Far from done.

src/main/java/io/github/rektroth/whiteout/mixin/mc235045/CommandManagerMixin.java

@@ -78,8 +78,12 @@ private void changeSuggestions(
 
 		if (requiredArgumentBuilder.getSuggestionsProvider() != null) {
 			requiredArgumentBuilder.suggests(SuggestionProviders.getLocalProvider(requiredArgumentBuilder.getSuggestionsProvider()));
-			registeredAskServerSuggestionsForTree = requiredArgumentBuilder.getSuggestionsProvider() == SuggestionProviders.ASK_SERVER;
-		} else if (!registeredAskServerSuggestionsForTree && requiredArgumentBuilder.getType() instanceof EntityArgumentType) {
+			registeredAskServerSuggestionsForTree =
+				requiredArgumentBuilder.getSuggestionsProvider() == SuggestionProviders.ASK_SERVER;
+		} else if (
+			!registeredAskServerSuggestionsForTree &&
+			requiredArgumentBuilder.getType() instanceof EntityArgumentType
+		) {
 			requiredArgumentBuilder.suggests(requiredArgumentBuilder.getType()::listSuggestions);
 			registeredAskServerSuggestionsForTree = true;
 		}

src/main/java/io/github/rektroth/whiteout/mixin/mc235045/EntitySelectorOptionsMixin.java

@@ -0,0 +1,118 @@
+/*
+ * Patch for MC-235045
+ *
+ * Authored for CraftBukkit/Spigot by Jake Potrebic <[email protected]> on May 13, 2020.
+ * Ported to Fabric by Rektroth <[email protected]> on August 25, 2024.
+ */
+
+package io.github.rektroth.whiteout.mixin.mc235045;
+
+import com.mojang.brigadier.exceptions.DynamicCommandExceptionType;
+import net.minecraft.command.CommandSource;
+import net.minecraft.command.EntitySelectorOptions;
+
+import net.minecraft.command.EntitySelectorReader;
+import net.minecraft.entity.EntityType;
+import net.minecraft.registry.Registries;
+import net.minecraft.registry.RegistryKeys;
+import net.minecraft.registry.tag.TagKey;
+import net.minecraft.text.Text;
+import net.minecraft.util.Identifier;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Shadow;
+import org.spongepowered.asm.mixin.Unique;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Redirect;
+
+import java.util.Objects;
+import java.util.function.Predicate;
+
+@Mixin(EntitySelectorOptions.class)
+public abstract class EntitySelectorOptionsMixin {
+	@Unique
+	private static final DynamicCommandExceptionType ERROR_ENTITY_TAG_INVALID = new DynamicCommandExceptionType((object) -> {
+		return Text.of("Invalid or unknown entity type tag '" + object + "'");
+		// missing hover text "You can disable this error in 'whiteout.properties'"
+	});
+
+	@Shadow
+	private static void putOption(
+		String id,
+		EntitySelectorOptions.SelectorHandler handler,
+		Predicate<EntitySelectorReader> condition,
+		Text description
+	) { }
+
+	@Redirect(
+		at = @At(
+			ordinal = 15,
+			target = "Lnet/minecraft/command/EntitySelectorOptions;putOption(Ljava/lang/String;Lnet/minecraft/command/EntitySelectorOptions$SelectorHandler;Ljava/util/function/Predicate;Lnet/minecraft/text/Text;)V",
+			value = "INVOKE"
+		),
+		method = "register"
+	)
+	private static void fixedPutOption(
+		String id,
+		EntitySelectorOptions.SelectorHandler handler,
+		Predicate<EntitySelectorReader> condition,
+		Text description
+	) {
+		putOption("type", (reader) -> {
+			reader.setSuggestionProvider((builder, consumer) -> {
+				CommandSource.suggestIdentifiers(Registries.ENTITY_TYPE.getIds(), builder, String.valueOf('!'));
+				CommandSource.suggestIdentifiers(Registries.ENTITY_TYPE.streamTags().map(TagKey::id), builder, "!#");
+
+				if (!reader.excludesEntityType()) {
+					CommandSource.suggestIdentifiers(Registries.ENTITY_TYPE.getIds(), builder);
+					CommandSource.suggestIdentifiers(Registries.ENTITY_TYPE.streamTags().map(TagKey::id), builder, String.valueOf('#'));
+				}
+
+				return builder.buildFuture();
+			});
+
+			int i = reader.getReader().getCursor();
+			boolean bl = reader.readNegationCharacter();
+
+			if (reader.excludesEntityType() && !bl) {
+				reader.getReader().setCursor(i);
+				throw EntitySelectorOptions.INAPPLICABLE_OPTION_EXCEPTION
+					.createWithContext(reader.getReader(), "type");
+			} else {
+				if (bl) {
+					reader.setExcludesEntityType();
+				}
+
+				if (reader.readTagCharacter()) {
+					TagKey<EntityType<?>> tagKey = TagKey.of(
+						RegistryKeys.ENTITY_TYPE,
+						Identifier.fromCommandInput(reader.getReader()));
+
+					if (/*reader.parsingEntityArgumentSuggestions &&*/ Registries.ENTITY_TYPE.getEntryList(tagKey).isEmpty()) {
+						reader.getReader().setCursor(i);
+						throw ERROR_ENTITY_TAG_INVALID.createWithContext(reader.getReader(), tagKey);
+					}
+
+					reader.addPredicate((entity) -> entity.getType().isIn(tagKey) != bl);
+				} else {
+					Identifier identifier = Identifier.fromCommandInput(reader.getReader());
+					EntityType<?> entityType = Registries.ENTITY_TYPE.getOrEmpty(identifier).orElseThrow(() -> {
+						reader.getReader().setCursor(i);
+						return EntitySelectorOptions.INVALID_TYPE_EXCEPTION
+							.createWithContext(reader.getReader(), identifier.toString());
+					});
+
+					if (Objects.equals(EntityType.PLAYER, entityType) && !bl) {
+						reader.setIncludesNonPlayers(false);
+					}
+
+					reader.addPredicate((entity) -> Objects.equals(entityType, entity.getType()) != bl);
+
+					if (!bl) {
+						reader.setEntityType(entityType);
+					}
+				}
+
+			}
+		}, (reader) -> !reader.selectsEntityType(), Text.translatable("argument.entity.options.type.description"));
+	}
+}

src/main/java/io/github/rektroth/whiteout/mixin/mc235045/EntitySelectorReaderMixin.java

@@ -0,0 +1,17 @@
+/*
+ * Patch for MC-235045
+ *
+ * Authored for CraftBukkit/Spigot by Jake Potrebic <[email protected]> on May 13, 2020.
+ * Ported to Fabric by Rektroth <[email protected]> on August 25, 2024.
+ */
+
+package io.github.rektroth.whiteout.mixin.mc235045;
+
+import net.minecraft.command.EntitySelectorReader;
+
+import org.spongepowered.asm.mixin.Mixin;
+
+@Mixin(EntitySelectorReader.class)
+public abstract class EntitySelectorReaderMixin {
+	// nothing yet
+}

src/main/resources/whiteout.mixins.json

@@ -30,6 +30,8 @@
     "mc224454.AzaleaBlockMixin",
     "mc235045.CommandManagerMixin",
     "mc235045.EntityArgumentTypeMixin",
+    "mc235045.EntitySelectorOptionsMixin",
+    "mc235045.EntitySelectorReaderMixin",
     "mc235045.ServerCommandSourceMixin",
     "mc248588.LeveledCauldronBlockMixin",
     "mc253721.DeOpCommandMixin",