null,
  "price": null,
  "from_previously_purchased": false,
  "note": "no relevant Costco result"
}

// Logged in: previously-purchased bias applied for matching items
{
  "logged_in": true,
  "previously_purchased_available": true,
  "items_added": [
    { "query": "milk", "product_name": "Kirkland Signature Organic Whole Milk, 8 fl oz, 18-count", "price": "$17.58", "from_previously_purchased": true }
  ]
}

// Auth wall hit (sign-in required but no credentials) — proceed as guest, not a hard failure
{
  "success": true,
  "logged_in": false,
  "previously_purchased_available": false,
  "previously_purchased_note": "Sign in via Costco.com requires member credentials; ran as guest",
  "cart_count": 3
}

create-cart