api/src/main/java/com/velocitypowered/api/network/ProtocolVersion.java

@@ -96,7 +96,8 @@ public enum ProtocolVersion implements Ordered<ProtocolVersion> {
   MINECRAFT_1_21_9(773, "1.21.9", "1.21.10"),
   MINECRAFT_1_21_11(774, "1.21.11"),
   MINECRAFT_26_1(775, "26.1", "26.1.1", "26.1.2"),
-  MINECRAFT_26_2(776, "26.2");
+  MINECRAFT_26_2(776, "26.2"),
+  MINECRAFT_26_3(777, "26.3");
 
   private static final int SNAPSHOT_BIT = 30;
 

proxy/src/main/java/com/velocitypowered/proxy/connection/MinecraftSessionHandler.java

@@ -23,6 +23,7 @@ import com.velocitypowered.proxy.protocol.packet.BossBarPacket;
 import com.velocitypowered.proxy.protocol.packet.BundleDelimiterPacket;
 import com.velocitypowered.proxy.protocol.packet.ClientSettingsPacket;
 import com.velocitypowered.proxy.protocol.packet.ClientboundCookieRequestPacket;
+import com.velocitypowered.proxy.protocol.packet.ClientboundPostEffectsPacket;
 import com.velocitypowered.proxy.protocol.packet.ClientboundSoundEntityPacket;
 import com.velocitypowered.proxy.protocol.packet.ClientboundStopSoundPacket;
 import com.velocitypowered.proxy.protocol.packet.ClientboundStoreCookiePacket;
@@ -404,4 +405,8 @@ public interface MinecraftSessionHandler {
   default boolean handle(ClientboundStopSoundPacket packet) {
     return false;
   }
+
+  default boolean handle(ClientboundPostEffectsPacket packet) {
+    return false;
+  }
 }

proxy/src/main/java/com/velocitypowered/proxy/protocol/ProtocolUtils.java

@@ -143,6 +143,7 @@ public enum ProtocolUtils {
           .build();
 
   public static final int DEFAULT_MAX_STRING_SIZE = 65536; // 64KiB
+  private static final int DEFAULT_MAX_KEY_ARRAY_SIZE = 1024;
   private static final int MAXIMUM_VARINT_SIZE = 5;
   private static final BinaryTagType<? extends BinaryTag>[] BINARY_TAG_TYPES = new BinaryTagType[] {
       BinaryTagTypes.END, BinaryTagTypes.BYTE, BinaryTagTypes.SHORT, BinaryTagTypes.INT,
@@ -349,15 +350,20 @@ public enum ProtocolUtils {
     writeString(buf, key.asMinimalString());
   }
 
+  public static Key[] readKeyArray(ByteBuf buf) {
+    return readKeyArray(buf, DEFAULT_MAX_KEY_ARRAY_SIZE);
+  }
+
   /**
    * Reads a standard Mojang Text namespaced:key array from the buffer.
    *
    * @param buf the buffer to read from
    * @return the decoded key array
    */
-  public static Key[] readKeyArray(ByteBuf buf) {
+  public static Key[] readKeyArray(ByteBuf buf, int cap) {
     int length = readVarInt(buf);
     checkFrame(length >= 0, "Got a negative-length array (%s)", length);
+    checkFrame(length <= cap, "Bad key array size (got %s, maximum is %s)", length, cap);
     checkFrame(buf.isReadable(length),
         "Trying to read an array that is too long (wanted %s, only have %s)", length,
         buf.readableBytes());

