From 6d3c58b29f0b2c3c3e1015bc182ef1adce94a96e Mon Sep 17 00:00:00 2001
From: BruceChen <hi@brucec.cn>
Date: Sat, 21 Mar 2026 14:11:23 +0800
Subject: [PATCH] fix: handle LpVec3 movement encoding in SpawnEntity for
 1.21.9+

Minecraft 1.21.9 changed the SpawnEntity (Add Entity) packet layout:
the velocity/movement field was moved before the angle fields and
switched from 3 x Short to a new variable-length LpVec3 encoding.

Add ReadNextLpVec3() to consume the LpVec3 wire format (1 byte header,
optionally 5+ more bytes with a continuation VarInt), and update
ReadNextEntity() to use the new field order for protocol >= 773.

Made-with: Cursor
---
 .../Protocol/Handlers/DataTypes.cs            | 61 +++++++++++++++----
 1 file changed, 49 insertions(+), 12 deletions(-)

diff --git a/MinecraftClient/Protocol/Handlers/DataTypes.cs b/MinecraftClient/Protocol/Handlers/DataTypes.cs
index db6b8c1e..d669fc68 100644
--- a/MinecraftClient/Protocol/Handlers/DataTypes.cs
+++ b/MinecraftClient/Protocol/Handlers/DataTypes.cs
@@ -559,11 +559,32 @@ namespace MinecraftClient.Protocol.Handlers
             int data = -1;
             byte entityPitch, entityYaw;
 
-            if (living)
+            if (protocolversion >= Protocol18Handler.MC_1_21_9_Version)
+            {
+                // 1.21.9+: LpVec3 movement before angles, unified format
+                ReadNextLpVec3(cache);                  // Movement (LpVec3)
+                entityPitch = ReadNextByte(cache);      // xRot
+                entityYaw = ReadNextByte(cache);        // yRot
+                ReadNextByte(cache);                    // yHeadRot
+                data = ReadNextVarInt(cache);            // Data
+            }
+            else if (living)
             {
                 entityYaw = ReadNextByte(cache); // Yaw
                 entityPitch = ReadNextByte(cache); // Pitch
                 entityPitch = ReadNextByte(cache); // Head Pitch
+
+                // Velocity (3 shorts)
+                if (protocolversion < Protocol18Handler.MC_1_9_Version)
+                {
+                    // no velocity for living entities in <1.9
+                }
+                else
+                {
+                    ReadNextShort(cache);
+                    ReadNextShort(cache);
+                    ReadNextShort(cache);
+                }
             }
             else
             {
@@ -577,24 +598,24 @@ namespace MinecraftClient.Protocol.Handlers
                 data = protocolversion >= Protocol18Handler.MC_1_19_Version
                     ? ReadNextVarInt(cache)
                     : ReadNextInt(cache);
-            }
 
-            // In 1.8 those 3 fields for Velocity are optional
-            if (protocolversion < Protocol18Handler.MC_1_9_Version)
-            {
-                if (data != 0)
+                // Velocity (3 shorts)
+                if (protocolversion < Protocol18Handler.MC_1_9_Version)
+                {
+                    if (data != 0)
+                    {
+                        ReadNextShort(cache);
+                        ReadNextShort(cache);
+                        ReadNextShort(cache);
+                    }
+                }
+                else
                 {
                     ReadNextShort(cache);
                     ReadNextShort(cache);
                     ReadNextShort(cache);
                 }
             }
-            else
-            {
-                ReadNextShort(cache);
-                ReadNextShort(cache);
-                ReadNextShort(cache);
-            }
 
             return new Entity(entityID, entityType, new Location(entityX, entityY, entityZ), entityYaw, entityPitch,
                 data);
@@ -945,6 +966,22 @@ namespace MinecraftClient.Protocol.Handlers
             }
         }
 
+        /// <summary>
+        /// Read an LpVec3 (low-precision vec3) from the cache (1.21.9+).
+        /// Variable-length encoding: first byte 0 = zero vector; otherwise
+        /// 2 bytes + 4 bytes (6 total), plus an optional VarInt continuation.
+        /// </summary>
+        public void ReadNextLpVec3(Queue<byte> cache)
+        {
+            int first = ReadNextByte(cache);
+            if (first == 0)
+                return;
+            ReadNextByte(cache);        // second byte
+            ReadData(4, cache);         // uint32
+            if ((first & 4) == 4)       // continuation bit set
+                ReadNextVarInt(cache);
+        }
+
         /// <summary>
         /// Consume bytes for a ResolvableProfile (1.21.9+).
         /// Wire: Either(GameProfile, Partial) + PlayerSkin.Patch