Google Merchant supplemental feeds (what we write, what we refuse to overwrite)

Feed tools that “take over primary” make great demos and terrible incidents. The safer automation surface is supplemental: labels, availability, diagnostics.

Why supplemental

You can segment and correct without destroying a feed the merchant or another app still owns. Diff tools show missing attributes and stale labels so the queue is visible.

Pair with attribute completeness and lag testing.

Labels are strategy

custom_label bands for cover and velocity only help if someone builds campaigns or reports that read them. The feed write is half the job. The media structure is the other half.

How to set up a supplemental feed in Google Merchant Center

Supplemental feeds are the one place you can automate feed data without risking the feed your store depends on. The setup takes about an hour and has exactly one step everybody forgets. If those corrections are triggered by outright disapprovals rather than routine gaps, walk through fixing Google Shopping disapprovals first so you patch the right attribute before touching the sheet.

1. Decide what you are overriding, and leave the rest alone. A supplemental feed is a patch, not a replacement. It can change or add attributes on products that already exist in your primary feed. It cannot create new products, and anything it does not mention stays exactly as the primary feed left it. That is the whole reason it is the safe surface: a bad supplemental feed is reversible, a bad primary feed is an incident.
2. Build the sheet with id first. Column one is id, and it must match the primary feed's id exactly, including case and any prefix Shopify adds (often shopify_GB_ style values). Then add only the columns you are overriding, for example custom_label_0 or availability. Do not paste your whole catalogue export in: every extra column is another thing that can silently overwrite something correct.
3. Add it as a data source in Merchant Center. In Merchant Center Next go to Data sources → Add supplemental data source; on older Merchant Center it is Products → Feeds → Supplemental feeds. Google Sheets is the easiest option to start with, scheduled fetch from a URL is the one you want once it is real.
4. Link it to the primary feed, or it does nothing. This is the step people miss. After creating the supplemental source you must select which primary feed or feeds it applies to. An unlinked supplemental feed uploads cleanly, reports no errors, and changes nothing at all.
5. Use custom labels to carry your stock reality. Custom labels 0 to 4 are free-form and are the natural home for operational truth the primary feed does not carry: a cover band, a velocity band, a margin tier, a season flag. Pick one meaning per label and write it down, because in six months nobody will remember whether custom_label_2 was margin or season.
6. Build the campaign structure that reads the labels. A label nobody targets is a rounding error. Once the label is populated, subdivide your Shopping product groups by it, or use it to build the asset group listing filters in PMax, so budget can actually move between bands. Writing the feed is half the job and the easier half.
7. Verify on the product, not on the upload confirmation. Wait for the next fetch, then open an affected product in Merchant Center and check the attribute shows the supplemental value and the source is attributed to the supplemental feed. A green upload only means the file parsed.
8. Re-check disapprovals after the first full cycle. Overriding availability or price is the fastest way to create a mismatch between the feed and the landing page, which Google will disapprove for. Check the diagnostics a day after your first real run, not a month later when the account has quietly halved its impressions.

If availability is still wrong after all this, the supplemental feed is not the problem. The bridge between Shopify inventory and the feed is, and that is measured in catalogue lag.

If Shopping is the problem: Ralph puts your Shopify stock next to Shopping spend every night and drafts the supplemental feed change or the campaign exclusion for you to approve. Your primary feed is never overwritten. See Ralph · The Shopping fix · Docs