Supplemental feed in GMC Next made simple

You have 4,000 SKUs in your primary Shopify feed. Google is disapproving 200 of them because GTINs are missing. You cannot get IT to backfill the metafield this month. A supplemental feed, or a rule in your product feed optimisation setup, lets you add those GTINs by hand in a spreadsheet and merge them with the primary feed by product id, no changes to your storefront.

What a supplemental feed actually does

It matches rows to your primary feed on the id attribute and overrides or adds attributes for the matching products. It cannot add new products. It cannot remove them. Everything not present in the primary feed is ignored.

When to use one

• Backfill missing GTINs, MPNs or brand values
• Override product_type or google_product_category on a subset
• Add custom_label_0 to 4 for bidding groups without touching the storefront
• Exclude a subset from Shopping with excluded_destination
• Add promotion_id links to your Merchant Promotions

Set one up in Merchant Center Next

1. Open Merchant Center Next, go to Data sources on the left rail
2. Click Add data source, choose Add products from a file
3. Toggle Add supplemental data (not a primary source)
4. Give it a name, choose the country and language, then pick Google Sheets, Scheduled fetch or Upload
5. Link it to the primary source it should merge with (this is the new bit in Next, you must pick the primary explicitly)
6. Save, then wait for the first fetch before checking a product in the Products section

Spreadsheet format

First row is a header. The id column is mandatory. Include only the attributes you want to add or override. A blank cell means do not change the primary value. To clear an attribute, some fields accept an empty string, most do not, use a placeholder or omit the column.

id,gtin,custom_label_0
SKU-001,5051234567890,best-seller
SKU-002,5051234567906,new-in
SKU-003,,clearance

Common gotchas

• The id must be identical to the primary feed, character for character, including case
• Merchant Center Next requires you to link a primary source, the old any-primary matching is gone
• A supplemental feed cannot add products that are absent from the primary
• If two supplemental feeds set the same attribute for the same id, the last one processed wins, keep one supplemental per use case

When a supplemental is the wrong tool

If you are overriding attributes for more than a third of your catalogue, or if the same overrides keep coming back every week, fix it in the source. AiFeedOptimiser does this automatically with rules - one rule ('brand contains X, set custom_label_0 = premium') runs on every feed refresh, no spreadsheet to maintain.