diff --git a/docs/workflow/testing/libraries/testing-wasm.md b/docs/workflow/testing/libraries/testing-wasm.md
index bb80a32876f9b8..7e2f3302e09ea5 100644
--- a/docs/workflow/testing/libraries/testing-wasm.md
+++ b/docs/workflow/testing/libraries/testing-wasm.md
@@ -195,26 +195,30 @@ stale staged assets can otherwise cause assembly-loading failures before tests s
 The `LibraryTestsCoreCLR_R2R` CI jobs use the same configuration and archive the published
 tests for execution on Helix. The existing interpreter jobs remain separate.
 
-To run a library suite with a single composite ReadyToRun image instead of per-assembly
-images, add `PublishReadyToRunComposite=true`:
+CoreCLR browser ReadyToRun tests publish a single composite image
+(`<test assembly>.r2r.wasm`) by default. It is staged into `wwwroot/_framework` beside
+the component WebCIL stubs and listed in the boot config.
+
+To run a library suite with per-assembly ReadyToRun images instead, add
+`PublishReadyToRunComposite=false`:
 
 ```bash
 XHARNESS_COMMAND=test-browser ./build.sh libs.tests -test -os browser -c Release \
   --projects $PWD/src/libraries/System.Memory/tests/System.Memory.Tests.csproj \
-  /p:RuntimeFlavor=CoreCLR /p:TestWasmReadyToRun=true /p:PublishReadyToRunComposite=true \
+  /p:RuntimeFlavor=CoreCLR /p:TestWasmReadyToRun=true /p:PublishReadyToRunComposite=false \
   /p:EnableAggressiveTrimming=true /p:Scenario=WasmTestOnChrome /p:InstallChromeForTests=true
 ```
 
-The composite image (`<test assembly>.r2r.wasm`) is staged into `wwwroot/_framework` beside
-the component WebCIL stubs and listed in the boot config. As with other mode switches, clean
-the project-specific `bin` and `obj` outputs when switching between composite and per-assembly
-images.
+As with other mode switches, clean the project-specific `bin` and `obj` outputs when
+switching between composite and per-assembly images.
 
-For CoreCLR browser applications, set `PublishReadyToRun=true` and
-`PublishReadyToRunComposite=true` to publish a single composite ReadyToRun image and
-component WebCIL stubs instead of per-assembly images. Publishing requires a wasm-capable
-crossgen2 and WebCIL enabled; the composite is loaded before the runtime starts.
-`ReadyToRunTests.PublishRunAllPagesComposite` exercises trimmed, untrimmed, and
+For CoreCLR browser applications, ReadyToRun remains opt-in: set
+`PublishReadyToRun=true` to publish a single composite ReadyToRun image and component
+WebCIL stubs. Set `PublishReadyToRunComposite=false` to retain per-assembly images.
+Publishing requires a wasm-capable crossgen2. Composite publishing additionally requires
+ReadyToRun SDK tasks that support wasm composite output naming and WebCIL enabled;
+the composite is loaded before the runtime starts.
+`ReadyToRunTests.PublishRunAllPagesComposite` exercises the default mode with trimmed, untrimmed, and
 native-relinked publishes in a browser, including asset fingerprinting when
 `USE_FINGERPRINTING_FOR_TESTS=true`.
 
