diff --git a/eng/testing/tests.browser.targets b/eng/testing/tests.browser.targets
index 794064b9e9eb28..cefc5b3b52caae 100644
--- a/eng/testing/tests.browser.targets
+++ b/eng/testing/tests.browser.targets
@@ -51,6 +51,10 @@
     <UseSystemResourceKeys Condition="'$(UseSystemResourceKeys)' == ''">false</UseSystemResourceKeys>
     <EventSourceSupport Condition="'$(EventSourceSupport)' == ''">true</EventSourceSupport>
     <NullabilityInfoContextSupport Condition="'$(NullabilityInfoContextSupport)' == ''">true</NullabilityInfoContextSupport>
+
+    <!-- Library tests run directly from bin/ without the static web assets middleware,
+         so copy framework files to the output directory. See https://github.com/dotnet/runtime/issues/127257. -->
+    <_WasmFrameworkCopyToOutputDirectory>PreserveNewest</_WasmFrameworkCopyToOutputDirectory>
   </PropertyGroup>
   <PropertyGroup Condition="'$(RuntimeFlavor)' == 'CoreCLR'">
     <WasmTestSupport>true</WasmTestSupport>
diff --git a/src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.targets b/src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.targets
index bd63688adeed59..52d869fca3df6a 100644
--- a/src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.targets
+++ b/src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.targets
@@ -354,6 +354,12 @@ Copyright (c) .NET Foundation. All rights reserved.
     <PropertyGroup>
       <_WasmBuildWebcilPath>$([MSBuild]::NormalizeDirectory($(IntermediateOutputPath), 'webcil'))</_WasmBuildWebcilPath>
       <_WasmBuildTmpWebcilPath>$([MSBuild]::NormalizeDirectory($(IntermediateOutputPath), 'tmp-webcil'))</_WasmBuildTmpWebcilPath>
+      <!-- Controls whether WASM framework/webcil static web assets are copied to bin/wwwroot/_framework/
+           at build time. Defaults to 'Never' (assets stay in obj/fx/ and are served via the static web
+           assets middleware). Set to 'PreserveNewest' to copy them to the output directory — this is
+           required for scenarios that run the app from bin/ without the static web assets middleware
+           (e.g. in-tree library tests). -->
+      <_WasmFrameworkCopyToOutputDirectory Condition="'$(_WasmFrameworkCopyToOutputDirectory)' == ''">Never</_WasmFrameworkCopyToOutputDirectory>
     </PropertyGroup>
 
     <ConvertDllsToWebcil Candidates="@(_BuildAssetsCandidates)" IntermediateOutputPath="$(_WasmBuildTmpWebcilPath)" OutputPath="$(_WasmBuildWebcilPath)" IsEnabled="$(_WasmEnableWebcil)" WebcilVersion="$(_WasmWebcilVersion)">
@@ -387,7 +393,7 @@ Copyright (c) .NET Foundation. All rights reserved.
       SourceType="Computed"
       AssetKind="Build"
       AssetRole="Primary"
-      CopyToOutputDirectory="Never"
+      CopyToOutputDirectory="$(_WasmFrameworkCopyToOutputDirectory)"
       CopyToPublishDirectory="Never"
       FingerprintCandidates="$(_WasmFingerprintAssets)"
       FingerprintPatterns="@(FingerprintPatterns);@(_WasmFingerprintPatterns)"
@@ -406,7 +412,7 @@ Copyright (c) .NET Foundation. All rights reserved.
       SourceType="Framework"
       AssetKind="Build"
       AssetRole="Primary"
-      CopyToOutputDirectory="Never"
+      CopyToOutputDirectory="$(_WasmFrameworkCopyToOutputDirectory)"
       CopyToPublishDirectory="Never"
       FingerprintCandidates="$(_WasmFingerprintAssets)"
       FingerprintPatterns="@(FingerprintPatterns);@(_WasmFingerprintPatterns)"
@@ -435,7 +441,7 @@ Copyright (c) .NET Foundation. All rights reserved.
          during dotnet run — they are not copied to bin/wwwroot/_framework/ at build time. -->
     <ItemGroup>
       <_WasmMaterializedFrameworkAssets Update="@(_WasmMaterializedFrameworkAssets)"
-        AssetMode="All" CopyToOutputDirectory="Never" />
+        AssetMode="All" CopyToOutputDirectory="$(_WasmFrameworkCopyToOutputDirectory)" />
       <WasmStaticWebAsset Include="@(_WasmMaterializedFrameworkAssets)" />
     </ItemGroup>