diff --git a/src/Common/Polyfills/System/Collections/Generic/CollectionExtensions.cs b/src/Common/Polyfills/System/Collections/Generic/CollectionExtensions.cs
index ae4f697bc..fe5e09931 100644
--- a/src/Common/Polyfills/System/Collections/Generic/CollectionExtensions.cs
+++ b/src/Common/Polyfills/System/Collections/Generic/CollectionExtensions.cs
@@ -1,3 +1,4 @@
+#if !NET
 using ModelContextProtocol;
 
 namespace System.Collections.Generic;
@@ -18,4 +19,5 @@ public static TValue GetValueOrDefault<TKey, TValue>(this IReadOnlyDictionary<TK
 
     public static Dictionary<TKey, TValue> ToDictionary<TKey, TValue>(this IEnumerable<KeyValuePair<TKey, TValue>> source) =>
         source.ToDictionary(kv => kv.Key, kv => kv.Value);
-}
\ No newline at end of file
+}
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/DynamicallyAccessedMemberTypes.cs b/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/DynamicallyAccessedMemberTypes.cs
index ee6fa51a3..fcb09e4ff 100644
--- a/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/DynamicallyAccessedMemberTypes.cs
+++ b/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/DynamicallyAccessedMemberTypes.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 namespace System.Diagnostics.CodeAnalysis;
 
 /// <summary>
@@ -162,3 +163,4 @@ internal enum DynamicallyAccessedMemberTypes
     /// </summary>
     All = ~None
 }
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/DynamicallyAccessedMembersAttribute.cs b/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/DynamicallyAccessedMembersAttribute.cs
index 2d0140477..c99bb8e0a 100644
--- a/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/DynamicallyAccessedMembersAttribute.cs
+++ b/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/DynamicallyAccessedMembersAttribute.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 namespace System.Diagnostics.CodeAnalysis;
 
 /// <summary>
@@ -48,3 +49,4 @@ public DynamicallyAccessedMembersAttribute(DynamicallyAccessedMemberTypes member
     /// </summary>
     public DynamicallyAccessedMemberTypes MemberTypes { get; }
 }
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/NullableAttributes.cs b/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/NullableAttributes.cs
index ef577a9ca..0e7425e01 100644
--- a/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/NullableAttributes.cs
+++ b/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/NullableAttributes.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 namespace System.Diagnostics.CodeAnalysis
 {
     /// <summary>Specifies that null is allowed as an input even if the corresponding type disallows it.</summary>
@@ -137,3 +138,4 @@ public MemberNotNullWhenAttribute(bool returnValue, params string[] members)
         public string[] Members { get; }
     }
 }
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/RequiresDynamicCodeAttribute.cs b/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/RequiresDynamicCodeAttribute.cs
index 817ec6eaa..554a699a4 100644
--- a/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/RequiresDynamicCodeAttribute.cs
+++ b/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/RequiresDynamicCodeAttribute.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 namespace System.Diagnostics.CodeAnalysis;
 
 /// <summary>
@@ -36,3 +37,4 @@ public RequiresDynamicCodeAttribute(string message)
     /// </summary>
     public string? Url { get; set; }
 }
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/RequiresUnreferencedCode.cs b/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/RequiresUnreferencedCode.cs
index 3e845a534..eb91908ec 100644
--- a/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/RequiresUnreferencedCode.cs
+++ b/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/RequiresUnreferencedCode.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 namespace System.Diagnostics.CodeAnalysis;
 
 /// <summary>
@@ -37,3 +38,4 @@ public RequiresUnreferencedCodeAttribute(string message)
     /// </summary>
     public string? Url { get; set; }
 }
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/SetsRequiredMembersAttribute.cs b/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/SetsRequiredMembersAttribute.cs
index 83d6793b3..b778c95fc 100644
--- a/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/SetsRequiredMembersAttribute.cs
+++ b/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/SetsRequiredMembersAttribute.cs
@@ -1,5 +1,7 @@
+#if !NET
 namespace System.Diagnostics.CodeAnalysis
 {
     [AttributeUsage(AttributeTargets.Constructor, AllowMultiple = false, Inherited = false)]
     internal sealed class SetsRequiredMembersAttribute : Attribute;
 }
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/StringSyntaxAttribute.cs b/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/StringSyntaxAttribute.cs
index a8ab9bd28..1b884fa14 100644
--- a/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/StringSyntaxAttribute.cs
+++ b/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/StringSyntaxAttribute.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 namespace System.Diagnostics.CodeAnalysis;
 