proxy/src/main/java/com/velocitypowered/proxy/protocol/StateRegistry.java

@@ -47,6 +47,7 @@ import static com.velocitypowered.api.network.ProtocolVersion.MINECRAFT_1_8;
 import static com.velocitypowered.api.network.ProtocolVersion.MINECRAFT_1_9;
 import static com.velocitypowered.api.network.ProtocolVersion.MINECRAFT_1_9_4;
 import static com.velocitypowered.api.network.ProtocolVersion.MINECRAFT_26_1;
+import static com.velocitypowered.api.network.ProtocolVersion.MINECRAFT_26_3;
 import static com.velocitypowered.api.network.ProtocolVersion.MINIMUM_VERSION;
 import static com.velocitypowered.api.network.ProtocolVersion.SUPPORTED_VERSIONS;
 import static com.velocitypowered.proxy.protocol.ProtocolUtils.Direction;
@@ -60,6 +61,7 @@ import com.velocitypowered.proxy.protocol.packet.BossBarPacket;
 import com.velocitypowered.proxy.protocol.packet.BundleDelimiterPacket;
 import com.velocitypowered.proxy.protocol.packet.ClientSettingsPacket;
 import com.velocitypowered.proxy.protocol.packet.ClientboundCookieRequestPacket;
+import com.velocitypowered.proxy.protocol.packet.ClientboundPostEffectsPacket;
 import com.velocitypowered.proxy.protocol.packet.ClientboundSoundEntityPacket;
 import com.velocitypowered.proxy.protocol.packet.ClientboundStopSoundPacket;
 import com.velocitypowered.proxy.protocol.packet.ClientboundStoreCookiePacket;
@@ -234,31 +236,44 @@ public enum StateRegistry {
           map(0x06, MINECRAFT_1_20_2, false),
           map(0x07, MINECRAFT_1_20_3, false),
           map(0x09, MINECRAFT_1_20_5, false));
+      clientbound.register(
+          ClientboundPostEffectsPacket.class, ClientboundPostEffectsPacket::new,
+          map(0x0A, MINECRAFT_26_3, false));
       clientbound.register(
           ClientboundStoreCookiePacket.class, ClientboundStoreCookiePacket::new,
-          map(0x0A, MINECRAFT_1_20_5, false));
+          map(0x0A, MINECRAFT_1_20_5, false),
+          map(0x0B, MINECRAFT_26_3, false));
       clientbound.register(TransferPacket.class, TransferPacket::new,
-          map(0x0B, MINECRAFT_1_20_5, false));
+          map(0x0B, MINECRAFT_1_20_5, false),
+          map(0x0C, MINECRAFT_26_3, false));
       clientbound.register(ActiveFeaturesPacket.class, ActiveFeaturesPacket::new,
           map(0x07, MINECRAFT_1_20_2, false),
           map(0x08, MINECRAFT_1_20_3, false),
-          map(0x0C, MINECRAFT_1_20_5, false));
+          map(0x0C, MINECRAFT_1_20_5, false),
+          map(0x0D, MINECRAFT_26_3, false));
       clientbound.register(TagsUpdatePacket.class, TagsUpdatePacket::new,
           map(0x08, MINECRAFT_1_20_2, false),
           map(0x09, MINECRAFT_1_20_3, false),
-          map(0x0D, MINECRAFT_1_20_5, false));
+          map(0x0D, MINECRAFT_1_20_5, false),
+          map(0x0E, MINECRAFT_26_3, false));
       clientbound.register(KnownPacksPacket.class, KnownPacksPacket::new,
-          map(0x0E, MINECRAFT_1_20_5, false));
+          map(0x0E, MINECRAFT_1_20_5, false),
+          map(0x0F, MINECRAFT_26_3, false));
       clientbound.register(ClientboundCustomReportDetailsPacket.class, ClientboundCustomReportDetailsPacket::new,
-          map(0x0F, MINECRAFT_1_21, false));
+          map(0x0F, MINECRAFT_1_21, false),
+          map(0x10, MINECRAFT_26_3, false));
       clientbound.register(ClientboundServerLinksPacket.class, ClientboundServerLinksPacket::new,
-          map(0x10, MINECRAFT_1_21, false));
+          map(0x10, MINECRAFT_1_21, false),
+          map(0x11, MINECRAFT_26_3, false));
       clientbound.register(DialogClearPacket.class, () -> DialogClearPacket.INSTANCE,
-          map(0x11, MINECRAFT_1_21_6, false));
+          map(0x11, MINECRAFT_1_21_6, false),
+          map(0x12, MINECRAFT_26_3, false));
       clientbound.register(DialogShowPacket.class, () -> new DialogShowPacket(this),
-          map(0x12, MINECRAFT_1_21_6, false));
+          map(0x12, MINECRAFT_1_21_6, false),
+          map(0x13, MINECRAFT_26_3, false));
       clientbound.register(CodeOfConductPacket.class, CodeOfConductPacket::new,
-          map(0x13, MINECRAFT_1_21_9, false));
+          map(0x13, MINECRAFT_1_21_9, false),
+          map(0x14, MINECRAFT_26_3, false));
     }
   },
   PLAY {
@@ -410,7 +425,8 @@ public enum StateRegistry {
           map(0x2D, MINECRAFT_1_21_2, false),
           map(0x2F, MINECRAFT_1_21_4, false),
           map(0x30, MINECRAFT_1_21_6, false),
-          map(0x31, MINECRAFT_26_1, false));
+          map(0x31, MINECRAFT_26_1, false),
+          map(0x32, MINECRAFT_26_3, false));
       serverbound.register(
           FinishedUpdatePacket.class, () -> FinishedUpdatePacket.INSTANCE,
           map(0x0B, MINECRAFT_1_20_2, false),
@@ -469,6 +485,9 @@ public enum StateRegistry {
