diff --git a/Lagrange.Proto.Test/NegativeVarIntTest.cs b/Lagrange.Proto.Test/NegativeVarIntTest.cs
new file mode 100644
index 000000000..056e708c0
--- /dev/null
+++ b/Lagrange.Proto.Test/NegativeVarIntTest.cs
@@ -0,0 +1,136 @@
+using Lagrange.Proto.Serialization;
+
+namespace Lagrange.Proto.Test;
+
+/// <summary>
+/// Regression tests for https://github.com/LagrangeDev/Lagrange.Core/issues/917
+/// Negative values of signed integer types with default (unsigned varint) handling
+/// corrupt the serialized payload.
+/// </summary>
+[TestFixture]
+public class NegativeVarIntTest
+{
+    [Test]
+    public void TestNegativeInt_Roundtrip_Reflection()
+    {
+        var obj = new NegativeVarIntMessage
+        {
+            IntField = default,
+            StringField1 = null,
+            StringField2 = null,
+            NegativeIntField = -1
+        };
+
+        byte[] bytes = ProtoSerializer.Serialize(obj);
+        var deserialized = ProtoSerializer.Deserialize<NegativeVarIntMessage>(bytes);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(deserialized.IntField, Is.EqualTo(obj.IntField));
+            Assert.That(deserialized.StringField1, Is.EqualTo(obj.StringField1));
+            Assert.That(deserialized.StringField2, Is.EqualTo(obj.StringField2));
+            Assert.That(deserialized.NegativeIntField, Is.EqualTo(obj.NegativeIntField));
+        });
+    }
+
+    [Test]
+    public void TestNegativeInt_Roundtrip_SourceGenerated()
+    {
+        var obj = new NegativeVarIntMessage
+        {
+            IntField = default,
+            StringField1 = null,
+            StringField2 = null,
+            NegativeIntField = -1
+        };
+
+        byte[] bytes = ProtoSerializer.SerializeProtoPackable(obj);
+        var deserialized = ProtoSerializer.DeserializeProtoPackable<NegativeVarIntMessage>(bytes);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(deserialized.IntField, Is.EqualTo(obj.IntField));
+            Assert.That(deserialized.StringField1, Is.EqualTo(obj.StringField1));
+            Assert.That(deserialized.StringField2, Is.EqualTo(obj.StringField2));
+            Assert.That(deserialized.NegativeIntField, Is.EqualTo(obj.NegativeIntField));
+        });
+    }
+
+    [Test]
+    public void TestNegativeInt_NestedObject_Roundtrip()
+    {
+        var obj = new NegativeVarIntNested
+        {
+            Inner = new NegativeVarIntMessage { IntField = 1, StringField1 = "t", StringField2 = "d", NegativeIntField = -1 }
+        };
+
+        byte[] bytes = ProtoSerializer.SerializeProtoPackable(obj);
+        var deserialized = ProtoSerializer.DeserializeProtoPackable<NegativeVarIntNested>(bytes);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(deserialized.Inner, Is.Not.Null);
+            Assert.That(deserialized.Inner!.IntField, Is.EqualTo(obj.Inner.IntField));
+            Assert.That(deserialized.Inner.NegativeIntField, Is.EqualTo(obj.Inner.NegativeIntField));
+        });
+    }
+
+    [Test]
+    public void TestNegativeValues_Boundary()
+    {
+        var obj = new NegativeVarIntBoundary
+        {
+            SByteValue = -1,
+            ShortValue = -1,
+            IntValue = -1,
+            LongValue = -1,
+            SByteMin = sbyte.MinValue,
+            ShortMin = short.MinValue,
+            IntMin = int.MinValue,
+            LongMin = long.MinValue
+        };
+
+        byte[] bytes = ProtoSerializer.SerializeProtoPackable(obj);
