diff --git a/eng/Packages.props b/eng/Packages.props
index 6817b951611..a5f52eecb45 100644
--- a/eng/Packages.props
+++ b/eng/Packages.props
@@ -10,6 +10,8 @@
   <ItemGroup>
     <PackageVersion Include="Microsoft.Build.NuGetSdkResolver" Version="$(NuGetBuildTasksVersion)" />
     <PackageVersion Include="Microsoft.CodeAnalysis.Collections" Version="$(MicrosoftCodeAnalysisCollectionsVersion)" />
+    <PackageVersion Include="Microsoft.CodeAnalysis.Contracts" Version="$(MicrosoftCodeAnalysisCollectionsVersion)" />
+    <PackageVersion Include="Microsoft.CodeAnalysis.PooledObjects" Version="$(MicrosoftCodeAnalysisCollectionsVersion)" />
     <PackageVersion Include="Microsoft.DotNet.XUnitExtensions" Version="$(MicrosoftDotNetXUnitExtensionsVersion)" />
     <PackageVersion Include="Microsoft.IO.Redist" Version="$(MicrosoftIORedistVersion)" />
     <PackageVersion Include="Microsoft.Net.Compilers.Toolset" Version="$(MicrosoftNetCompilersToolsetVersion)" Condition="'$(UsingToolMicrosoftNetCompilers)' != 'true'" />
diff --git a/eng/Versions.props b/eng/Versions.props
index 94f3c6786bc..0826882f7da 100644
--- a/eng/Versions.props
+++ b/eng/Versions.props
@@ -79,7 +79,7 @@
     <!-- DotNetCliVersion MUST match the dotnet version in global.json.
          Otherwise, this version of dotnet will not be installed and the build will error out. -->
     <DotNetCliVersion>$([System.Text.RegularExpressions.Regex]::Match($([System.IO.File]::ReadAllText('$(MSBuildThisFileDirectory)..\global.json')), '"dotnet": "([^"]*)"').Groups.get_Item(1))</DotNetCliVersion>
-    <MicrosoftCodeAnalysisCollectionsVersion>4.2.0-1.22102.8</MicrosoftCodeAnalysisCollectionsVersion>
+    <MicrosoftCodeAnalysisCollectionsVersion>5.0.0-1.25277.114</MicrosoftCodeAnalysisCollectionsVersion>
     <MicrosoftDotNetXUnitExtensionsVersion>10.0.0-beta.25358.3</MicrosoftDotNetXUnitExtensionsVersion>
     <NuGetBuildTasksVersion>6.15.0-preview.1.86</NuGetBuildTasksVersion>
     <MicrosoftNetCompilersToolsetVersion>5.0.0-1.25361.3</MicrosoftNetCompilersToolsetVersion>
diff --git a/src/Build/Microsoft.Build.csproj b/src/Build/Microsoft.Build.csproj
index 0c139e64683..0c9cd1c3da9 100644
--- a/src/Build/Microsoft.Build.csproj
+++ b/src/Build/Microsoft.Build.csproj
@@ -38,6 +38,8 @@
 
     <PackageReference Include="Microsoft.IO.Redist" Condition="'$(FeatureMSIORedist)' == 'true'" />
 
+    <PackageReference Include="Microsoft.CodeAnalysis.Collections" PrivateAssets="all" />
+    <PackageReference Include="Microsoft.CodeAnalysis.PooledObjects" PrivateAssets="all" />
     <PackageReference Include="Microsoft.BuildXL.Processes" Condition="'$(FeatureReportFileAccesses)' == 'true'" PrivateAssets="all" />
     <!-- Remove the direct NETStandard.Library dependency when Microsoft.BuildXL.Processes stops bringing in netstandard1.x dependencies
          or when a .NET 10 SDK is used (NuGet Package Pruning eliminates netstandard1.x dependencies). -->
@@ -124,9 +126,6 @@
     <Compile Include="..\Shared\TaskLoggingHelperExtension.cs">
       <Link>BackEnd\Components\RequestBuilder\IntrinsicTasks\TaskLoggingHelperExtension.cs</Link>
     </Compile>
-    <Compile Include="..\Shared\IsExternalInit.cs">
-      <Link>BuildCheck\Utilities\IsExternalInit.cs</Link>
-    </Compile>
     <Compile Include="..\Shared\TaskParameter.cs" />
     <Compile Include="..\Shared\TaskParameterTypeVerifier.cs" />
     <Compile Include="..\Shared\TranslatorHelpers.cs" />
diff --git a/src/Framework/AssemblyUtilities.cs b/src/Framework/AssemblyUtilities.cs
index 5eb6e996a6e..6b21b7d22eb 100644
--- a/src/Framework/AssemblyUtilities.cs
+++ b/src/Framework/AssemblyUtilities.cs
@@ -10,15 +10,8 @@
 using Microsoft.Build.Framework;
 #endif
 