           ClientboundCookieRequestPacket.class, ClientboundCookieRequestPacket::new,
           map(0x16, MINECRAFT_1_20_5, false),
           map(0x15, MINECRAFT_1_21_5, false));
+      clientbound.register(
+          ClientboundPostEffectsPacket.class, ClientboundPostEffectsPacket::new,
+          map(0x53, MINECRAFT_26_3, false));
       clientbound.register(
           ClientboundSoundEntityPacket.class, ClientboundSoundEntityPacket::new,
           map(0x5D, MINECRAFT_1_19_3, true),
@@ -479,7 +498,8 @@ public enum StateRegistry {
           map(0x6E, MINECRAFT_1_21_2, true),
           map(0x6D, MINECRAFT_1_21_5, true),
           map(0x72, MINECRAFT_1_21_9, true),
-          map(0x74, MINECRAFT_26_1, true));
+          map(0x74, MINECRAFT_26_1, true),
+          map(0x76, MINECRAFT_26_3, true));
       clientbound.register(
           ClientboundStopSoundPacket.class, ClientboundStopSoundPacket::new,
           map(0x5F, MINECRAFT_1_19_3, true),
@@ -490,7 +510,8 @@ public enum StateRegistry {
           map(0x71, MINECRAFT_1_21_2, true),
           map(0x70, MINECRAFT_1_21_5, true),
           map(0x75, MINECRAFT_1_21_9, true),
-          map(0x77, MINECRAFT_26_1, true));
+          map(0x77, MINECRAFT_26_1, true),
+          map(0x79, MINECRAFT_26_3, true));
       clientbound.register(
           PluginMessagePacket.class,
           PluginMessagePacket::new,
@@ -548,7 +569,8 @@ public enum StateRegistry {
           map(0x27, MINECRAFT_1_21_2, false),
           map(0x26, MINECRAFT_1_21_5, false),
           map(0x2B, MINECRAFT_1_21_9, false),
-          map(0x2C, MINECRAFT_26_1, false));
+          map(0x2C, MINECRAFT_26_1, false),
+          map(0x2D, MINECRAFT_26_3, false));
       clientbound.register(
           JoinGamePacket.class,
           JoinGamePacket::new,
@@ -569,7 +591,8 @@ public enum StateRegistry {
           map(0x2C, MINECRAFT_1_21_2, false),
           map(0x2B, MINECRAFT_1_21_5, false),
           map(0x30, MINECRAFT_1_21_9, false),
-          map(0x31, MINECRAFT_26_1, false));
+          map(0x31, MINECRAFT_26_1, false),
+          map(0x32, MINECRAFT_26_3, false));
       clientbound.register(
           RespawnPacket.class,
           RespawnPacket::new,
@@ -593,7 +616,8 @@ public enum StateRegistry {
           map(0x4C, MINECRAFT_1_21_2, true),
           map(0x4B, MINECRAFT_1_21_5, true),
           map(0x50, MINECRAFT_1_21_9, true),
-          map(0x52, MINECRAFT_26_1, true));
+          map(0x52, MINECRAFT_26_1, true),
+          map(0x54, MINECRAFT_26_3, true));
       clientbound.register(
           RemoveResourcePackPacket.class,
           RemoveResourcePackPacket::new,
@@ -602,7 +626,8 @@ public enum StateRegistry {
           map(0x4A, MINECRAFT_1_21_2, false),
           map(0x49, MINECRAFT_1_21_5, false),
           map(0x4E, MINECRAFT_1_21_9, false),
-          map(0x50, MINECRAFT_26_1, false));
+          map(0x50, MINECRAFT_26_1, false),
+          map(0x51, MINECRAFT_26_3, false));
       clientbound.register(
           ResourcePackRequestPacket.class,
           ResourcePackRequestPacket::new,
@@ -626,7 +651,8 @@ public enum StateRegistry {
           map(0x4B, MINECRAFT_1_21_2, false),
           map(0x4A, MINECRAFT_1_21_5, false),
           map(0x4F, MINECRAFT_1_21_9, false),
-          map(0x51, MINECRAFT_26_1, false));
+          map(0x51, MINECRAFT_26_1, false),
+          map(0x52, MINECRAFT_26_3, false));
       clientbound.register(
           HeaderAndFooterPacket.class,
           HeaderAndFooterPacket::new,
@@ -651,7 +677,8 @@ public enum StateRegistry {
           map(0x74, MINECRAFT_1_21_2, true),
           map(0x73, MINECRAFT_1_21_5, true),
           map(0x78, MINECRAFT_1_21_9, true),
-          map(0x7A, MINECRAFT_26_1, true));
+          map(0x7A, MINECRAFT_26_1, true),
+          map(0x7D, MINECRAFT_26_3, true));
       clientbound.register(
           LegacyTitlePacket.class,
           LegacyTitlePacket::new,
@@ -675,7 +702,8 @@ public enum StateRegistry {
           map(0x6A, MINECRAFT_1_21_2, true),
           map(0x69, MINECRAFT_1_21_5, true),
           map(0x6E, MINECRAFT_1_21_9, true),
-          map(0x70, MINECRAFT_26_1, true));
+          map(0x70, MINECRAFT_26_1, true),
+          map(0x72, MINECRAFT_26_3, true));
       clientbound.register(
           TitleTextPacket.class,
           TitleTextPacket::new,
@@ -690,7 +718,8 @@ public enum StateRegistry {
           map(0x6C, MINECRAFT_1_21_2, true),
           map(0x6B, MINECRAFT_1_21_5, true),
           map(0x70, MINECRAFT_1_21_9, true),
-          map(0x72, MINECRAFT_26_1, true));
+          map(0x72, MINECRAFT_26_1, true),
+          map(0x74, MINECRAFT_26_3, true));
