diff --git a/internal/ucp/checkout.go b/internal/ucp/checkout.go
index 0e1a963..2e2006b 100644
--- a/internal/ucp/checkout.go
+++ b/internal/ucp/checkout.go
@@ -45,12 +45,70 @@ func (s *CheckoutServer) handleCreateCheckout(w http.ResponseWriter, r *http.Req
 		writeError(w, http.StatusBadRequest, "invalid request body: "+err.Error())
 		return
 	}
-	if req.CartId == "" {
-		writeError(w, http.StatusBadRequest, "cartId is required")
+
+	items := req.Items
+	if len(items) == 0 && len(req.LineItems) > 0 {
+		items = req.LineItems
+	}
+
+	var cartIdStr string
+	if req.CartId != "" {
+		cartIdStr = req.CartId
+	} else if len(items) > 0 {
+		// Fetch cart base URL
+		cartBaseURL := os.Getenv("CART_INTERNAL_URL")
+		if cartBaseURL == "" {
+			cartBaseURL = "http://cart:8080"
+		}
+
+		// Prepare POST request to /ucp/v1/carts/ to create and seed the cart
+		createReqBody := CreateCartRequest{
+			Items: items,
+		}
+		bodyBytes, err := json.Marshal(createReqBody)
+		if err != nil {
+			writeError(w, http.StatusInternalServerError, "failed to marshal create cart request: "+err.Error())
+			return
+		}
+
+		client := &http.Client{Timeout: 10 * time.Second}
+		cartReq, err := http.NewRequestWithContext(r.Context(), "POST", fmt.Sprintf("%s/ucp/v1/carts/", cartBaseURL), bytes.NewReader(bodyBytes))
+		if err != nil {
+			writeError(w, http.StatusInternalServerError, "failed to create post cart request: "+err.Error())
+			return
+		}
+		cartReq.Header.Set("Content-Type", "application/json")
+
+		profile := os.Getenv("UCP_AGENT_PROFILE")
+		if profile == "" {
+			profile = DefaultUCPAgentProfile
+		}
+		cartReq.Header.Set("UCP-Agent", `profile="`+profile+`"`)
+
+		cartResp, err := client.Do(cartReq)
+		if err != nil {
+			writeError(w, http.StatusInternalServerError, "failed to create implicit cart: "+err.Error())
+			return
+		}
+		defer cartResp.Body.Close()
+
+		if cartResp.StatusCode != http.StatusCreated && cartResp.StatusCode != http.StatusOK {
+			writeError(w, http.StatusBadRequest, fmt.Sprintf("cart service returned status: %d when creating implicit cart", cartResp.StatusCode))
+			return
+		}
+
+		var cartRespBody CartResponse
+		if err := json.NewDecoder(cartResp.Body).Decode(&cartRespBody); err != nil {
+			writeError(w, http.StatusInternalServerError, "failed to decode implicit cart response: "+err.Error())
+			return
+		}
+		cartIdStr = cartRespBody.Id
+	} else {
+		writeError(w, http.StatusBadRequest, "cartId or items/line_items is required")
 		return
 	}
 
