Shopify's agent-commerce category filter doesn't filter. We checked 190 stores.

Since 2026 every Shopify storefront answers an agent-commerce endpoint at POST /api/ucp/mcp , advertised at GET /.well-known/ucp . Merchants did not turn it on and it is not in their admin. It speaks the Universal Commerce Protocol over JSON-RPC, and the tool that matters is search_catalog : an AI shopping agent asks a store for its catalogue and gets structured product data back - integer prices in minor units with a currency, variants, SKUs, canonical URLs, and a Shopify taxonomy category per product. Fetch the tool list from any store and search_catalog declares catalog.filters.categories , an array of strings documented as "category filters combined with OR logic", next to catalog.filters.price.{min,max} . So an agent should be able to ask for running shoes and get running shoes. We were about to write a paragraph about what it costs a merchant to leave the category field blank. Then we tried it. Method 200 stores, drawn deterministically from a corpus of 10,099 known Shopify storefronts: sort the hostnames, take every Nth. Reproducible, so nobody has to take "we picked 200 stores" on trust. Run on 2 September 2026. Every store got the same five calls, 10 products requested each time: # Call Filter sent A working filter would 1 Control none return products 2 Impossible category gid://shopify/TaxonomyCategory/zz-99-99-99 return nothing 3 The store's own category a category the control's products carry return at least that product 4 Same, unwrapped the bare id without gid://... the other form an agent would try 5 Price control price.max = 1 return nothing - nothing costs a cent Calls 2 and 3 only mean something together. Call 2 alone cannot distinguish "the filter is ignored" from "the filter rejects everything". Those are opposite findings, and both happen. The query matters more than it looks. Generic words ("gift", "set", "new") surface a catalogue's odd corners rather than its catalogue, and produce numbers that are measured honestly and still wrong. Every que