       clientbound.register(
           TitleActionbarPacket.class,
           TitleActionbarPacket::new,
@@ -705,7 +734,8 @@ public enum StateRegistry {
           map(0x51, MINECRAFT_1_21_2, true),
           map(0x50, MINECRAFT_1_21_5, true),
           map(0x55, MINECRAFT_1_21_9, true),
-          map(0x57, MINECRAFT_26_1, true));
+          map(0x57, MINECRAFT_26_1, true),
+          map(0x59, MINECRAFT_26_3, true));
       clientbound.register(
           TitleTimesPacket.class,
           TitleTimesPacket::new,
@@ -720,7 +750,8 @@ public enum StateRegistry {
           map(0x6D, MINECRAFT_1_21_2, true),
           map(0x6C, MINECRAFT_1_21_5, true),
           map(0x71, MINECRAFT_1_21_9, true),
-          map(0x73, MINECRAFT_26_1, true));
+          map(0x73, MINECRAFT_26_1, true),
+          map(0x75, MINECRAFT_26_3, true));
       clientbound.register(
           TitleClearPacket.class,
           TitleClearPacket::new,
@@ -752,7 +783,8 @@ public enum StateRegistry {
           map(0x3F, MINECRAFT_1_21_2, false),
           map(0x3E, MINECRAFT_1_21_5, false),
           map(0x43, MINECRAFT_1_21_9, false),
-          map(0x45, MINECRAFT_26_1, false));
+          map(0x45, MINECRAFT_26_1, false),
+          map(0x46, MINECRAFT_26_3, false));
       clientbound.register(
           UpsertPlayerInfoPacket.class,
           UpsertPlayerInfoPacket::new,
@@ -763,14 +795,16 @@ public enum StateRegistry {
           map(0x40, MINECRAFT_1_21_2, false),
           map(0x3F, MINECRAFT_1_21_5, false),
           map(0x44, MINECRAFT_1_21_9, false),
-          map(0x46, MINECRAFT_26_1, false));
+          map(0x46, MINECRAFT_26_1, false),
+          map(0x47, MINECRAFT_26_3, false));
       clientbound.register(
           ClientboundStoreCookiePacket.class, ClientboundStoreCookiePacket::new,
           map(0x6B, MINECRAFT_1_20_5, false),
           map(0x72, MINECRAFT_1_21_2, false),
           map(0x71, MINECRAFT_1_21_5, false),
           map(0x76, MINECRAFT_1_21_9, false),
-          map(0x78, MINECRAFT_26_1, false));
+          map(0x78, MINECRAFT_26_1, false),
+          map(0x7A, MINECRAFT_26_3, false));
       clientbound.register(
           SystemChatPacket.class,
           SystemChatPacket::new,
@@ -784,7 +818,8 @@ public enum StateRegistry {
           map(0x73, MINECRAFT_1_21_2, true),
           map(0x72, MINECRAFT_1_21_5, true),
           map(0x77, MINECRAFT_1_21_9, true),
-          map(0x79, MINECRAFT_26_1, true));
+          map(0x79, MINECRAFT_26_1, true),
+          map(0x7C, MINECRAFT_26_3, true));
       clientbound.register(
           PlayerChatCompletionPacket.class,
           PlayerChatCompletionPacket::new,
@@ -807,7 +842,8 @@ public enum StateRegistry {
           map(0x50, MINECRAFT_1_21_2, false),
           map(0x4F, MINECRAFT_1_21_5, false),
           map(0x54, MINECRAFT_1_21_9, false),
-          map(0x56, MINECRAFT_26_1, false));
+          map(0x56, MINECRAFT_26_1, false),
+          map(0x58, MINECRAFT_26_3, false));
       clientbound.register(
           StartUpdatePacket.class,
           () -> StartUpdatePacket.INSTANCE,
@@ -817,7 +853,8 @@ public enum StateRegistry {
           map(0x70, MINECRAFT_1_21_2, false),
           map(0x6F, MINECRAFT_1_21_5, false),
           map(0x74, MINECRAFT_1_21_9, false),
-          map(0x76, MINECRAFT_26_1, false));
+          map(0x76, MINECRAFT_26_1, false),
+          map(0x78, MINECRAFT_26_3, false));
       clientbound.register(
           BundleDelimiterPacket.class,
           () -> BundleDelimiterPacket.INSTANCE,
@@ -828,21 +865,24 @@ public enum StateRegistry {
           map(0x73, MINECRAFT_1_20_5, false),
           map(0x7A, MINECRAFT_1_21_2, false),
           map(0x7F, MINECRAFT_1_21_9, false),
-          map(0x81, MINECRAFT_26_1, false));
+          map(0x81, MINECRAFT_26_1, false),
+          map(0x84, MINECRAFT_26_3, false));
       clientbound.register(
           ClientboundCustomReportDetailsPacket.class,
           ClientboundCustomReportDetailsPacket::new,
           map(0x7A, MINECRAFT_1_21, false),
           map(0x81, MINECRAFT_1_21_2, false),
           map(0x86, MINECRAFT_1_21_9, false),
-          map(0x88, MINECRAFT_26_1, false));
+          map(0x88, MINECRAFT_26_1, false),
+          map(0x8B, MINECRAFT_26_3, false));
       clientbound.register(
           ClientboundServerLinksPacket.class,
           ClientboundServerLinksPacket::new,
           map(0x7B, MINECRAFT_1_21, false),
           map(0x82, MINECRAFT_1_21_2, false),
           map(0x87, MINECRAFT_1_21_9, false),
-          map(0x89, MINECRAFT_26_1, false));
+          map(0x89, MINECRAFT_26_1, false),
+          map(0x8C, MINECRAFT_26_3, false));
     }
   },
   LOGIN {

