{
  "id": "C06",
  "url": "https://cartforagents.com/guides/gtin-mpn-sku",
  "title": "GTIN, MPN, SKU and UPID: choose the correct product identifier",
  "description": "Use an identifier decision table to distinguish manufacturer, merchant and catalog identities, including legitimate missing-ID cases.",
  "answer": "Send a GTIN only when the brand owner has assigned one through GS1 to that exact variant, copied digit for digit, and send the manufacturer's MPN alongside the brand. Keep your SKU as your internal ID, never in a GTIN or MPN field; Shopify assigns the UPID, so you don't set it. When a product legitimately has no GTIN, leave the field empty and say so where the channel asks, never with a guess or a placeholder.",
  "take": "A wrong GTIN is worse than none, because it can match your listing to a different product. Send real identifiers or nothing, and never put your SKU in a GTIN field.",
  "summary": "Four codes, four owners. A GTIN says which trade item this is, an MPN says which part its maker means, a SKU says which shelf in your stockroom, and a UPID is the catalog's own label. The table below tells you which to send, where, and when the honest answer is none.",
  "claims": [
    {
      "text": "Google says unique product identifiers are assigned to each product by the manufacturer; not to make up, guess or include values from similar products; that incorrect identifiers can get products disapproved; and not to put internal SKUs or other store-specific values in the GTIN, brand or MPN attributes.",
      "source_title": "Google Merchant Center Help: About unique product identifiers",
      "source_url": "https://support.google.com/merchants/answer/160161?hl=en",
      "source_date": "2026-10-27",
      "method": "documented",
      "limitation": "Merchant Center rules. Other channels set their own, though the ones we read agree on not inventing values."
    },
    {
      "text": "Google's GTIN rules say each product and variant (different colors or sizes) has its own GTIN; that if you are the only seller of a product or it is a store brand, it generally won't have a GTIN; that the check digit must be correct; and that GTINs with the restricted prefixes 02, 04 or 2, or the coupon prefixes 05, 98 or 99, should not be submitted.",
      "source_title": "Google Merchant Center Help: GTIN [gtin]",
      "source_url": "https://support.google.com/merchants/answer/6324461?hl=en",
      "source_date": "2026-10-27",
      "method": "documented",
      "limitation": "Merchant Center rules. Requirements vary by country and product type."
    },
    {
      "text": "Google says MPN is required for products without a manufacturer-assigned GTIN, not to use a value you created unless you are the manufacturer, and that different apparel sizes often share one MPN. For custom or homemade goods with no official brand, it says to use the store name as brand with an MPN of your choice, and to set identifier_exists to no only when you are certain no identifier is assigned.",
      "source_title": "Google Merchant Center Help: MPN [mpn]; About unique product identifiers; Identifier exists [identifier_exists]",
      "source_url": "https://support.google.com/merchants/answer/6324482?hl=en",
      "source_date": "2026-10-27",
      "method": "documented",
      "limitation": "The same pages call MPN optional for custom-made products. Read them for your own category."
    },
    {
      "text": "OpenAI's file-upload spec says a GTIN must be exactly 8, 12, 13 or 14 digits with a valid check digit, preserving leading zeros, with no spaces or dashes, and omitted if unassigned; and says not to invent an MPN to replace a missing GTIN.",
      "source_title": "OpenAI Commerce: Products (file upload), Variants",
      "source_url": "https://developers.openai.com/commerce/specs/file-upload/products",
      "source_date": "2026-10-27",
      "method": "documented",
      "limitation": "Living specification for ChatGPT's file upload, which currently targets the US."
    },
    {
      "text": "Schema.org defines gtin as a Global Trade Item Number that should be an all-numeric 8, 12, 13 or 14 digit string (or a GS1 Digital Link URL) with a valid check digit, and says left-padding is not required or encouraged; it defines mpn as the Manufacturer Part Number and sku as a merchant-specific identifier.",
      "source_title": "Schema.org: gtin, mpn, sku",
      "source_url": "https://schema.org/gtin",
      "source_date": "2026-10-27",
      "method": "documented",
      "limitation": "A vocabulary definition. It does not say how any search engine or assistant uses the values."
    },
    {
      "text": "Shopify says SKUs are an optional code that identifies the product within your business; that a product or variant can have up to 20 barcodes, each typed as Custom, GTIN, UPC, EAN, ISBN, ASIN or NS-PID; that Custom is the default type; and that some sales channels might require a GTIN before a product can be published.",
      "source_title": "Shopify Help Center: Product details page (Inventory: SKUs and Barcodes)",
      "source_url": "https://help.shopify.com/en/manual/products/details/product-details-page",
      "source_date": "2026-10-27",
      "method": "documented",
      "limitation": "Describes the admin fields, not how each channel reads them."
    },
    {
      "text": "Shopify's Global Catalog clusters search results by Universal Product ID (UPID) and includes offers from multiple merchants; lookup_catalog accepts gid://shopify/p/{upid}, variant IDs and Shopify product URLs, and groups IDs that resolve to the same product.",
      "source_title": "Shopify: Global Catalog MCP (product discovery flow; lookup_catalog)",
      "source_url": "https://shopify.dev/docs/agents/catalog/global-catalog",
      "source_date": "2026-10-27",
      "method": "documented",
      "limitation": "The page does not document how a UPID is assigned or which product data decides that two listings are the same product."
