better results

cmd/cart/main.go

@@ -202,59 +202,17 @@ func main() {
 		actor.NewMutationProcessor(func(ctx context.Context, g *cart.CartGrain) error {
 			_, span := tracer.Start(ctx, "Totals and promotions")
 			defer span.End()
-
-			// Clear bypass flags first
-			for _, v := range g.Vouchers {
-				if v != nil {
-					v.BypassedByPromotions = false
-				}
-			}
-
-			g.UpdateTotals()
-			promotionCtx := promotions.NewContextFromCart(g, promotions.WithNow(time.Now()))
-			results, _ := promotionService.EvaluateAll(promotionStore.Snapshot(), promotionCtx)
-
-			// Check if any qualified promotion rule has a coupon_code condition matching our vouchers
-			hasBypassed := false
-			for _, res := range results {
-				if res.Applicable {
-					promotions.WalkConditions(res.Rule.Conditions, func(c promotions.Condition) bool {
-						if bc, ok := c.(promotions.BaseCondition); ok && bc.Type == promotions.CondCouponCode {
-							var codes []string
-							if s, ok := bc.Value.AsString(); ok {
-								codes = append(codes, strings.ToLower(s))
-							} else if arr, ok := bc.Value.AsStringSlice(); ok {
-								for _, s := range arr {
-									codes = append(codes, strings.ToLower(s))
-								}
-							}
-							for _, code := range codes {
-								for _, v := range g.Vouchers {
-									if v != nil && strings.ToLower(v.Code) == code {
-										if !v.BypassedByPromotions {
-											v.BypassedByPromotions = true
-											hasBypassed = true
-										}
-									}
-								}
-							}
-						}
-						return true
-					})
-				}
-			}
-
-			if hasBypassed {
-				// Re-evaluate with bypassed vouchers
-				g.UpdateTotals()
-				promotionCtx = promotions.NewContextFromCart(g, promotions.WithNow(time.Now()))
-				results, _ = promotionService.EvaluateAll(promotionStore.Snapshot(), promotionCtx)
-			}
-
-			// ApplyResults applies qualifying actions in priority order and records
-			// every effect — both applied discounts and pending "spend X more for ..."
-			// nudges with their progress — in g.AppliedPromotions.
-			promotionService.ApplyResults(g, results, promotionCtx)
+			// Canonical promotion pipeline — same call site as the
+			// /promotions/evaluate-with-cart preview handler in
+			// promotions_evaluate.go. Going through the shared
+			// PromotionService.EvaluateAndApply means the bypass
+			// loop, the re-eval trigger, and ApplyResults stay in
+			// lockstep with the preview so a what-if always matches
+			// post-add behavior (including the coupon+promotion-
+			// overlap edge case). If you find yourself adding
+			// promotion logic here, add it to EvaluateAndApply
+			// instead — the preview must follow.
+			promotionService.EvaluateAndApply(promotionStore.Snapshot(), g, promotions.WithNow(time.Now()))
 			g.Version++
 			return nil
 		}),
@@ -490,6 +448,19 @@ func main() {
 	// without creating or mutating a real cart.
 	mux.HandleFunc("POST /promotions/evaluate", newPromotionEvaluateHandler(promotionStore, promotionService))
 
+	// Cart-aware promotion evaluation: reads the cartid cookie the storefront
+	// already maintains, fetches the actual cart grain (cross-pod via
+	// GetAnywhere), merges with the request's items (the items the user is
+	// considering adding), and runs the engine on the merged line list. The
+	// cart is the source of truth (vouchers, customer segment, customer
+	// lifetime value, total) so a serialised copy from the browser can race
+	// the live cart — letting the backend read the cookie + fetch the grain
+	// means exactly one source of truth and no race. Missing cookie or fetch
+	// failure gracefully degrades to a stateless evaluation of the request
+	// items alone. See newPromotionEvaluateWithCartHandler in
+	// promotions_evaluate.go for the full contract.
+	mux.HandleFunc("POST /promotions/evaluate-with-cart", newPromotionEvaluateWithCartHandler(promotionStore, promotionService, syncedServer))
+
 	// only for local
 	mux.HandleFunc("GET /add/remote/{host}", func(w http.ResponseWriter, r *http.Request) {
 		pool.AddRemote(r.PathValue("host"))