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/ClientboundPostEffectsPacket.java

@@ -0,0 +1,53 @@
+/*
+ * Copyright (C) 2018-2026 Velocity Contributors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package com.velocitypowered.proxy.protocol.packet;
+
+import com.velocitypowered.api.network.ProtocolVersion;
+import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
+import com.velocitypowered.proxy.protocol.MinecraftPacket;
+import com.velocitypowered.proxy.protocol.ProtocolUtils;
+import com.velocitypowered.proxy.protocol.ProtocolUtils.Direction;
+import io.netty.buffer.ByteBuf;
+import net.kyori.adventure.key.Key;
+
+public final class ClientboundPostEffectsPacket implements MinecraftPacket {
+
+  private Key[] postEffects;
+
+  public ClientboundPostEffectsPacket() {
+  }
+
+  public ClientboundPostEffectsPacket(Key[] postEffects) {
+    this.postEffects = postEffects;
+  }
+
+  @Override
+  public void decode(ByteBuf buf, Direction direction, ProtocolVersion protocolVersion) {
+    this.postEffects = ProtocolUtils.readKeyArray(buf);
+  }
+
+  @Override
+  public void encode(ByteBuf buf, Direction direction, ProtocolVersion protocolVersion) {
+    ProtocolUtils.writeKeyArray(buf, this.postEffects);
+  }
+
+  @Override
+  public boolean handle(MinecraftSessionHandler handler) {
+    return handler.handle(this);
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/JoinGamePacket.java