 /// <summary>Specifies the syntax used in a string.</summary>
@@ -65,4 +66,5 @@ public StringSyntaxAttribute(string syntax, params object?[] arguments)
 
     /// <summary>The syntax identifier for strings containing XML.</summary>
     public const string Xml = nameof(Xml);
-}
\ No newline at end of file
+}
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/UnconditionalSuppressMessageAttribute.cs b/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/UnconditionalSuppressMessageAttribute.cs
index b06d9ed1a..db80655a7 100644
--- a/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/UnconditionalSuppressMessageAttribute.cs
+++ b/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/UnconditionalSuppressMessageAttribute.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 namespace System.Diagnostics.CodeAnalysis;
 
 /// <summary>
@@ -81,4 +82,5 @@ public UnconditionalSuppressMessageAttribute(string category, string checkId)
     /// Gets or sets the justification for suppressing the code analysis message.
     /// </summary>
     public string? Justification { get; set; }
-}
\ No newline at end of file
+}
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/IO/StreamExtensions.cs b/src/Common/Polyfills/System/IO/StreamExtensions.cs
index 4dc8e2a5a..452b80321 100644
--- a/src/Common/Polyfills/System/IO/StreamExtensions.cs
+++ b/src/Common/Polyfills/System/IO/StreamExtensions.cs
@@ -3,6 +3,7 @@
 using System.Runtime.InteropServices;
 using System.Text;
 
+#if !NET
 namespace System.IO;
 
 internal static class StreamExtensions
@@ -61,4 +62,5 @@ static async ValueTask<int> ReadAsyncCore(Stream stream, Memory<byte> buffer, Ca
             }
         }
     }
-}
\ No newline at end of file
+}
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/IO/TextWriterExtensions.cs b/src/Common/Polyfills/System/IO/TextWriterExtensions.cs
index 637cc09b0..a8dabd1fc 100644
--- a/src/Common/Polyfills/System/IO/TextWriterExtensions.cs
+++ b/src/Common/Polyfills/System/IO/TextWriterExtensions.cs
@@ -1,3 +1,4 @@
+#if !NET
 namespace System.IO;
 
 internal static class TextWriterExtensions
@@ -7,4 +8,5 @@ public static async Task FlushAsync(this TextWriter writer, CancellationToken ca
         cancellationToken.ThrowIfCancellationRequested();
         await writer.FlushAsync();
     }
-}
\ No newline at end of file
+}
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/Net/Http/HttpClientExtensions.cs b/src/Common/Polyfills/System/Net/Http/HttpClientExtensions.cs
index 85612b1d5..96a2948fb 100644
--- a/src/Common/Polyfills/System/Net/Http/HttpClientExtensions.cs
+++ b/src/Common/Polyfills/System/Net/Http/HttpClientExtensions.cs
@@ -1,3 +1,4 @@
+#if !NET
 using ModelContextProtocol;
 
 namespace System.Net.Http;
@@ -19,4 +20,5 @@ public static async Task<string> ReadAsStringAsync(this HttpContent content, Can
         cancellationToken.ThrowIfCancellationRequested();
         return await content.ReadAsStringAsync();
     }
-}
\ No newline at end of file
+}
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/PasteArguments.cs b/src/Common/Polyfills/System/PasteArguments.cs
index 32eb4c69f..d838ec023 100644
--- a/src/Common/Polyfills/System/PasteArguments.cs
+++ b/src/Common/Polyfills/System/PasteArguments.cs
@@ -5,6 +5,7 @@
 // https://github.com/dotnet/runtime/blob/d2650b6ae7023a2d9d2c74c56116f1f18472ab04/src/libraries/System.Private.CoreLib/src/System/PasteArguments.cs
 // and changed from using ValueStringBuilder to StringBuilder.
 
+#if !NET
 using System.Text;
 
 namespace System;
@@ -98,4 +99,5 @@ private static bool ContainsNoWhitespaceOrQuotes(string s)
 
     private const char Quote = '\"';
     private const char Backslash = '\\';
-}
\ No newline at end of file
+}
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/Runtime/CompilerServices/CallerArgumentExpressionAttribute.cs b/src/Common/Polyfills/System/Runtime/CompilerServices/CallerArgumentExpressionAttribute.cs
index 968c31e8a..553afbea1 100644
--- a/src/Common/Polyfills/System/Runtime/CompilerServices/CallerArgumentExpressionAttribute.cs
+++ b/src/Common/Polyfills/System/Runtime/CompilerServices/CallerArgumentExpressionAttribute.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 namespace System.Runtime.CompilerServices;
 
