How to add a supplemental feed in Google Merchant Center

Who this is for

Store owners and marketing teams whose main feed comes from a platform (Shopify, SHOPLINE, a plugin or an agency) and can't easily be edited — but whose titles, brands or descriptions need work so Google's Shopping results and AI answers describe the products well.

• Use a supplemental source when: you want to fix or add attributes for existing products, and keep price, stock and links coming from your platform.
• Don't use it for: adding new products or removing old ones — only a primary source can do that.

Primary vs supplemental data sources

Google's wording: a supplemental data source "can't be used to add or remove products or be used as a standalone data source".

How to add a supplemental data source

1. In Merchant Center, open Settings → Add-ons and turn on Advanced data source management. The Supplemental sources tab only appears after this.
2. Go to Settings → Data sources → Supplemental sources and select Add supplemental product data.
3. Choose how to provide it: a file (uploaded, or fetched from a URL on a schedule), a Google Sheets template, or the API.
4. Enter the file name or URL, then set the data source label and language to match your primary source.
5. Choose the primary data source to attach it to and select Create data source.

Fetching from a URL: the URL must point directly to the file (http, https, ftp or sftp), the file must be under 4 GB, and Googlebot and AdsBot-Google must not be blocked from it. You set how often it's fetched in Scheduled updates.

The step most people miss: matching IDs

Every row in a supplemental source must carry the same ID as the product in your primary source. Google asks you to match the ID "exactly". A row whose ID doesn't exist in the primary source changes nothing — and there's no obvious error telling you so.

• Check one product first: open a product in Merchant Center, copy its ID, and confirm the same ID is in your supplemental file.
• Platform apps may rename IDs: Shopify's Google & YouTube app, for example, sends one item per variant with IDs like shopify_TW_<product>_<variant> — different from the plain product ID in a store export. A supplemental file keyed by the plain ID would match nothing.
• Label and language must match too: the supplemental source's label and language should equal the primary source's, or the two won't link.

What to put in a supplemental feed

Only the columns you want to change, plus id. Keep to Google's limits:

Leave price and availability to your primary source so they stay in sync with your store.

Doing this with AI Catalog

AI Catalog writes the supplemental file for you: it fills brand, category, size and clearer titles and descriptions from your own product data, every change cites its source, and nothing ships until you approve it. It also checks your Merchant Center IDs (including Shopify's variant format) so the fixes actually apply. Check your catalog.

Questions