diff --git a/src/StreamJsonRpc.Analyzers/GeneratorModels/InterfaceModel.cs b/src/StreamJsonRpc.Analyzers/GeneratorModels/InterfaceModel.cs
index 96b43572..53108042 100644
--- a/src/StreamJsonRpc.Analyzers/GeneratorModels/InterfaceModel.cs
+++ b/src/StreamJsonRpc.Analyzers/GeneratorModels/InterfaceModel.cs
@@ -18,7 +18,7 @@ namespace StreamJsonRpc.Analyzers.GeneratorModels;
 /// <param name="Methods">The methods in the interface.</param>
 /// <param name="Events">The events in the interface.</param>
 /// <param name="HasUnsupportedMemberTypes">Indicates whether the interface has additional members that are not supported.</param>
-internal record InterfaceModel(string FullName, string Name, ImmutableEquatableArray<string> TypeParameters, Container? Container, ImmutableEquatableArray<MethodModel> Methods, ImmutableEquatableArray<EventModel> Events, bool HasUnsupportedMemberTypes)
+internal record InterfaceModel(string FullName, string Name, ImmutableEquatableArray<(VarianceKind Variance, string Identifier)> TypeParameters, Container? Container, ImmutableEquatableArray<MethodModel> Methods, ImmutableEquatableArray<EventModel> Events, bool HasUnsupportedMemberTypes)
 {
     internal required bool IsPartial { get; init; }
 
@@ -77,7 +77,7 @@ internal static InterfaceModel Create(INamedTypeSymbol iface, KnownSymbols symbo
         return new InterfaceModel(
             iface.ToDisplayString(ProxyGenerator.FullyQualifiedNoGlobalWithNullableFormat),
             iface.Name,
-            [.. iface.TypeParameters.Select(tp => tp.Name)],
+            [.. iface.TypeParameters.Select(tp => (tp.Variance, tp.Name))],
             Container.CreateFor((INamespaceOrTypeSymbol?)iface.ContainingType ?? iface.ContainingNamespace, cancellationToken),
             methods.ToImmutableEquatableArray(),
             events.ToImmutableEquatableArray(),
diff --git a/src/StreamJsonRpc.Analyzers/GeneratorModels/ProxyModel.cs b/src/StreamJsonRpc.Analyzers/GeneratorModels/ProxyModel.cs
index fa085795..50071307 100644
--- a/src/StreamJsonRpc.Analyzers/GeneratorModels/ProxyModel.cs
+++ b/src/StreamJsonRpc.Analyzers/GeneratorModels/ProxyModel.cs
@@ -74,7 +74,7 @@ internal ProxyModel(ImmutableEquatableSet<InterfaceModel> interfaces, string? ex
     internal void WriteInterfaceMapping(SourceWriter writer, InterfaceModel iface)
     {
         string genericTypeParameters = iface.TypeParameters.Length > 0
-            ? $"<{string.Join(", ", iface.TypeParameters)}>"
+            ? $"<{string.Join(", ", iface.TypeParameters.Select(WriteTypeParameter))}>"
             : string.Empty;
         writer.WriteLine($$"""
             [global::StreamJsonRpc.Reflection.JsonRpcProxyMappingAttribute(typeof({{ProxyGenerator.GenerationNamespace}}.{{this.Name}}{{this.GenericTypeDefinitionSuffix}}))]
@@ -291,4 +291,12 @@ private static string CreateProxyName(ImmutableEquatableSet<InterfaceModel> inte
         string additionalInterfaceHashString = Convert.ToBase64String(additionalInterfaceHash).TrimEnd('=').Replace('+', '_').Replace('/', '_');
         return $"{sorted[0]}{additionalInterfaceHashString[..8]}";
     }
+
+    private static string WriteTypeParameter((VarianceKind Variance, string Identifier) typeParameter) => typeParameter.Variance switch
+    {
+        VarianceKind.None => typeParameter.Identifier,
+        VarianceKind.In => $"in {typeParameter.Identifier}",
+        VarianceKind.Out => $"out {typeParameter.Identifier}",
+        _ => throw new InvalidOperationException($"Unknown variance kind: {typeParameter.Variance}."),