@@ -32,7 +32,7 @@ public class JoinGamePacket implements MinecraftPacket {
 
   private static final BinaryTagIO.Reader JOINGAME_READER = BinaryTagIO.reader(4 * 1024 * 1024);
   private int entityId;
-  private short gamemode;
+  private int gamemode;
   private int dimension;
   private long partialHashedSeed; // 1.15+
   private short difficulty;
@@ -47,7 +47,7 @@ public class JoinGamePacket implements MinecraftPacket {
   private CompoundBinaryTag registry; // 1.16+
   private DimensionInfo dimensionInfo; // 1.16+
   private CompoundBinaryTag currentDimensionData; // 1.16.2+
-  private short previousGamemode; // 1.16+
+  private int previousGamemode; // 1.16+
   private int simulationDistance; // 1.18+
   private @Nullable Pair<String, Long> lastDeathPosition; // 1.19+
   private int portalCooldown; // 1.20+
@@ -63,11 +63,11 @@ public class JoinGamePacket implements MinecraftPacket {
     this.entityId = entityId;
   }
 
-  public short getGamemode() {
+  public int getGamemode() {
     return gamemode;
   }
 
-  public void setGamemode(short gamemode) {
+  public void setGamemode(int gamemode) {
     this.gamemode = gamemode;
   }
 
@@ -131,11 +131,11 @@ public class JoinGamePacket implements MinecraftPacket {
     this.dimensionInfo = dimensionInfo;
   }
 
-  public short getPreviousGamemode() {
+  public int getPreviousGamemode() {
     return previousGamemode;
   }
 
-  public void setPreviousGamemode(short previousGamemode) {
+  public void setPreviousGamemode(int previousGamemode) {
     this.previousGamemode = previousGamemode;
   }
 
@@ -345,8 +345,13 @@ public class JoinGamePacket implements MinecraftPacket {
     String levelName = ProtocolUtils.readString(buf);
     this.partialHashedSeed = buf.readLong();
 
-    this.gamemode = buf.readByte();
+    if (version.noLessThan(ProtocolVersion.MINECRAFT_26_3)) {
-    this.previousGamemode = buf.readByte();
+      this.gamemode = ProtocolUtils.readVarInt(buf);
+      this.previousGamemode = ProtocolUtils.readVarInt(buf); // game mode + 1 or 0
+    } else {
+      this.gamemode = buf.readByte();
+      this.previousGamemode = buf.readByte(); // game mode or -1
+    }
 
     boolean isDebug = buf.readBoolean();
     boolean isFlat = buf.readBoolean();
@@ -498,8 +503,13 @@ public class JoinGamePacket implements MinecraftPacket {
     ProtocolUtils.writeString(buf, dimensionInfo.getLevelName());
     buf.writeLong(partialHashedSeed);
 
-    buf.writeByte(gamemode);
+    if (version.noLessThan(ProtocolVersion.MINECRAFT_26_3)) {
-    buf.writeByte(previousGamemode);
+      ProtocolUtils.writeVarInt(buf, this.gamemode);
+      ProtocolUtils.writeVarInt(buf, this.previousGamemode);
+    } else {
+      buf.writeByte(this.gamemode);
+      buf.writeByte(this.previousGamemode);
+    }
 
     buf.writeBoolean(dimensionInfo.isDebugType());
     buf.writeBoolean(dimensionInfo.isFlat());