-	cartId, ok := cart.ParseCartId(req.CartId)
+	cartId, ok := cart.ParseCartId(cartIdStr)
 	if !ok {
 		writeError(w, http.StatusBadRequest, "invalid cartId")
 		return
diff --git a/internal/ucp/checkout_test.go b/internal/ucp/checkout_test.go
new file mode 100644
index 0000000..9ae5a62
--- /dev/null
+++ b/internal/ucp/checkout_test.go
@@ -0,0 +1,153 @@
+package ucp
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"strings"
+	"testing"
+	"time"
+
+	"git.k6n.net/mats/go-cart-actor/pkg/actor"
+	"git.k6n.net/mats/go-cart-actor/pkg/cart"
+	"git.k6n.net/mats/go-cart-actor/pkg/checkout"
+	checkoutMessages "git.k6n.net/mats/go-cart-actor/proto/checkout"
+	"google.golang.org/protobuf/proto"
+)
+
+type testCheckoutApplier struct {
+	grains map[uint64]*checkout.CheckoutGrain
+}
+
+func newTestCheckoutApplier() *testCheckoutApplier {
+	return &testCheckoutApplier{
+		grains: make(map[uint64]*checkout.CheckoutGrain),
+	}
+}
+
+func (a *testCheckoutApplier) Get(_ context.Context, id uint64) (*checkout.CheckoutGrain, error) {
+	g, ok := a.grains[id]
+	if !ok {
+		g = checkout.NewCheckoutGrain(id, 0, 0, time.UnixMilli(1000000), nil)
+		a.grains[id] = g
+	}
+	return g, nil
+}
+
+func (a *testCheckoutApplier) Apply(ctx context.Context, id uint64, msgs ...proto.Message) (*actor.MutationResult[checkout.CheckoutGrain], error) {
+	g, err := a.Get(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+	for _, msg := range msgs {
+		switch m := msg.(type) {
+		case *checkoutMessages.InitializeCheckout:
+			g.CartId = cart.CartId(m.CartId)
+		}
+	}
+	return &actor.MutationResult[checkout.CheckoutGrain]{
+		Result: *g,
+	}, nil
+}
+
+func TestCheckoutHandler_CreateWithCartId(t *testing.T) {
+	// Start a mock Cart server.
+	mockCartServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if !strings.Contains(r.URL.Path, "/cart/byid/") {
+			t.Errorf("expected path to contain /cart/byid/, got %q", r.URL.Path)
+		}
+		// Return dummy cart state bytes.
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`{}`))
+	}))
+	defer mockCartServer.Close()
+
+	os.Setenv("CART_INTERNAL_URL", mockCartServer.URL)
+	defer os.Unsetenv("CART_INTERNAL_URL")
+
+	applier := newTestCheckoutApplier()
+	handler := CheckoutHandler(applier)
+
+	// Create a dummy cart ID.
+	dummyCartId, _ := cart.NewCartId()
+
+	reqBody := fmt.Sprintf(`{"cartId": %q}`, dummyCartId.String())
+	req := httptest.NewRequest("POST", "/", strings.NewReader(reqBody))
+	req.Header.Set("Content-Type", "application/json")
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusCreated {
+		t.Fatalf("expected 201 Created, got %d. Body: %s", rec.Code, rec.Body.String())
+	}
+
+	var resp CheckoutResponse
+	if err := json.NewDecoder(rec.Body).Decode(&resp); err != nil {
+		t.Fatalf("failed to decode response: %v", err)
+	}
+	if resp.Id == "" {
+		t.Fatal("expected non-empty checkout session id")
+	}
+	if resp.CartId != dummyCartId.String() {
+		t.Fatalf("expected cartId %q, got %q", dummyCartId.String(), resp.CartId)
+	}
+}
+
+func TestCheckoutHandler_CreateWithLineItemsFallback(t *testing.T) {
+	// Start a mock Cart server that supports both creating a cart and getting it.
+	var createdCartId cart.CartId
+	mockCartServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method == "POST" && strings.Contains(r.URL.Path, "/ucp/v1/carts/") {
+			var err error
+			createdCartId, err = cart.NewCartId()
+			if err != nil {
+				w.WriteHeader(http.StatusInternalServerError)
+				return
+			}
+			w.Header().Set("Content-Type", "application/json")
+			w.WriteHeader(http.StatusCreated)
+			w.Write([]byte(fmt.Sprintf(`{"id": %q, "status": "active"}`, createdCartId.String())))
+			return
+		}
+
+		if r.Method == "GET" && strings.Contains(r.URL.Path, "/cart/byid/") {
+			w.WriteHeader(http.StatusOK)
+			w.Write([]byte(`{}`))
+			return
+		}
+
+		w.WriteHeader(http.StatusNotFound)
+	}))
+	defer mockCartServer.Close()
+
+	os.Setenv("CART_INTERNAL_URL", mockCartServer.URL)
+	defer os.Unsetenv("CART_INTERNAL_URL")
+
+	applier := newTestCheckoutApplier()
+	handler := CheckoutHandler(applier)
+
+	// POST with line_items and no cartId.
+	reqBody := `{"line_items": [{"sku": "SKU123", "quantity": 2}]}`
+	req := httptest.NewRequest("POST", "/", strings.NewReader(reqBody))
+	req.Header.Set("Content-Type", "application/json")
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusCreated {
+		t.Fatalf("expected 201 Created, got %d. Body: %s", rec.Code, rec.Body.String())
+	}
+
+	var resp CheckoutResponse
+	if err := json.NewDecoder(rec.Body).Decode(&resp); err != nil {
+		t.Fatalf("failed to decode response: %v", err)
+	}
+	if resp.Id == "" {
+		t.Fatal("expected non-empty checkout session id")
+	}
+	if resp.CartId != createdCartId.String() {
+		t.Fatalf("expected cartId %q to match created implicit cart %q", resp.CartId, createdCartId.String())
+	}
+}
diff --git a/internal/ucp/types.go b/internal/ucp/types.go
index 57a5c6b..ff2247d 100644
--- a/internal/ucp/types.go
+++ b/internal/ucp/types.go
@@ -143,10 +143,11 @@ type Totals struct {
 
 // CreateCheckoutRequest is the body for POST /checkout-sessions.
 type CreateCheckoutRequest struct {
-	CartId string         `json:"cartId"`
-	Items  []CartItemInput `json:"items,omitempty"`
-	Country string        `json:"country,omitempty"`
-	Currency string       `json:"currency,omitempty"`
+	CartId    string          `json:"cartId"`
+	Items     []CartItemInput `json:"items,omitempty"`
+	LineItems []CartItemInput `json:"line_items,omitempty"`
+	Country   string          `json:"country,omitempty"`
+	Currency  string          `json:"currency,omitempty"`
 }
 
 // UpdateCheckoutRequest is the body for PUT /checkout-sessions/{id}.