diff --git a/src/AcDream.App/Input/PlayerMovementController.cs b/src/AcDream.App/Input/PlayerMovementController.cs
index bfdaa69..e96269c 100644
--- a/src/AcDream.App/Input/PlayerMovementController.cs
+++ b/src/AcDream.App/Input/PlayerMovementController.cs
@@ -32,7 +32,8 @@ public readonly record struct MovementResult(
     float? ForwardSpeed,
     float? SidestepSpeed,
     float? TurnSpeed,
-    float? JumpExtent = null);  // non-null when a jump was triggered this frame
+    float? JumpExtent = null,       // non-null when a jump was triggered this frame
+    Vector3? JumpVelocity = null);  // world-space launch velocity (sent in jump packet)
 
 /// <summary>
 /// Portal-space state for the player movement controller.
@@ -259,6 +260,7 @@ public sealed class PlayerMovementController
         // Release to execute: jump(extent) validates + sets JumpExtent,
         // then LeaveGround() applies the scaled velocity via get_leave_ground_velocity.
         float? outJumpExtent = null;
+        Vector3? outJumpVelocity = null;
 
         if (input.Jump && _body.OnWalkable)
         {
@@ -278,6 +280,7 @@ public sealed class PlayerMovementController
             {
                 _motion.LeaveGround();
                 outJumpExtent = _jumpExtent;
+                outJumpVelocity = _body.Velocity; // capture after LeaveGround applies it
             }
             _jumpCharging = false;
             _jumpExtent   = 0f;
@@ -411,6 +414,7 @@ public sealed class PlayerMovementController
             ForwardSpeed: outForwardSpeed,
             SidestepSpeed: outSidestepSpeed,
             TurnSpeed: outTurnSpeed,
-            JumpExtent: outJumpExtent);
+            JumpExtent: outJumpExtent,
+            JumpVelocity: outJumpVelocity);
     }
 }
diff --git a/src/AcDream.App/Rendering/GameWindow.cs b/src/AcDream.App/Rendering/GameWindow.cs
index 981070b..d425bcb 100644
--- a/src/AcDream.App/Rendering/GameWindow.cs
+++ b/src/AcDream.App/Rendering/GameWindow.cs
@@ -1936,11 +1936,18 @@ public sealed class GameWindow : IDisposable
                     _liveSession.SendGameAction(body);
                 }
 
-                if (result.JumpExtent.HasValue)
+                if (result.JumpExtent.HasValue && result.JumpVelocity.HasValue)
                 {
-                    // TODO: send jump packet to server (format needs research from holtburger).
-                    // Local jump physics work without server acknowledgment for now.
-                    Console.WriteLine($"jump: extent={result.JumpExtent.Value:F2}");
+                    var seq = _liveSession.NextGameActionSequence();
+                    var jumpBody = AcDream.Core.Net.Messages.JumpAction.Build(
+                        gameActionSequence:     seq,
+                        extent:                 result.JumpExtent.Value,
+                        velocity:               result.JumpVelocity.Value,
+                        instanceSequence:       _liveSession.InstanceSequence,
+                        serverControlSequence:  _liveSession.ServerControlSequence,
+                        teleportSequence:       _liveSession.TeleportSequence,
+                        forcePositionSequence:  _liveSession.ForcePositionSequence);
+                    _liveSession.SendGameAction(jumpBody);
                 }
             }
 
diff --git a/src/AcDream.Core.Net/Messages/JumpAction.cs b/src/AcDream.Core.Net/Messages/JumpAction.cs
new file mode 100644
index 0000000..d5bc6d7
--- /dev/null
+++ b/src/AcDream.Core.Net/Messages/JumpAction.cs
@@ -0,0 +1,57 @@
+using System.Numerics;
+using AcDream.Core.Net.Packets;
+
+namespace AcDream.Core.Net.Messages;
+
+/// <summary>
+/// Outbound <c>GameAction(Jump)</c> message — opcode <c>0xF61B</c>.
+/// Sent when the player releases the spacebar to jump. The server uses
+/// the extent + velocity to validate the jump and replicate it to nearby
+/// clients.
+///
+/// Wire layout (from holtburger JumpActionData::pack):
+///   u32 0xF7B1 (GameAction envelope)
+///   u32 sequence
+///   u32 0xF61B (Jump sub-opcode)
+///   f32 extent (0.0–1.0 charge power)
+///   f32 velocity.x, f32 velocity.y, f32 velocity.z
+///   u16 instanceSequence
+///   u16 serverControlSequence
+///   u16 teleportSequence
+///   u16 forcePositionSequence
+///   u32 objectGuid (0 for normal jump)
+///   u32 spellId (0 for normal jump)
+/// </summary>
+public static class JumpAction
+{
+    public const uint GameActionOpcode = 0xF7B1u;
+    public const uint JumpOpcode       = 0xF61Bu;
+
+    public static byte[] Build(
+        uint gameActionSequence,
+        float extent,
+        Vector3 velocity,
+        ushort instanceSequence,
+        ushort serverControlSequence,
+        ushort teleportSequence,
+        ushort forcePositionSequence)
+    {
+        var w = new PacketWriter(48);
+
+        w.WriteUInt32(GameActionOpcode);
+        w.WriteUInt32(gameActionSequence);
+        w.WriteUInt32(JumpOpcode);
+        w.WriteFloat(extent);
+        w.WriteFloat(velocity.X);
+        w.WriteFloat(velocity.Y);
+        w.WriteFloat(velocity.Z);
+        w.WriteUInt16(instanceSequence);
+        w.WriteUInt16(serverControlSequence);
+        w.WriteUInt16(teleportSequence);
+        w.WriteUInt16(forcePositionSequence);
+        w.WriteUInt32(0); // objectGuid — 0 for normal jump
+        w.WriteUInt32(0); // spellId — 0 for normal jump
+
+        return w.ToArray();
+    }
+}