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/RespawnPacket.java

@@ -33,11 +33,11 @@ public class RespawnPacket implements MinecraftPacket {
   private int dimension;
   private long partialHashedSeed;
   private short difficulty;
-  private short gamemode;
+  private int gamemode;
   private String levelType = "";
   private byte dataToKeep; // 1.16+
   private DimensionInfo dimensionInfo; // 1.16-1.16.1
-  private short previousGamemode; // 1.16+
+  private int previousGamemode; // 1.16+
   private CompoundBinaryTag currentDimensionData; // 1.16.2+
   private @Nullable Pair<String, Long> lastDeathPosition; // 1.19+
   private int portalCooldown; // 1.20+
@@ -46,9 +46,9 @@ public class RespawnPacket implements MinecraftPacket {
   public RespawnPacket() {
   }
 
-  public RespawnPacket(int dimension, long partialHashedSeed, short difficulty, short gamemode,
+  public RespawnPacket(int dimension, long partialHashedSeed, short difficulty, int gamemode,
                        String levelType, byte dataToKeep, DimensionInfo dimensionInfo,
-                       short previousGamemode, CompoundBinaryTag currentDimensionData,
+                       int previousGamemode, CompoundBinaryTag currentDimensionData,
                        @Nullable Pair<String, Long> lastDeathPosition, int portalCooldown,
                        int seaLevel) {
     this.dimension = dimension;
@@ -97,11 +97,11 @@ public class RespawnPacket implements MinecraftPacket {
     this.difficulty = difficulty;
   }
 
-  public short getGamemode() {
+  public int getGamemode() {
     return gamemode;
   }
 
-  public void setGamemode(short gamemode) {
+  public void setGamemode(int gamemode) {
     this.gamemode = gamemode;
   }
 
@@ -121,11 +121,11 @@ public class RespawnPacket implements MinecraftPacket {
     this.dataToKeep = dataToKeep;
   }
 
-  public short getPreviousGamemode() {
+  public int getPreviousGamemode() {
     return previousGamemode;
   }
 
-  public void setPreviousGamemode(short previousGamemode) {
+  public void setPreviousGamemode(int previousGamemode) {
     this.previousGamemode = previousGamemode;
   }
 
@@ -197,9 +197,17 @@ public class RespawnPacket implements MinecraftPacket {
     if (version.noLessThan(ProtocolVersion.MINECRAFT_1_15)) {
       this.partialHashedSeed = buf.readLong();
     }
-    this.gamemode = buf.readByte();
+    if (version.noLessThan(ProtocolVersion.MINECRAFT_26_3)) {
+      this.gamemode = ProtocolUtils.readVarInt(buf);
+    } else {
+      this.gamemode = buf.readByte();
+    }
     if (version.noLessThan(ProtocolVersion.MINECRAFT_1_16)) {
-      this.previousGamemode = buf.readByte();
+      if (version.noLessThan(ProtocolVersion.MINECRAFT_26_3)) {
+        this.previousGamemode = ProtocolUtils.readVarInt(buf);
+      } else {
+        this.previousGamemode = buf.readByte();
+      }
       boolean isDebug = buf.readBoolean();
       boolean isFlat = buf.readBoolean();
       this.dimensionInfo = new DimensionInfo(dimensionKey, levelName, isFlat, isDebug, version);
@@ -249,9 +257,17 @@ public class RespawnPacket implements MinecraftPacket {
     if (version.noLessThan(ProtocolVersion.MINECRAFT_1_15)) {
       buf.writeLong(partialHashedSeed);
     }
-    buf.writeByte(gamemode);
+    if (version.noLessThan(ProtocolVersion.MINECRAFT_26_3)) {
+      ProtocolUtils.writeVarInt(buf, this.gamemode);
+    } else {
+      buf.writeByte(this.gamemode);
+    }
     if (version.noLessThan(ProtocolVersion.MINECRAFT_1_16)) {
-      buf.writeByte(previousGamemode);
+      if (version.noLessThan(ProtocolVersion.MINECRAFT_26_3)) {
+        ProtocolUtils.writeVarInt(buf, this.previousGamemode);
+      } else {
+        buf.writeByte(this.previousGamemode);
+      }
       buf.writeBoolean(dimensionInfo.isDebugType());
       buf.writeBoolean(dimensionInfo.isFlat());
       if (version.lessThan(ProtocolVersion.MINECRAFT_1_19_3)) {

