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cmd/cart/pool-server.go

@@ -75,6 +75,7 @@ func (s *PoolServer) GetCartHandler(w http.ResponseWriter, r *http.Request, id c
 
 func (s *PoolServer) AddSkuToCartHandler(w http.ResponseWriter, r *http.Request, id cart.CartId) error {
 	sku := r.PathValue("sku")
+	country := getCountryFromHost(r.Host)
 	if s.idx != nil && s.idx.IsDeleted(sku) {
 		// Bus-deleted SKU — skip the HTTP fetch and reject the add. Mirrors the
 		// product-fetcher's "product service returned %d for sku %s" shape so
@@ -86,7 +87,21 @@ func (s *PoolServer) AddSkuToCartHandler(w http.ResponseWriter, r *http.Request,
 		http.Error(w, fmt.Sprintf("product service returned %d for sku %s (bus-deleted)", 404, sku), http.StatusNotFound)
 		return nil
 	}
-	msg, err := GetItemAddMessage(r.Context(), sku, 1, getCountryFromHost(r.Host), nil)
+
+	// Cache-only fast path: AddSkuToCartHandler is a SINGLE top-level add
+	// (qty=1, no children). When the projection carries full authoritative
+	// fields, we skip PRODUCT_BASE_URL entirely and build the AddItem
+	// directly from the cache. This eliminates one HTTP round-trip per
+	// add-to-cart under steady-state bus-fed conditions.
+	var msg *messages.AddItem
+	if s.idx != nil {
+		if p, ok := s.idx.Get(sku); ok && HasRequiredFields(p) {
+			msg = BuildAddItemFromCache(sku, 1, country, nil, p)
+		}
+	}
+	if msg == nil {
+		var err error
+		msg, err = GetItemAddMessage(r.Context(), sku, 1, country, nil)
 		if err != nil {
 			return err
 		}
@@ -95,6 +110,7 @@ func (s *PoolServer) AddSkuToCartHandler(w http.ResponseWriter, r *http.Request,
 				ApplyProjectionOverlay(msg, p)
 			}
 		}
+	}
 
 	data, err := s.ApplyLocal(r.Context(), id, msg)
 	if err != nil {

cmd/cart/projection_overlay.go

@@ -34,6 +34,82 @@ func resolveTaxBp(class string) int {
 	return 2500
 }
 
+// HasRequiredFields asserts the catalog Projection carries enough authoritative
+// fields for the cart to build a complete AddItem WITHOUT an HTTP fallback to
+// PRODUCT_BASE_URL. Required:
+//
+//	PriceIncVat > 0   (treats zero as "unbus-fed", not "free"; positive-only
+//	                     matches every other cache-authoritative field)
+//	TaxClass != ""    (string identifier; numeric rate resolved via
+//	                     resolveTaxBp at the same scale as the HTTP path)
+//	ItemID != 0       (uint32 dedup key; zero means the producer didn't
+//	                     publish one, so we don't claim we know the id)
+//
+// Display fields (Title, Image) and fulfillment flags (InventoryTracked,
+// DropShip) are optional — when present they populate the line, when absent
+// the proto-zero value is acceptable.
+func HasRequiredFields(p catalog.Projection) bool {
+	return p.PriceIncVat.Int64() > 0 &&
+		p.TaxClass != "" &&
+		p.ItemID != 0
+}
+
+// BuildAddItemFromCache constructs a complete AddItem message directly from a
+// cache hit that satisfies HasRequiredFields. The caller has already confirmed
+// the projection is well-formed; we map every cache-visible carrier onto the
+// proto fields the existing HTTP path would have constructed.
+//
+// Fields mapped:
+//
+//	Sku              ← input
+//	ItemId           ← p.ItemID (uint32 dedup key)
+//	Quantity         ← input
+//	Country          ← input
+//	StoreId          ← input
+//	Price            ← p.PriceIncVat.Int64()
+//	Tax              ← int32(resolveTaxBp(p.TaxClass))
+//	Name             ← p.Title (may be empty)
+//	Image            ← p.Image (may be empty)
+//	InventoryTracked ← p.InventoryTracked
+//	DropShip         ← p.DropShip
+//
+// Fields intentionally left blank (cache-only gap; the schema doesn't yet
+// carry them and the audience for those fields is downstream of the cart line
+// — they are NOT required for line dedup or pricing):
+//
+//	SellerId / SellerName     — marketplace split; not on Projection yet.
+//	Stock                     — queried synchronously from inventory at
+//	                            decision points (per docs/inventory-shape.md).
+//	OrgPrice / Discount       — pre-discount price; a future Projection
+//	                            extension when promotion previews need it.
+//	ExtraJson                 — dynamic product data (configurator
+//	                            width/height etc.); a future schema extension
+//	                            for cache-only-skip-HTTP for groups with
+//	                            configurator children.
+//
+// IMPORTANT: this helper is for TOP-LEVEL lines only. Configurator children
+// re-price based on parent dimensions (ToItemAddMessage's parent != nil
+// path); since the Projection does not yet carry width/height, callers must
+// keep the existing HTTP-fetch flow for groups with children.
+//
+// Tombstoned SKUs never reach this helper; callers check idx.IsDeleted(sku)
+// first and short-circuit.
+func BuildAddItemFromCache(sku string, qty int, country string, storeId *string, p catalog.Projection) *messages.AddItem {
+	return &messages.AddItem{
+		Sku:              sku,
+		ItemId:           p.ItemID,
+		Quantity:         int32(qty),
+		Country:          country,
+		StoreId:          storeId,
+		Price:            p.PriceIncVat.Int64(),
+		Tax:              int32(resolveTaxBp(p.TaxClass)),
+		Name:             p.Title,
+		Image:            p.Image,
+		InventoryTracked: p.InventoryTracked,
+		DropShip:         p.DropShip,
+	}
+}
+
 // ApplyProjectionOverlay overlays the cache's authoritative catalog facts onto
 // an AddItem message that was just produced by a PRODUCT_BASE_URL HTTP fetch.
 //
@@ -61,10 +137,12 @@ func resolveTaxBp(class string) int {
 // ExtraJson product data (configurator options — width/height used by
 // accessory child-pricing).
 //
-// Now that ItemID round-trips from the bus, a future "cache-only" build path
-// (skipping PRODUCT_BASE_URL entirely for known SKUs) is unblocked — the
-// only fields left HTTP-only in that future path are Sellers, Stock, OrgPrice
-// and configurator extras, none of which affect line dedup.
+// SKUs that satisfy HasRequiredFields AND have no configurator children
+// route through BuildAddItemFromCache instead — see the cache-only fast path
+// in buildItemGroups / AddSkuToCartHandler. ApplyProjectionOverlay remains
+// for the cold-grace case (unbus-fed cache entry) AND for groups with
+// children (the parent's HTTP fetch is required for child dimension-driven
+// pricing; Overlay is applied after the fetch).
 //
 // Tombstoned SKUs (catalog.projection_published with deleted=true) → the
 // caller checks idx.IsDeleted(sku) before this helper and short-circuits.