diff --git a/src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.CoreCLR.targets b/src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.CoreCLR.targets
index d5d6a22615f67a..0f18cc5dac3515 100644
--- a/src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.CoreCLR.targets
+++ b/src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.CoreCLR.targets
@@ -30,8 +30,6 @@ Copyright (c) .NET Foundation. All rights reserved.
     <!-- Override only the incompatible SDK default: net11+ defaults ContainerFormat=pe (unloadable on wasm).
          An explicit unsupported value is left as-is and rejected by _WasmCoreClrValidateReadyToRun. -->
     <PublishReadyToRunContainerFormat Condition="'$(PublishReadyToRunContainerFormat)' == '' or '$(PublishReadyToRunContainerFormat)' == 'pe'">wasm</PublishReadyToRunContainerFormat>
-    <!-- Keep the existing per-assembly behavior unless composite is explicitly requested. -->
-    <PublishReadyToRunComposite Condition="'$(PublishReadyToRunComposite)' == ''">false</PublishReadyToRunComposite>
     <!-- Compile every assembly into one shared (large) version bubble. -->
     <PublishReadyToRunCrossgen2ExtraArgs>$(PublishReadyToRunCrossgen2ExtraArgs);--opt-cross-module:*</PublishReadyToRunCrossgen2ExtraArgs>
     <PublishReadyToRunCrossgen2ExtraArgs Condition="'$(WasmEnableRelaxedSimd)' == 'true'">$(PublishReadyToRunCrossgen2ExtraArgs);--instruction-set:relaxed-simd</PublishReadyToRunCrossgen2ExtraArgs>
@@ -83,7 +81,7 @@ Copyright (c) .NET Foundation. All rights reserved.
     <Error Condition="'@(_WasmMissingReadyToRunPgoFile)' != ''"
            Text="PublishReadyToRunPgoFiles item '@(_WasmMissingReadyToRunPgoFile)' does not exist. Specify an existing .mibc file generated for the assemblies being published." />
     <Error Condition="'$(PublishReadyToRunComposite)' == 'true' and '$(_WasmEnableWebcil)' != 'true'"
-           Text="PublishReadyToRunComposite for CoreCLR browser-wasm requires WebCIL-in-Wasm assemblies; enable WasmEnableWebcil." />
+           Text="PublishReadyToRunComposite for CoreCLR browser-wasm requires WebCIL-in-Wasm assemblies; enable WasmEnableWebcil. To opt out of composite mode, set PublishReadyToRunComposite=false." />
     <Error Condition="'$(PublishReadyToRunContainerFormat)' != 'wasm'"
            Text="PublishReadyToRunContainerFormat '$(PublishReadyToRunContainerFormat)' is not supported for CoreCLR browser-wasm; only 'wasm' is supported." />
     <Error Condition="('$(_IsPublishing)' == 'true' or '$(WasmBuildingForNestedPublish)' == 'true' or '$(WasmBuildOnlyAfterPublish)' == 'true') and ('$(Crossgen2InBuildDir)' == '' or !Exists('$(Crossgen2InBuildDir)')) and '@(ResolvedCrossgen2Pack)' == ''"
@@ -101,7 +99,7 @@ Copyright (c) .NET Foundation. All rights reserved.
       <_WasmCompositeOwnerEntry Include="@(_ReadyToRunCompileList->WithMetadataValue('CreateCompositeImage', 'true'))" />
     </ItemGroup>
     <Error Condition="'@(_WasmCompositeOwnerEntry)' != '' and !$([System.String]::Copy('%(_WasmCompositeOwnerEntry.OutputR2RImage)').EndsWith('.r2r.wasm'))"
-           Text="PublishReadyToRunComposite for CoreCLR browser-wasm requires ReadyToRun SDK tasks that name the composite image '&lt;entry&gt;.r2r.wasm', but they planned '$([System.IO.Path]::GetFileName('%(_WasmCompositeOwnerEntry.OutputR2RImage)'))'. Use an SDK with wasm ReadyToRun output naming (dotnet/sdk#56395), or set Crossgen2SdkOverridePropsPath and Crossgen2SdkOverrideTargetsPath to the dotnet/runtime Crossgen2Tasks." />
+           Text="PublishReadyToRunComposite for CoreCLR browser-wasm requires ReadyToRun SDK tasks that name the composite image '&lt;entry&gt;.r2r.wasm', but they planned '$([System.IO.Path]::GetFileName('%(_WasmCompositeOwnerEntry.OutputR2RImage)'))'. Use an SDK with wasm ReadyToRun output naming (dotnet/sdk#56395), or set Crossgen2SdkOverridePropsPath and Crossgen2SdkOverrideTargetsPath to the dotnet/runtime Crossgen2Tasks. To opt out of composite mode, set PublishReadyToRunComposite=false." />
   </Target>
 
   <!-- PublishTrimmed selects the staged closure (untrimmed pack vs linker output) exactly like
diff --git a/src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.props b/src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.props
index 30cfa34ccc3695..3b44a521a85a16 100644
--- a/src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.props
+++ b/src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.props
@@ -24,6 +24,8 @@ Copyright (c) .NET Foundation. All rights reserved.
     <!-- Turn off symbol publishing by default -->
     <CopyOutputSymbolsToPublishDirectory>false</CopyOutputSymbolsToPublishDirectory>
 
