diff --git a/shortcuts/doc/docs_update.go b/shortcuts/doc/docs_update.go
index 5c64b7cc..64298721 100644
--- a/shortcuts/doc/docs_update.go
+++ b/shortcuts/doc/docs_update.go
@@ -62,6 +62,9 @@ var DocsUpdate = common.Shortcut{
 		if needsSelection[mode] && selEllipsis == "" && selTitle == "" {
 			return common.FlagErrorf("--%s mode requires --selection-with-ellipsis or --selection-by-title", mode)
 		}
+		if err := validateSelectionByTitle(selTitle); err != nil {
+			return err
+		}
 
 		return nil
 	},
@@ -156,3 +159,17 @@ func normalizeBoardTokens(raw interface{}) []string {
 		return []string{}
 	}
 }
+
+func validateSelectionByTitle(title string) error {
+	if title == "" {
+		return nil
+	}
+	trimmed := strings.TrimSpace(title)
+	if strings.Contains(trimmed, "\n") || strings.Contains(trimmed, "\r") {
+		return common.FlagErrorf("--selection-by-title must be a single heading line (for example: '## Section')")
+	}
+	if strings.HasPrefix(trimmed, "#") {
+		return nil
+	}
+	return common.FlagErrorf("--selection-by-title must include markdown heading prefix '#'. Example: --selection-by-title '## Section'")
+}
diff --git a/shortcuts/doc/docs_update_test.go b/shortcuts/doc/docs_update_test.go
index d0845e36..b4270def 100644
--- a/shortcuts/doc/docs_update_test.go
+++ b/shortcuts/doc/docs_update_test.go
@@ -4,6 +4,7 @@ package doc
 
 import (
 	"reflect"
+	"strings"
 	"testing"
 )
 
@@ -76,3 +77,56 @@ func TestNormalizeDocsUpdateResult(t *testing.T) {
 		}
 	})
 }
+
+func TestValidateSelectionByTitle(t *testing.T) {
+	t.Run("empty title passes", func(t *testing.T) {
+		if err := validateSelectionByTitle(""); err != nil {
+			t.Fatalf("expected nil error, got %v", err)
+		}
+	})
+
+	t.Run("heading style title passes", func(t *testing.T) {
+		if err := validateSelectionByTitle("## 第二章"); err != nil {
+			t.Fatalf("expected nil error, got %v", err)
+		}
+	})
+
+	t.Run("plain text title fails with guidance", func(t *testing.T) {
+		err := validateSelectionByTitle("第二章")
+		if err == nil {
+			t.Fatalf("expected validation error")
+		}
+		if got := err.Error(); got == "" || !containsAll(got, "selection-by-title", "heading prefix") {
+			t.Fatalf("unexpected error: %v", err)
+		}
+	})
+
+	t.Run("multi-line heading still fails", func(t *testing.T) {
+		err := validateSelectionByTitle("## 第二章\n## 第三章")
+		if err == nil {
+			t.Fatalf("expected validation error")
+		}
+		if got := err.Error(); got == "" || !containsAll(got, "single heading line") {
+			t.Fatalf("unexpected error: %v", err)
+		}
+	})
+
+	t.Run("multi-line title fails", func(t *testing.T) {
+		err := validateSelectionByTitle("第二章\n第三章")
+		if err == nil {
+			t.Fatalf("expected validation error")
+		}
+		if got := err.Error(); got == "" || !containsAll(got, "single heading line") {
+			t.Fatalf("unexpected error: %v", err)
+		}
+	})
+}
+
+func containsAll(s string, tokens ...string) bool {
+	for _, token := range tokens {
+		if !strings.Contains(s, token) {
+			return false
+		}
+	}
+	return true
+}