    },
    {
      "text": "In our October 26 Shopify Catalog snapshot, all 1,620 result occurrences carried a product ID in the gid://shopify/p/ form (1,612 distinct IDs), and none of the 1,620 first variants included a SKU or barcode field in the search response.",
      "source_title": "Cart for Agents: Shopify agent-catalog snapshot, Oct 26, 2026 (How we checked, this page)",
      "source_url": "https://cartforagents.com/guides/gtin-mpn-sku#how-we-checked",
      "source_date": "2026-10-26",
      "method": "observed",
      "limitation": "One unpersonalized search context, first variant only. Fields absent from a search response may still exist in the merchant's data or in other catalog calls."
    },
    {
      "text": "GS1 US says barcodes from GS1 identify your company as the brand owner of that product, that each variation of each product you sell requires a unique barcode (its example: 3 sizes x 3 colors = 9), and listed a single GS1 US GTIN at $30 with no annual renewal fee on the day we checked.",
      "source_title": "GS1 US: Get Your U.P.C. Barcodes from GS1 US (get started guide)",
      "source_url": "https://www.gs1us.org/upcs-barcodes-prefixes/get-started-guide",
      "source_date": "2026-10-27",
      "method": "documented",
      "limitation": "US pricing and terms, which GS1 US says it can change. We did not check GS1 Canada or other member organizations."
    },
    {
      "text": "Google says shopping results labeled Top recommendations are selected based on relevance to the search, ratings, price and product features, using Shopping data aggregated from brands, stores and other content providers.",
      "source_title": "Google Shopping Help: Understand how shopping results are generated",
      "source_url": "https://support.google.com/googleshopping/answer/15456476?hl=en",
      "source_date": "2026-10-27",
      "method": "documented",
      "limitation": "Does not name identifiers as a ranking input or give weights."
    }
  ],
  "sources": [
    {
      "title": "Google Merchant Center Help: About unique product identifiers",
      "url": "https://support.google.com/merchants/answer/160161?hl=en",
      "accessed": "2026-10-27"
    },
    {
      "title": "Google Merchant Center Help: GTIN [gtin]",
      "url": "https://support.google.com/merchants/answer/6324461?hl=en",
      "accessed": "2026-10-27"
    },
    {
      "title": "Google Merchant Center Help: MPN [mpn]",
      "url": "https://support.google.com/merchants/answer/6324482?hl=en",
      "accessed": "2026-10-27"
    },
    {
      "title": "Google Merchant Center Help: Identifier exists [identifier_exists]",
      "url": "https://support.google.com/merchants/answer/6324478?hl=en",
      "accessed": "2026-10-27"
    },
    {
      "title": "OpenAI Commerce: Products (file upload)",
      "url": "https://developers.openai.com/commerce/specs/file-upload/products",
      "accessed": "2026-10-27"
    },
    {
      "title": "Schema.org: gtin",
      "url": "https://schema.org/gtin",
      "accessed": "2026-10-27"
    },
    {
      "title": "Schema.org: mpn",
      "url": "https://schema.org/mpn",
      "accessed": "2026-10-27"
    },
    {
      "title": "Schema.org: sku",
      "url": "https://schema.org/sku",
      "accessed": "2026-10-27"
    },
    {
      "title": "Shopify Help Center: Product details page",
      "url": "https://help.shopify.com/en/manual/products/details/product-details-page",
      "accessed": "2026-10-27"
    },
    {
      "title": "Shopify Help Center: Using CSV files to import and export products",
      "url": "https://help.shopify.com/en/manual/products/import-export/using-csv",
      "accessed": "2026-10-27"
    },
    {
      "title": "Shopify: Global Catalog MCP",
      "url": "https://shopify.dev/docs/agents/catalog/global-catalog",
      "accessed": "2026-10-27"
    },
    {
      "title": "Universal Commerce Protocol: variant schema (2026-08-25)",
      "url": "https://ucp.dev/2026-08-25/schemas/shopping/types/variant.json",
      "accessed": "2026-10-27"
    },
    {
      "title": "GS1 US: Get started guide",
      "url": "https://www.gs1us.org/upcs-barcodes-prefixes/get-started-guide",
      "accessed": "2026-10-27"
    },
    {
      "title": "Google Shopping Help: Understand how shopping results are generated",
      "url": "https://support.google.com/googleshopping/answer/15456476?hl=en",
      "accessed": "2026-10-27"
    }
  ],
  "limitations": [
    "Platform and standards pages were checked on 2026-10-27. Identifier rules and catalog behavior change; re-check before acting.",
    "The decision table is our proposed summary of the rules quoted. It is not a GS1, Google, OpenAI or Shopify document, and we don't give legal advice about who counts as a brand owner.",
    "No platform we read says identifiers improve ranking in AI answers. Google says products with an assigned but missing GTIN may have limited visibility in its own Shopping surfaces; we found no equivalent statement for ChatGPT, Copilot or Perplexity.",
    "How Shopify assigns a UPID, and whether it uses GTINs to group listings, is not documented on the page we read.",
    "The snapshot observation covers one unpersonalized search context and the first variant of each result. It says nothing about fields the merchants hold privately.",
    "The worked case is a hypothetical store. The GTIN shown is the example value from Shopify's own CSV help, used only to demonstrate the check digit; never put it on a product.",
    "We checked GS1 US only. Canadian and other sellers should check their own GS1 member organization."
  ],
  "related": [
    "https://cartforagents.com/guides/agent-ready-product-data",
    "https://cartforagents.com/guides/product-feed-optimization",
    "https://cartforagents.com/guides/ai-product-descriptions",
    "https://cartforagents.com/guides/product-constraint-coverage",
    "https://cartforagents.com/guides/product-schema-markup",
    "https://cartforagents.com/guides/shopify-global-catalog"
  ],
  "published": "2026-10-27",
  "updated": "2026-10-27",
  "checked": "2026-10-27",
  "kind": "guide"
}