-
-// Declare this to get init properties. See https://github.com/dotnet/roslyn/issues/45510#issuecomment-694977239
 #nullable disable
 
-namespace System.Runtime.CompilerServices
-{
-    internal static class IsExternalInit { }
-}
-
 namespace Microsoft.Build.Shared
 {
     /// <summary>
diff --git a/src/Framework/Microsoft.Build.Framework.csproj b/src/Framework/Microsoft.Build.Framework.csproj
index da61a612686..296bff029bd 100644
--- a/src/Framework/Microsoft.Build.Framework.csproj
+++ b/src/Framework/Microsoft.Build.Framework.csproj
@@ -15,7 +15,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.CodeAnalysis.Collections" PrivateAssets="all" />
+    <PackageReference Include="Microsoft.CodeAnalysis.Contracts" PrivateAssets="all" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETFramework'">
@@ -29,8 +29,9 @@
     <PackageReference Include="Microsoft.VisualStudio.OpenTelemetry.ClientExtensions" PrivateAssets="all" />
   </ItemGroup>
 
-  <!-- Framework and standard don't have DiagnosticSource. -->
+  <!-- Framework and standard don't have these. -->
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETFramework' OR '$(TargetFrameworkIdentifier)' == '.NETStandard'">
+    <PackageReference Include="System.Collections.Immutable" />
     <PackageReference Include="System.Diagnostics.DiagnosticSource" />
   </ItemGroup>
 
diff --git a/src/MSBuildTaskHost/MSBuildTaskHost.csproj b/src/MSBuildTaskHost/MSBuildTaskHost.csproj
index 8512aae44f0..83345b0c8ae 100644
--- a/src/MSBuildTaskHost/MSBuildTaskHost.csproj
+++ b/src/MSBuildTaskHost/MSBuildTaskHost.csproj
@@ -70,6 +70,7 @@
       <Link>CopyOnWriteDictionary.cs</Link>
     </Compile>
     <Compile Include="..\Shared\Constants.cs" />
+    <Compile Include="..\Shared\IsExternalInit.cs" />
     <Compile Include="..\Shared\EnvironmentUtilities.cs" Link="EnvironmentUtilities.cs" />
     <Compile Include="..\Shared\ReadOnlyEmptyDictionary.cs" />
     <Compile Include="..\Framework\ErrorUtilities.cs">
@@ -202,6 +203,7 @@
     <Compile Include="Concurrent\ConcurrentDictionary.cs" />
     <Compile Include="Concurrent\ConcurrentQueue.cs" />
     <Compile Include="Immutable\ImmutableDictionary.cs" />
+    <Compile Include="NullableAttributes.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="TypeLoader.cs" />
     <Compile Include="..\MSBuild\LogMessagePacket.cs">
diff --git a/src/Framework/Polyfills/NullableAttributes.cs b/src/MSBuildTaskHost/NullableAttributes.cs
similarity index 97%
rename from src/Framework/Polyfills/NullableAttributes.cs
rename to src/MSBuildTaskHost/NullableAttributes.cs
index b6698fce618..2c086ba5622 100644
--- a/src/Framework/Polyfills/NullableAttributes.cs
+++ b/src/MSBuildTaskHost/NullableAttributes.cs
@@ -3,6 +3,8 @@
 
 // This was copied from https://github.com/dotnet/runtime/blob/39b9607807f29e48cae4652cd74735182b31182e/src/libraries/System.Private.CoreLib/src/System/Diagnostics/CodeAnalysis/NullableAttributes.cs
 // and updated to have the scope of the attributes be internal.
+// This file is used only by MSBuildTaskHost; other projects should get the polyfills from the
+// Microsoft.CodeAnalysis.Contracts package, through the Microsoft.Build.Framework project.
 
 #nullable disable
 
diff --git a/src/Shared/IsExternalInit.cs b/src/Shared/IsExternalInit.cs
index ae2ffb321cd..f0a0588d1df 100644
--- a/src/Shared/IsExternalInit.cs
+++ b/src/Shared/IsExternalInit.cs
@@ -1,9 +1,22 @@
 // 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.Runtime.CompilerServices;
+
+// Type-forward to the inbox class where available, in order to maintain binary compatibility
+// between the .NET and .NET Standard 2.0 assemblies.
+[assembly: TypeForwardedTo(typeof(IsExternalInit))]
+
+#else
+
+using System.ComponentModel;
+
 namespace System.Runtime.CompilerServices
 {
     // Needed so we can use init setters in full fw or netstandard
     //  (details: https://developercommunity.visualstudio.com/t/error-cs0518-predefined-type-systemruntimecompiler/1244809)
+    [EditorBrowsable(EditorBrowsableState.Never)]
     internal static class IsExternalInit { }
 }
+#endif