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/brigadier/ArgumentPropertyRegistry.java

@@ -25,6 +25,7 @@ import static com.velocitypowered.api.network.ProtocolVersion.MINECRAFT_1_20_5;
 import static com.velocitypowered.api.network.ProtocolVersion.MINECRAFT_1_21_5;
 import static com.velocitypowered.api.network.ProtocolVersion.MINECRAFT_1_21_6;
 import static com.velocitypowered.api.network.ProtocolVersion.MINECRAFT_26_2;
+import static com.velocitypowered.api.network.ProtocolVersion.MINECRAFT_26_3;
 import static com.velocitypowered.proxy.protocol.packet.brigadier.ArgumentIdentifier.id;
 import static com.velocitypowered.proxy.protocol.packet.brigadier.ArgumentIdentifier.mapSet;
 import static com.velocitypowered.proxy.protocol.packet.brigadier.DoubleArgumentPropertySerializer.DOUBLE;
@@ -273,16 +274,21 @@ public class ArgumentPropertyRegistry {
     empty(id("minecraft:heightmap", mapSet(MINECRAFT_1_21_6, 51), mapSet(MINECRAFT_1_21_5, 50), mapSet(MINECRAFT_1_20_3, 49),
         mapSet(MINECRAFT_1_19_4, 47))); // 1.19.4
 
-    empty(id("minecraft:uuid", mapSet(MINECRAFT_1_21_6, 56), mapSet(MINECRAFT_1_21_5, 54),mapSet(MINECRAFT_1_20_5, 53), mapSet(MINECRAFT_1_20_3, 48),
+    empty(id("minecraft:uuid", mapSet(MINECRAFT_26_3, 61), mapSet(MINECRAFT_1_21_6, 56), mapSet(MINECRAFT_1_21_5, 54), mapSet(MINECRAFT_1_20_5, 53),
-        mapSet(MINECRAFT_1_19_4, 48), mapSet(MINECRAFT_1_19, 47))); // added in 1.16
+        mapSet(MINECRAFT_1_20_3, 48), mapSet(MINECRAFT_1_19_4, 48), mapSet(MINECRAFT_1_19, 47))); // added in 1.16
 
     empty(id("minecraft:loot_table", mapSet(MINECRAFT_1_21_6, 52), mapSet(MINECRAFT_1_21_5, 51), mapSet(MINECRAFT_1_20_5, 50)));
     empty(id("minecraft:loot_predicate", mapSet(MINECRAFT_1_21_6, 53), mapSet(MINECRAFT_1_21_5, 52), mapSet(MINECRAFT_1_20_5, 51)));
     empty(id("minecraft:loot_modifier", mapSet(MINECRAFT_1_21_6, 54), mapSet(MINECRAFT_1_21_5, 53), mapSet(MINECRAFT_1_20_5, 52)));
 
     empty(id("minecraft:hex_color", mapSet(MINECRAFT_1_21_6, 17))); // added in 1.21.6
-    empty(id("minecraft:dialog", mapSet(MINECRAFT_1_21_6, 55))); // added in 1.21.6
+    empty(id("minecraft:dialog", mapSet(MINECRAFT_26_3, 58), mapSet(MINECRAFT_1_21_6, 55))); // added in 1.21.6
     empty(id("minecraft:team_color", mapSet(MINECRAFT_26_2, 16))); // renamed from color in 26.2
+    empty(id("minecraft:context_float_provider", mapSet(MINECRAFT_26_3, 55))); // added in 26.3
+    empty(id("minecraft:context_int_provider", mapSet(MINECRAFT_26_3, 56))); // added in 26.3
+    empty(id("minecraft:slot_source", mapSet(MINECRAFT_26_3, 57))); // added in 26.3
+    empty(id("minecraft:feature", mapSet(MINECRAFT_26_3, 59))); // added in 26.3
+    empty(id("minecraft:swing_animation", mapSet(MINECRAFT_26_3, 60))); // added in 26.3
 
     // Crossstitch support
     register(id("crossstitch:mod_argument", mapSet(MINECRAFT_1_19, -256)), ModArgumentProperty.class, MOD);