 [AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false, Inherited = false)]
@@ -13,3 +14,4 @@ public CallerArgumentExpressionAttribute(string parameterName)
 
     public string ParameterName { get; }
 }
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/Runtime/CompilerServices/CompilerFeatureRequiredAttribute.cs b/src/Common/Polyfills/System/Runtime/CompilerServices/CompilerFeatureRequiredAttribute.cs
index 12f3e5d2d..1df9c1c21 100644
--- a/src/Common/Polyfills/System/Runtime/CompilerServices/CompilerFeatureRequiredAttribute.cs
+++ b/src/Common/Polyfills/System/Runtime/CompilerServices/CompilerFeatureRequiredAttribute.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 namespace System.Runtime.CompilerServices
 {
     /// <summary>
@@ -30,3 +31,4 @@ public CompilerFeatureRequiredAttribute(string featureName)
         public const string RequiredMembers = nameof(RequiredMembers);
     }
 }
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/Runtime/CompilerServices/DefaultInterpolatedStringHandler.cs b/src/Common/Polyfills/System/Runtime/CompilerServices/DefaultInterpolatedStringHandler.cs
index 244f0875e..24622096f 100644
--- a/src/Common/Polyfills/System/Runtime/CompilerServices/DefaultInterpolatedStringHandler.cs
+++ b/src/Common/Polyfills/System/Runtime/CompilerServices/DefaultInterpolatedStringHandler.cs
@@ -5,6 +5,7 @@
 // https://github.com/dotnet/runtime/blob/dd75c45c123055baacd7aa4418f425f412797a29/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/DefaultInterpolatedStringHandler.cs
 // and then modified to build on netstandard2.0.
 
+#if !NET
 using System.Buffers;
 using System.Diagnostics;
 using System.Globalization;
@@ -614,4 +615,5 @@ private static uint Clamp(uint value, uint min, uint max)
             return value;
         }
     }
-}
\ No newline at end of file
+}
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/Runtime/CompilerServices/IsExternalInit.cs b/src/Common/Polyfills/System/Runtime/CompilerServices/IsExternalInit.cs
index 70443090c..9ae535381 100644
--- a/src/Common/Polyfills/System/Runtime/CompilerServices/IsExternalInit.cs
+++ b/src/Common/Polyfills/System/Runtime/CompilerServices/IsExternalInit.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 using System.ComponentModel;
 
 namespace System.Runtime.CompilerServices
@@ -12,3 +13,8 @@ namespace System.Runtime.CompilerServices
     [EditorBrowsable(EditorBrowsableState.Never)]
     internal static class IsExternalInit;
 }
+#else
+// The compiler emits a reference to the internal copy of this type in the non-.NET builds,
+// so we must include a forward to be compatible.
+[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Runtime.CompilerServices.IsExternalInit))]
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/Runtime/CompilerServices/RequiredMemberAttribute.cs b/src/Common/Polyfills/System/Runtime/CompilerServices/RequiredMemberAttribute.cs
index 6930dc4f1..35b6948a7 100644
--- a/src/Common/Polyfills/System/Runtime/CompilerServices/RequiredMemberAttribute.cs
+++ b/src/Common/Polyfills/System/Runtime/CompilerServices/RequiredMemberAttribute.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 using System.ComponentModel;
 
 namespace System.Runtime.CompilerServices
@@ -10,3 +11,4 @@ namespace System.Runtime.CompilerServices
     [EditorBrowsable(EditorBrowsableState.Never)]
     internal sealed class RequiredMemberAttribute : Attribute;
 }
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/Threading/CancellationTokenSourceExtensions.cs b/src/Common/Polyfills/System/Threading/CancellationTokenSourceExtensions.cs
index 95acac96c..d5508153f 100644
--- a/src/Common/Polyfills/System/Threading/CancellationTokenSourceExtensions.cs
+++ b/src/Common/Polyfills/System/Threading/CancellationTokenSourceExtensions.cs
@@ -1,3 +1,4 @@
+#if !NET
 using ModelContextProtocol;
 
 namespace System.Threading.Tasks;
@@ -11,4 +12,5 @@ public static Task CancelAsync(this CancellationTokenSource cancellationTokenSou
         cancellationTokenSource.Cancel();
         return Task.CompletedTask;
     }
-}
\ No newline at end of file
+}
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/Threading/Channels/ChannelExtensions.cs b/src/Common/Polyfills/System/Threading/Channels/ChannelExtensions.cs
index 89822eff1..6cda43e94 100644
--- a/src/Common/Polyfills/System/Threading/Channels/ChannelExtensions.cs
+++ b/src/Common/Polyfills/System/Threading/Channels/ChannelExtensions.cs
@@ -1,3 +1,4 @@
+#if !NET
 using System.Runtime.CompilerServices;
 
