diff --git a/proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/brigadier/ArgumentPropertyRegistry.java b/proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/brigadier/ArgumentPropertyRegistry.java
index c175f8dc4..b98365d3a 100644
--- a/proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/brigadier/ArgumentPropertyRegistry.java
+++ b/proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/brigadier/ArgumentPropertyRegistry.java
@@ -5,6 +5,7 @@ import static com.velocitypowered.proxy.protocol.packet.brigadier.EmptyArgumentP
 import static com.velocitypowered.proxy.protocol.packet.brigadier.FloatArgumentPropertySerializer.FLOAT;
 import static com.velocitypowered.proxy.protocol.packet.brigadier.IntegerArgumentPropertySerializer.INTEGER;
 import static com.velocitypowered.proxy.protocol.packet.brigadier.LongArgumentPropertySerializer.LONG;
+import static com.velocitypowered.proxy.protocol.packet.brigadier.ModArgumentPropertySerializer.MOD;
 import static com.velocitypowered.proxy.protocol.packet.brigadier.StringArgumentPropertySerializer.STRING;
 
 import com.mojang.brigadier.arguments.ArgumentType;
@@ -76,6 +77,10 @@ public class ArgumentPropertyRegistry {
       if (property.getResult() != null) {
         property.getSerializer().serialize(property.getResult(), buf);
       }
+    } else if (type instanceof ModArgumentProperty) {
+      ModArgumentProperty property = (ModArgumentProperty) type;
+      ProtocolUtils.writeString(buf, property.getIdentifier());
+      buf.writeBytes(property.getData());
     } else {
       ArgumentPropertySerializer serializer = byClass.get(type.getClass());
       String id = classToId.get(type.getClass());
@@ -98,6 +103,9 @@ public class ArgumentPropertyRegistry {
         GenericArgumentPropertySerializer.create(BoolArgumentType::bool));
     register("brigadier:long", LongArgumentType.class, LONG);
 
+    // Crossstitch support
+    register("crossstitch:mod_argument", ModArgumentProperty.class, MOD);
+
     // Minecraft argument types with extra properties
     empty("minecraft:entity", ByteArgumentPropertySerializer.BYTE);
     empty("minecraft:score_holder", ByteArgumentPropertySerializer.BYTE);
diff --git a/proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/brigadier/ModArgumentProperty.java b/proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/brigadier/ModArgumentProperty.java
new file mode 100644
index 000000000..f82133260
--- /dev/null
+++ b/proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/brigadier/ModArgumentProperty.java
@@ -0,0 +1,47 @@
+package com.velocitypowered.proxy.protocol.packet.brigadier;
+
+import com.mojang.brigadier.StringReader;
+import com.mojang.brigadier.arguments.ArgumentType;
+import com.mojang.brigadier.context.CommandContext;
+import com.mojang.brigadier.exceptions.CommandSyntaxException;
+import com.mojang.brigadier.suggestion.Suggestions;
+import com.mojang.brigadier.suggestion.SuggestionsBuilder;
+import io.netty.buffer.ByteBuf;
+import io.netty.buffer.Unpooled;
+import java.util.Collection;
+import java.util.concurrent.CompletableFuture;
+
+public class ModArgumentProperty implements ArgumentType<ByteBuf> {
+
+  private final String identifier;
+  private final ByteBuf data;
+
+  public ModArgumentProperty(String identifier, ByteBuf data) {
+    this.identifier = identifier;
+    this.data = Unpooled.unreleasableBuffer(data.asReadOnly());
+  }
+
+  public String getIdentifier() {
+    return identifier;
+  }
+
+  public ByteBuf getData() {
+    return data.slice();
+  }
+
+  @Override
+  public ByteBuf parse(StringReader reader) throws CommandSyntaxException {
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
+  public <S> CompletableFuture<Suggestions> listSuggestions(CommandContext<S> context,
+      SuggestionsBuilder builder) {
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
+  public Collection<String> getExamples() {
+    throw new UnsupportedOperationException();
+  }
+}
diff --git a/proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/brigadier/ModArgumentPropertySerializer.java b/proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/brigadier/ModArgumentPropertySerializer.java
new file mode 100644
index 000000000..6c6eebbd0
--- /dev/null
+++ b/proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/brigadier/ModArgumentPropertySerializer.java
@@ -0,0 +1,27 @@
+package com.velocitypowered.proxy.protocol.packet.brigadier;
+
+import com.velocitypowered.proxy.protocol.ProtocolUtils;
+import io.netty.buffer.ByteBuf;
+import io.netty.buffer.Unpooled;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
+class ModArgumentPropertySerializer implements ArgumentPropertySerializer<ModArgumentProperty> {
+  static final ModArgumentPropertySerializer MOD = new ModArgumentPropertySerializer();
+
+  private ModArgumentPropertySerializer() {
+
+  }
+
+  @Override
+  public @Nullable ModArgumentProperty deserialize(ByteBuf buf) {
+    String identifier = ProtocolUtils.readString(buf);
+    byte[] extraData = ProtocolUtils.readByteArray(buf);
+    return new ModArgumentProperty(identifier, Unpooled.wrappedBuffer(extraData));
+  }
+
+  @Override
+  public void serialize(ModArgumentProperty object, ByteBuf buf) {
+    // This is special-cased by ArgumentPropertyRegistry
+    throw new UnsupportedOperationException();
+  }
+}