PaperMC/paper-server/patches/unapplied/net/minecraft/network/chat/ComponentUtils.java.patch

--- a/net/minecraft/network/chat/ComponentUtils.java
+++ b/net/minecraft/network/chat/ComponentUtils.java
@@ -33,14 +33,39 @@
         }
     }
 
+    @io.papermc.paper.annotation.DoNotUse // Paper - validate separators - right now this method is only used for separator evaluation. Error on build if this changes to re-evaluate.
     public static Optional<MutableComponent> updateForEntity(@Nullable CommandSourceStack source, Optional<Component> text, @Nullable Entity sender, int depth) throws CommandSyntaxException {
         return text.isPresent() ? Optional.of(updateForEntity(source, text.get(), sender, depth)) : Optional.empty();
     }
 
+    // Paper start - validate separator
+    public static Optional<MutableComponent> updateSeparatorForEntity(@Nullable CommandSourceStack source, Optional<Component> text, @Nullable Entity sender, int depth) throws CommandSyntaxException {
+        if (text.isEmpty() || !isValidSelector(text.get())) return Optional.empty();
+        return Optional.of(updateForEntity(source, text.get(), sender, depth));
+    }
+    public static boolean isValidSelector(final Component component) {
+        final ComponentContents contents = component.getContents();
+
+        if (contents instanceof net.minecraft.network.chat.contents.NbtContents || contents instanceof net.minecraft.network.chat.contents.SelectorContents) return false;
+        if (contents instanceof final net.minecraft.network.chat.contents.TranslatableContents translatableContents) {
+            for (final Object arg : translatableContents.getArgs()) {
+                if (arg instanceof final Component argumentAsComponent && !isValidSelector(argumentAsComponent)) return false;
+            }
+        }
+
+        return true;
+    }
+    // Paper end - validate separator
+
     public static MutableComponent updateForEntity(@Nullable CommandSourceStack source, Component text, @Nullable Entity sender, int depth) throws CommandSyntaxException {
         if (depth > 100) {
             return text.copy();
         } else {
+            // Paper start - adventure; pass actual vanilla component
+            if (text instanceof io.papermc.paper.adventure.AdventureComponent adventureComponent) {
+                text = adventureComponent.deepConverted();
+            }
+            // Paper end - adventure; pass actual vanilla component
             MutableComponent mutableComponent = text.getContents().resolve(source, sender, depth + 1);
 
             for (Component component : text.getSiblings()) {