+    };
 }
diff --git a/test/StreamJsonRpc.Analyzer.Tests/ProxyGeneratorTests.cs b/test/StreamJsonRpc.Analyzer.Tests/ProxyGeneratorTests.cs
index de1f6204..630c4825 100644
--- a/test/StreamJsonRpc.Analyzer.Tests/ProxyGeneratorTests.cs
+++ b/test/StreamJsonRpc.Analyzer.Tests/ProxyGeneratorTests.cs
@@ -375,6 +375,30 @@ public partial interface IGenericMarshalable<T>
             """);
     }
 
+    [Fact]
+    public async Task RpcMarshalable_Generic_WithInModifier()
+    {
+        await VerifyCS.RunDefaultAsync("""
+            [RpcMarshalable]
+            public partial interface IGenericMarshalable<in T>
+            {
+                Task DoSomethingWithParameterAsync(T parameter);
+            }
+            """);
+    }
+
+    [Fact]
+    public async Task RpcMarshalable_Generic_WithOutModifier()
+    {
+        await VerifyCS.RunDefaultAsync("""
+            [RpcMarshalable]
+            public partial interface IGenericMarshalable<out T>
+            {
+                Task DoSomethingWithParameterAsync();
+            }
+            """);
+    }
+
     [Fact]
     public async Task RpcMarshalable_GenericWithClosedPrescriptions()
     {
diff --git a/test/StreamJsonRpc.Analyzer.Tests/Resources/RpcMarshalable_Generic_WithInModifier/IGenericMarshalable_T_.g.cs b/test/StreamJsonRpc.Analyzer.Tests/Resources/RpcMarshalable_Generic_WithInModifier/IGenericMarshalable_T_.g.cs
new file mode 100644
index 00000000..f0a406ed
--- /dev/null
+++ b/test/StreamJsonRpc.Analyzer.Tests/Resources/RpcMarshalable_Generic_WithInModifier/IGenericMarshalable_T_.g.cs
@@ -0,0 +1,56 @@
+// <auto-generated/>
+
+#nullable enable
+#pragma warning disable CS0436 // prefer local types to imported ones
+
+[global::StreamJsonRpc.Reflection.JsonRpcProxyMappingAttribute(typeof(StreamJsonRpc.Generated.IGenericMarshalable_Proxy<>))]
+partial interface IGenericMarshalable<in T>
+{
+}
+
+namespace StreamJsonRpc.Generated
+{
+	
+	[global::System.CodeDom.Compiler.GeneratedCodeAttribute("StreamJsonRpc.Analyzers", "x.x.x.x")]
+	internal class IGenericMarshalable_Proxy<T> : global::StreamJsonRpc.Reflection.ProxyBase
+		, global::IGenericMarshalable<T>
+	{
+		
+		private static readonly global::System.Collections.Generic.IReadOnlyDictionary<string, global::System.Type> DoSomethingWithParameterAsyncNamedArgumentDeclaredTypes1 = new global::System.Collections.Generic.Dictionary<string, global::System.Type>
+		{
+			["parameter"] = typeof(T),
+		};
+		
+		private static readonly global::System.Collections.Generic.IReadOnlyList<global::System.Type> DoSomethingWithParameterAsyncPositionalArgumentDeclaredTypes1 = new global::System.Collections.Generic.List<global::System.Type>
+		{
+			typeof(T),
+		};
+		
+		private string? transformedDoSomethingWithParameterAsync1;
+		
+		public IGenericMarshalable_Proxy(global::StreamJsonRpc.JsonRpc client, global::StreamJsonRpc.Reflection.ProxyInputs inputs)
+		    : base(client, inputs)
+		{
+		}
+		
+		global::System.Threading.Tasks.Task global::IGenericMarshalable<T>.DoSomethingWithParameterAsync(T parameter)
+		{
+			if (this.IsDisposed) throw new global::System.ObjectDisposedException(this.GetType().FullName);
+			
+			this.OnCallingMethod("DoSomethingWithParameterAsync");
+			string rpcMethodName = this.transformedDoSomethingWithParameterAsync1 ??= this.TransformMethodName("DoSomethingWithParameterAsync", typeof(global::IGenericMarshalable<T>));
+			global::System.Threading.Tasks.Task result = this.Options.ServerRequiresNamedArguments ?
