diff --git a/backend/handlers.go b/backend/handlers.go
index c427bf1..025d670 100644
--- a/backend/handlers.go
+++ b/backend/handlers.go
@@ -60,6 +60,21 @@ func (h *bookmarkHandler) put(w http.ResponseWriter, r *http.Request) {
 			}
 		}
 	}
+
+	// An empty status is "no opinion" and Upsert keeps the stored bucket.
+	// Finishing a series is a web-UI decision, so the JSON API refuses it
+	// rather than trusting every client to leave it alone.
+	switch b.Status {
+	case "", statusReading, statusArchived:
+	case statusFinished:
+		http.Error(w, "status "+statusFinished+" can only be set from the web UI",
+			http.StatusBadRequest)
+		return
+	default:
+		http.Error(w, "invalid status", http.StatusBadRequest)
+		return
+	}
+
 	// Candidate timestamp, not a decision: Upsert keeps the stored one unless
 	// reading progress actually moved. Any client value is ignored.
 	b.UpdatedAt = time.Now().UnixMilli()
diff --git a/backend/main_test.go b/backend/main_test.go
index 90145bb..d199eb7 100644
--- a/backend/main_test.go
+++ b/backend/main_test.go
@@ -1,6 +1,11 @@
 package main
 
 import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"strings"
 	"testing"
 	"time"
 )
@@ -113,3 +118,45 @@ func TestLoadLatestPollClampsAndFallsBack(t *testing.T) {
 		})
 	}
 }
+
+func TestPutStatusValidation(t *testing.T) {
+	cases := []struct {
+		name   string
+		status string
+		want   int
+	}{
+		{"empty is no opinion", "", http.StatusOK},
+		{"reading", "reading", http.StatusOK},
+		{"archived", "archived", http.StatusOK},
+		{"finished is web-only", "finished", http.StatusBadRequest},
+		{"garbage", "dropped", http.StatusBadRequest},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			srv := newTestServer(t)
+			body := fmt.Sprintf(`{"title":"Solo","status":%q}`, tc.status)
+			req := auth(httptest.NewRequest(http.MethodPut, "/bookmarks/asura:solo",
+				strings.NewReader(body)))
+			rr := httptest.NewRecorder()
+			srv.ServeHTTP(rr, req)
+
+			if rr.Code != tc.want {
+				t.Fatalf("status = %d, want %d (body %s)", rr.Code, tc.want, rr.Body.String())
+			}
+			if tc.want != http.StatusOK {
+				return
+			}
+			var got Bookmark
+			if err := json.Unmarshal(rr.Body.Bytes(), &got); err != nil {
+				t.Fatalf("decode: %v", err)
+			}
+			want := tc.status
+			if want == "" {
+				want = "reading"
+			}
+			if got.Status != want {
+				t.Fatalf("stored status = %q, want %q", got.Status, want)
+			}
+		})
+	}
+}