+        var deserialized = ProtoSerializer.DeserializeProtoPackable<NegativeVarIntBoundary>(bytes);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(deserialized.SByteValue, Is.EqualTo(obj.SByteValue));
+            Assert.That(deserialized.ShortValue, Is.EqualTo(obj.ShortValue));
+            Assert.That(deserialized.IntValue, Is.EqualTo(obj.IntValue));
+            Assert.That(deserialized.LongValue, Is.EqualTo(obj.LongValue));
+            Assert.That(deserialized.SByteMin, Is.EqualTo(obj.SByteMin));
+            Assert.That(deserialized.ShortMin, Is.EqualTo(obj.ShortMin));
+            Assert.That(deserialized.IntMin, Is.EqualTo(obj.IntMin));
+            Assert.That(deserialized.LongMin, Is.EqualTo(obj.LongMin));
+        });
+    }
+}
+
+[ProtoPackable]
+public partial class NegativeVarIntMessage
+{
+    [ProtoMember(1)] public int IntField { get; set; }
+    [ProtoMember(2)] public string? StringField1 { get; set; }
+    [ProtoMember(3)] public string? StringField2 { get; set; }
+    [ProtoMember(4)] public int NegativeIntField { get; set; }
+}
+
+[ProtoPackable]
+public partial class NegativeVarIntNested
+{
+    [ProtoMember(1)] public NegativeVarIntMessage? Inner { get; set; }
+}
+
+[ProtoPackable]
+public partial class NegativeVarIntBoundary
+{
+    [ProtoMember(1)] public sbyte SByteValue { get; set; }
+    [ProtoMember(2)] public short ShortValue { get; set; }
+    [ProtoMember(3)] public int IntValue { get; set; }
+    [ProtoMember(4)] public long LongValue { get; set; }
+    [ProtoMember(5)] public sbyte SByteMin { get; set; }
+    [ProtoMember(6)] public short ShortMin { get; set; }
+    [ProtoMember(7)] public int IntMin { get; set; }
+    [ProtoMember(8)] public long LongMin { get; set; }
+}
diff --git a/Lagrange.Proto/Primitives/ProtoWriter.cs b/Lagrange.Proto/Primitives/ProtoWriter.cs
index de938fdee..27eef16ae 100644
--- a/Lagrange.Proto/Primitives/ProtoWriter.cs
+++ b/Lagrange.Proto/Primitives/ProtoWriter.cs
@@ -97,7 +97,7 @@ public void EncodeVarInt<T>(T value) where T : unmanaged, INumber<T>
     {
         if (_memory.Length - BytesPending >= 10)
         {
-            if (value < T.CreateTruncating(0x80))
+            if (ulong.CreateTruncating(value) < 0x80) // signed negative values must not take the single-byte path
             {
                 Unsafe.Add(ref MemoryMarshal.GetReference(_memory.Span), BytesPending++) = byte.CreateTruncating(value);
                 return;
diff --git a/Lagrange.Proto/Utility/ProtoHelper.cs b/Lagrange.Proto/Utility/ProtoHelper.cs
index 9ca22e99d..e54405bf3 100644
--- a/Lagrange.Proto/Utility/ProtoHelper.cs
+++ b/Lagrange.Proto/Utility/ProtoHelper.cs
@@ -1,4 +1,4 @@
-using System.Numerics;
+using System.Numerics;
 using System.Runtime.CompilerServices;
 using System.Text;
 using Lagrange.Proto.Primitives;
@@ -40,12 +40,12 @@ public static unsafe int GetVarIntLength<T>(T value) where T : unmanaged, INumbe
         
         if (sizeof(T) <= 4)
         {
-            int leadingZeros = BitOperations.LeadingZeroCount(uint.CreateSaturating(value));
+            int leadingZeros = BitOperations.LeadingZeroCount(uint.CreateTruncating(value));
             return VarIntLengths32[leadingZeros];
         }
         else
         {
-            int leadingZeros = BitOperations.LeadingZeroCount(ulong.CreateSaturating(value));
+            int leadingZeros = BitOperations.LeadingZeroCount(ulong.CreateTruncating(value));
             return VarIntLengths64[leadingZeros];
         }
     }