 namespace System.Threading.Channels;
@@ -14,4 +15,5 @@ public static async IAsyncEnumerable<T> ReadAllAsync<T>(this ChannelReader<T> re
             }
         }
     }
-}
\ No newline at end of file
+}
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/Threading/ForceYielding.cs b/src/Common/Polyfills/System/Threading/ForceYielding.cs
index a25baa977..4ce99c87c 100644
--- a/src/Common/Polyfills/System/Threading/ForceYielding.cs
+++ b/src/Common/Polyfills/System/Threading/ForceYielding.cs
@@ -1,3 +1,4 @@
+#if !NET
 using System.Runtime.CompilerServices;
 
 namespace System.Threading;
@@ -14,4 +15,5 @@ namespace System.Threading;
     public void OnCompleted(Action continuation) => ThreadPool.QueueUserWorkItem(a => ((Action)a!)(), continuation);
     public void UnsafeOnCompleted(Action continuation) => ThreadPool.UnsafeQueueUserWorkItem(a => ((Action)a!)(), continuation);
     public void GetResult() { }
-}
\ No newline at end of file
+}
+#endif
\ No newline at end of file
diff --git a/src/Common/Polyfills/System/Threading/Tasks/TaskExtensions.cs b/src/Common/Polyfills/System/Threading/Tasks/TaskExtensions.cs
index bee89a25d..68eb073d7 100644
--- a/src/Common/Polyfills/System/Threading/Tasks/TaskExtensions.cs
+++ b/src/Common/Polyfills/System/Threading/Tasks/TaskExtensions.cs
@@ -1,3 +1,4 @@
+#if !NET
 using ModelContextProtocol;
 
 namespace System.Threading.Tasks;
@@ -49,4 +50,5 @@ public static async Task WaitAsync(this Task task, TimeSpan timeout, Cancellatio
 
         await task.ConfigureAwait(false);
     }
-}
\ No newline at end of file
+}
+#endif
\ No newline at end of file
diff --git a/src/Directory.Build.props b/src/Directory.Build.props
index b8408bacd..7859ba39a 100644
--- a/src/Directory.Build.props
+++ b/src/Directory.Build.props
@@ -23,6 +23,10 @@
     <PackageReference Include="Microsoft.SourceLink.GitHub" PrivateAssets="All" />
   </ItemGroup>
 
+  <ItemGroup>
+    <Compile Include="..\Common\Polyfills\**\*.cs" />
+  </ItemGroup>
+
   <ItemGroup>
     <None Include="$(RepoRoot)\logo.png" Pack="true" PackagePath="\" />
   </ItemGroup>
diff --git a/src/ModelContextProtocol.Core/ModelContextProtocol.Core.csproj b/src/ModelContextProtocol.Core/ModelContextProtocol.Core.csproj
index f3ab71819..07a5ec1b0 100644
--- a/src/ModelContextProtocol.Core/ModelContextProtocol.Core.csproj
+++ b/src/ModelContextProtocol.Core/ModelContextProtocol.Core.csproj
@@ -20,7 +20,6 @@
 
   <!-- Dependencies only needed by netstandard2.0 -->
   <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
-    <Compile Include="..\Common\Polyfills\**\*.cs" />
     <Compile Include="..\Common\CancellableStreamReader\**\*.cs" />
     <PackageReference Include="Microsoft.Bcl.Memory" />
     <PackageReference Include="System.Diagnostics.DiagnosticSource" />
diff --git a/src/ModelContextProtocol/ModelContextProtocol.csproj b/src/ModelContextProtocol/ModelContextProtocol.csproj
index 994f3dcc5..963ba0fed 100644
--- a/src/ModelContextProtocol/ModelContextProtocol.csproj
+++ b/src/ModelContextProtocol/ModelContextProtocol.csproj
@@ -17,10 +17,6 @@
     <Compile Include="..\Common\Throw.cs" Link="Throw.cs" />
   </ItemGroup>
 
-  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
-    <Compile Include="..\Common\Polyfills\**\*.cs" />
-  </ItemGroup>
-
   <ItemGroup>
     <ProjectReference Include="..\ModelContextProtocol.Core\ModelContextProtocol.Core.csproj" />
   </ItemGroup>