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Exercise: Design the request and response payloads for the "fetch cart" call triggered when a user taps the cart icon, plus the follow-up mutation APIs.

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Read this first: scope & honesty notes

• This is a design artifact, not a reproduction. Flipkart's real internal cart API is not publicly documented. Everything below is a reasonable, defensible design of the kind a reviewer would expect — it is not a copy of Flipkart's actual endpoints, field names, or behaviour. Do not present it as "how Flipkart really works."
• Only 2 of the 9 items are real. The book (A Promised Land) and the chair (Rastogi ... Ergonomic Chair) are taken from the provided screenshot. The other 7 items (4 grocery, 1 quick, plus 2 extra Flipkart items to reach the shown count of 4) are invented but plausible. They are marked "_note": "INVENTED" inline so no one mistakes them for real data.
• The date year is inferred. The screenshot shows "Mon Feb 7" and "Wed Feb 9" with no year. I assumed 2022 because Feb 7 falls on a Monday in 2022 — this is an inference, not a confirmed fact. Change it if you know the real year.
• Totals/price-summary block is deliberately omitted, per the decision to model only what's visible on the screen. A real cart response would almost certainly carry a totals block for the checkout bar; if you add one later, keep money in minor units (see below).

1. Fetch cart

Triggered when the user taps the cart icon.

1a. Request

A cart fetch is a read, so it's a GET. The only inputs that change the result are the delivery address / pincode (they affect serviceability and delivery promises), so those go in query params. Identity and plumbing go in headers. There is no request body.

GET /v3/cart?address_id=ADDR_001&pincode=400093

Headers

{
  "Authorization": "Bearer <access_token>",
  "X-Request-Id": "req_a1b2c3d4e5",
  "X-Device-Platform": "android",
  "X-App-Version": "8.12.3",
  "Accept": "application/json",
  "Accept-Language": "en-IN",
  "If-None-Match": "\"cart-v42\""
}

Query params

{
  "address_id": "ADDR_001",
  "pincode": "400093"
}

1b. Response (200 OK)

All 9 items populated. Money in paise. null for absent fields. No envelope. Invented items flagged inline.

{
  "cart_id": "CART_789",
  "version": 42,
  "currency": "INR",
  "address": {
    "address_id": "ADDR_001",
    "display_name": "Pankaj Ku...",
    "full_name": "Pankaj Kumar",
    "pincode": "400093",
    "type": "HOME"
  },
  "groups": [
    {
      "type": "flipkart",
      "label": "Flipkart",
      "count": 4,
      "lines": [
        {
          "line_id": "LINE_001",
          "product_id": "PROD_BOOK_123",
          "listing_id": "LST_88",
          "title": "A Promised Land (English, Hardcover)",
          "subtitle": "Hardcover, Obama Barack",
          "image_url": "<https://example.cdn/a-promised-land.jpg>",
          "rating": { "average": 4.5, "count": 50 },
          "price": { "mrp_minor": 99900, "sale_minor": 38400, "discount_pct": 61 },
          "offers": { "applied": 1, "available": 1 },
          "delivery": { "promise_date": "2022-02-07", "fee_minor": 5000, "fee_waived": false, "original_fee_minor": null },
          "quantity": { "value": 1, "min": 1, "max": 10, "step": 1 },
          "availability": { "status": "in_stock", "units_left": null, "message": null },
          "seller": { "seller_id": "SELL_BOOKS", "name": null, "assured": false },
          "actions": ["save_for_later", "remove", "update_quantity"]
        },
        {
          "line_id": "LINE_002",
          "product_id": "PROD_CHAIR_456",
          "listing_id": "LST_91",
          "title": "Rastogi Office Revolving Mesh Ergonomic Chair",
          "subtitle": "Black, Pre-assembled",
          "image_url": "<https://example.cdn/rastogi-chair.jpg>",
          "rating": null,
          "price": { "mrp_minor": 1500000, "sale_minor": 899900, "discount_pct": 40 },
          "offers": { "applied": 1, "available": 1 },
          "delivery": { "promise_date": "2022-02-09", "fee_minor": 0, "fee_waived": true, "original_fee_minor": 4000 },
          "quantity": { "value": 1, "min": 1, "max": 5, "step": 1 },
          "availability": { "status": "low_stock", "units_left": 5, "message": "Only 5 left" },
          "seller": { "seller_id": "SELL_RASTOGI", "name": "RastogiSteelFurniture", "assured": true },
          "actions": ["save_for_later", "remove", "update_quantity"]
        }
      ]
    },
    { "type": "grocery", "label": "Grocery", "count": 4, "lines": [] },
    { "type": "quick",   "label": "Quick",   "count": 1, "lines": [] }
  ]
}