+    <PublishReadyToRunComposite Condition="'$(PublishReadyToRunComposite)' == ''">true</PublishReadyToRunComposite>
+
     <!-- Trimmer defaults -->
     <TrimMode Condition="'$(TrimMode)' == '' and '$(UsingMicrosoftNETSdkBlazorWebAssembly)' == 'true'">partial</TrimMode>
     <TrimMode Condition="'$(TrimMode)' == '' and '$(UsingMicrosoftNETSdkBlazorWebAssembly)' != 'true'">full</TrimMode>
diff --git a/src/mono/sample/wasm/Directory.Build.props b/src/mono/sample/wasm/Directory.Build.props
index b63362260ba5e2..03138cf9ff35a0 100644
--- a/src/mono/sample/wasm/Directory.Build.props
+++ b/src/mono/sample/wasm/Directory.Build.props
@@ -53,6 +53,7 @@
   <PropertyGroup>
     <RuntimeFlavor Condition="'$(Nested_RuntimeFlavor)' != ''">$(Nested_RuntimeFlavor)</RuntimeFlavor>
     <PublishReadyToRun Condition="'$(PublishReadyToRun)' == '' and '$(Nested_PublishReadyToRun)' != ''">$(Nested_PublishReadyToRun)</PublishReadyToRun>
+    <PublishReadyToRunComposite Condition="'$(PublishReadyToRunComposite)' == '' and '$(Nested_PublishReadyToRunComposite)' != ''">$(Nested_PublishReadyToRunComposite)</PublishReadyToRunComposite>
     <PublishReadyToRun Condition="'$(PublishReadyToRun)' == ''">false</PublishReadyToRun>
   </PropertyGroup>
 
diff --git a/src/mono/sample/wasm/Directory.Build.targets b/src/mono/sample/wasm/Directory.Build.targets
index bd4f2de1aa2de6..e7f33cf2022e0a 100644
--- a/src/mono/sample/wasm/Directory.Build.targets
+++ b/src/mono/sample/wasm/Directory.Build.targets
@@ -34,6 +34,7 @@
     <NestedBuildProperty Include="InvariantTimezone" />
     <NestedBuildProperty Include="PublishTrimmed" />
     <NestedBuildProperty Include="PublishReadyToRun" />
+    <NestedBuildProperty Include="PublishReadyToRunComposite" />
     <NestedBuildProperty Include="RunAOTCompilation" />
     <NestedBuildProperty Include="UsingNativeAOT" />
     <NestedBuildProperty Include="WasmBuildNative" />
diff --git a/src/mono/wasm/Wasm.Build.Tests/NativeLibraryTests.cs b/src/mono/wasm/Wasm.Build.Tests/NativeLibraryTests.cs
index 21307e7197e75b..62a102cc0e5e1a 100644
--- a/src/mono/wasm/Wasm.Build.Tests/NativeLibraryTests.cs
+++ b/src/mono/wasm/Wasm.Build.Tests/NativeLibraryTests.cs
@@ -41,7 +41,7 @@ private async Task ProjectWithNativeReferenceCore(Configuration config, bool aot
             string extraItems = $"<NativeFileReference Include=\"{objectFilename}\" />";
             string extraProperties = "<WasmBuildNative>true</WasmBuildNative>";
             if (readyToRun)
-                extraProperties += "<PublishReadyToRun>true</PublishReadyToRun>";
+                extraProperties += "<PublishReadyToRun>true</PublishReadyToRun><PublishReadyToRunComposite>false</PublishReadyToRunComposite>";
             string insertAtEnd = readyToRun
                 ? """
                     <Target Name="PrintReadyToRunPInvokeModules" AfterTargets="_WasmConfigureReadyToRunPInvokes">
diff --git a/src/mono/wasm/Wasm.Build.Tests/ReadyToRunTests.cs b/src/mono/wasm/Wasm.Build.Tests/ReadyToRunTests.cs
index c6a3ddb6b5e508..1334dd1c09de24 100644
--- a/src/mono/wasm/Wasm.Build.Tests/ReadyToRunTests.cs
+++ b/src/mono/wasm/Wasm.Build.Tests/ReadyToRunTests.cs
@@ -86,8 +86,9 @@ private async Task PublishRunAllPagesCore(Configuration config, bool trimmed, bo
             string extraItems = composite
                 ? """<ProjectReference Include="../R2rSuffixLibrary/R2rSuffixLibrary.csproj" />"""
                 : string.Empty;
+            string compositeProperty = composite ? string.Empty : "<PublishReadyToRunComposite>false</PublishReadyToRunComposite>";
             ProjectInfo info = CopyTestAsset(config, aot: false, TestAsset.BlazorBasicTestApp, label,
-                extraProperties: $"<PublishReadyToRun>true</PublishReadyToRun><PublishReadyToRunComposite>{(composite ? "true" : "false")}</PublishReadyToRunComposite><PublishTrimmed>{(trimmed ? "true" : "false")}</PublishTrimmed>",
+                extraProperties: $"<PublishReadyToRun>true</PublishReadyToRun>{compositeProperty}<PublishTrimmed>{(trimmed ? "true" : "false")}</PublishTrimmed>",
                 extraItems: extraItems);
             if (composite)
                 LogR2RSuffixLibraryMarker();