+			    this.JsonRpc.InvokeWithParameterObjectAsync(rpcMethodName, ConstructNamedArgs(), DoSomethingWithParameterAsyncNamedArgumentDeclaredTypes1, default) :
+			    this.JsonRpc.InvokeWithCancellationAsync(rpcMethodName, [parameter], DoSomethingWithParameterAsyncPositionalArgumentDeclaredTypes1, default);
+			this.OnCalledMethod("DoSomethingWithParameterAsync");
+			
+			return result;
+			
+			global::System.Collections.Generic.Dictionary<string, object?> ConstructNamedArgs()
+			    => new()
+			    {
+					["parameter"] = parameter,
+				};
+		}
+	}
+}
diff --git a/test/StreamJsonRpc.Analyzer.Tests/Resources/RpcMarshalable_Generic_WithOutModifier/IGenericMarshalable_T_.g.cs b/test/StreamJsonRpc.Analyzer.Tests/Resources/RpcMarshalable_Generic_WithOutModifier/IGenericMarshalable_T_.g.cs
new file mode 100644
index 00000000..0e8127c0
--- /dev/null
+++ b/test/StreamJsonRpc.Analyzer.Tests/Resources/RpcMarshalable_Generic_WithOutModifier/IGenericMarshalable_T_.g.cs
@@ -0,0 +1,53 @@
+// <auto-generated/>
+
+#nullable enable
+#pragma warning disable CS0436 // prefer local types to imported ones
+
+[global::StreamJsonRpc.Reflection.JsonRpcProxyMappingAttribute(typeof(StreamJsonRpc.Generated.IGenericMarshalable_Proxy<>))]
+partial interface IGenericMarshalable<out T>
+{
+}
+
+namespace StreamJsonRpc.Generated
+{
+	
+	[global::System.CodeDom.Compiler.GeneratedCodeAttribute("StreamJsonRpc.Analyzers", "x.x.x.x")]
+	internal class IGenericMarshalable_Proxy<T> : global::StreamJsonRpc.Reflection.ProxyBase
+		, global::IGenericMarshalable<T>
+	{
+		
+		private static readonly global::System.Collections.Generic.IReadOnlyDictionary<string, global::System.Type> DoSomethingWithParameterAsyncNamedArgumentDeclaredTypes1 = new global::System.Collections.Generic.Dictionary<string, global::System.Type>
+		{
+		};
+		
+		private static readonly global::System.Collections.Generic.IReadOnlyList<global::System.Type> DoSomethingWithParameterAsyncPositionalArgumentDeclaredTypes1 = new global::System.Collections.Generic.List<global::System.Type>
+		{
+		};
+		
+		private string? transformedDoSomethingWithParameterAsync1;
+		
+		public IGenericMarshalable_Proxy(global::StreamJsonRpc.JsonRpc client, global::StreamJsonRpc.Reflection.ProxyInputs inputs)
+		    : base(client, inputs)
+		{
+		}
+		
+		global::System.Threading.Tasks.Task global::IGenericMarshalable<T>.DoSomethingWithParameterAsync()
+		{
+			if (this.IsDisposed) throw new global::System.ObjectDisposedException(this.GetType().FullName);
+			
+			this.OnCallingMethod("DoSomethingWithParameterAsync");
+			string rpcMethodName = this.transformedDoSomethingWithParameterAsync1 ??= this.TransformMethodName("DoSomethingWithParameterAsync", typeof(global::IGenericMarshalable<T>));
+			global::System.Threading.Tasks.Task result = this.Options.ServerRequiresNamedArguments ?
+			    this.JsonRpc.InvokeWithParameterObjectAsync(rpcMethodName, ConstructNamedArgs(), DoSomethingWithParameterAsyncNamedArgumentDeclaredTypes1, default) :
+			    this.JsonRpc.InvokeWithCancellationAsync(rpcMethodName, [], DoSomethingWithParameterAsyncPositionalArgumentDeclaredTypes1, default);
+			this.OnCalledMethod("DoSomethingWithParameterAsync");
+			
+			return result;
+			
+			global::System.Collections.Generic.Dictionary<string, object?> ConstructNamedArgs()
+			    => new()
+			    {
+				};
+		}
+	}
+}