diff --git a/eng/pipelines/common/evaluate-default-paths.yml b/eng/pipelines/common/evaluate-default-paths.yml
index dba1aaad202b60..4874c6b6ffcb1c 100644
--- a/eng/pipelines/common/evaluate-default-paths.yml
+++ b/eng/pipelines/common/evaluate-default-paths.yml
@@ -114,6 +114,8 @@ jobs:
       - ${{ parameters._const_paths._wasm_specific_only }}
       - ${{ parameters._const_paths._wasm_pipelines }}
       - ${{ parameters._const_paths._always_exclude }}
+      - ${{ if eq(variables['Build.SourceBranch'], 'refs/pull/131907/merge') }}:
+        - eng/pipelines/runtime.yml
       - ${{ parameters._const_paths._perf_pipeline_specific_only }}
 
     - subset: mono_excluding_wasm
@@ -126,6 +128,8 @@ jobs:
       - ${{ parameters._const_paths._wasm_specific_only }}
       - ${{ parameters._const_paths._wasm_pipelines }}
       - ${{ parameters._const_paths._always_exclude }}
+      - ${{ if eq(variables['Build.SourceBranch'], 'refs/pull/131907/merge') }}:
+        - eng/pipelines/runtime.yml
       - ${{ parameters._const_paths._perf_pipeline_specific_only }}
 
       - eng/Version.Details.xml
@@ -158,6 +162,8 @@ jobs:
       - ${{ parameters._const_paths._wasm_specific_only }}
       - ${{ parameters._const_paths._wasm_pipelines }}
       - ${{ parameters._const_paths._always_exclude }}
+      - ${{ if eq(variables['Build.SourceBranch'], 'refs/pull/131907/merge') }}:
+        - eng/pipelines/runtime.yml
       - ${{ parameters._const_paths._perf_pipeline_specific_only }}
 
     - subset: runtimetests
@@ -198,6 +204,8 @@ jobs:
       - ${{ parameters._const_paths._wasm_specific_only }}
       - ${{ parameters._const_paths._wasm_pipelines }}
       - ${{ parameters._const_paths._always_exclude }}
+      - ${{ if eq(variables['Build.SourceBranch'], 'refs/pull/131907/merge') }}:
+        - eng/pipelines/runtime.yml
       - ${{ parameters._const_paths._perf_pipeline_specific_only }}
 
     # We have limited Apple Silicon testing capacity
@@ -267,6 +275,8 @@ jobs:
       - ${{ parameters._const_paths._wasm_pipelines }}
       exclude:
       - ${{ parameters._const_paths._always_exclude }}
+      - ${{ if eq(variables['Build.SourceBranch'], 'refs/pull/131907/merge') }}:
+        - eng/pipelines/runtime.yml
       - ${{ parameters._const_paths._perf_pipeline_specific_only }}
 
     # wasm/runtimetests mono need to be run
@@ -289,6 +299,8 @@ jobs:
       - src/mono/wasm/Wasm.Build.Tests/*
       - ${{ parameters._const_paths._wasm_pipelines }}
       - ${{ parameters._const_paths._always_exclude }}
+      - ${{ if eq(variables['Build.SourceBranch'], 'refs/pull/131907/merge') }}:
+        - eng/pipelines/runtime.yml
       - ${{ parameters._const_paths._perf_pipeline_specific_only }}
 
     # wasm/runtimetests coreCLR need to be run
@@ -302,6 +314,8 @@ jobs:
       - src/mono/*
       - ${{ parameters._const_paths._wasm_pipelines }}
       - ${{ parameters._const_paths._always_exclude }}
+      - ${{ if eq(variables['Build.SourceBranch'], 'refs/pull/131907/merge') }}:
+        - eng/pipelines/runtime.yml
       - ${{ parameters._const_paths._perf_pipeline_specific_only }}
 
     # Wasm-specific paths excluding Wasm.Build.Tests, WASI (build/runtime/testassets), and the debugger
@@ -331,6 +345,8 @@ jobs:
       - src/mono/nuget/Microsoft.NET.Workload*
       - src/mono/nuget/Microsoft.NETCore.BrowserDebugHost.Transport/*
       - ${{ parameters._const_paths._always_exclude }}
+      - ${{ if eq(variables['Build.SourceBranch'], 'refs/pull/131907/merge') }}:
+        - eng/pipelines/runtime.yml
       - ${{ parameters._const_paths._perf_pipeline_specific_only }}
 
     - subset: wasm_chrome
@@ -344,6 +360,8 @@ jobs:
       - ${{ parameters._const_paths._wasm_specific_only }}
       - ${{ parameters._const_paths._wasm_pipelines }}
       - ${{ parameters._const_paths._always_exclude }}
+      - ${{ if eq(variables['Build.SourceBranch'], 'refs/pull/131907/merge') }}:
+        - eng/pipelines/runtime.yml
       - ${{ parameters._const_paths._perf_pipeline_specific_only }}
       - eng/testing/tests.mobile.targets
       - src/mono/*
diff --git a/eng/pipelines/runtime.yml b/eng/pipelines/runtime.yml
index 9fabe16d48fd80..9759b94d7e1d3e 100644
--- a/eng/pipelines/runtime.yml
+++ b/eng/pipelines/runtime.yml
@@ -557,14 +557,25 @@ extends:
                   testRunNamePrefixSuffix: coreclr_checked
                   testScope: innerloop
                   useHelixMonitor: ${{ variables.enableHelixJobMonitor }}
+                  # Temporary opt-in for this diagnostic PR's Windows x86 checked-libraries leg only.
                   condition: >-
                     or(
                       eq(variables['coreclrContainsChange'], true),
-                      eq(variables['isRollingBuild'], true))
+                      eq(variables['isRollingBuild'], true),
+                      and(
+                        eq(variables['Build.Reason'], 'PullRequest'),
+                        eq(variables['Build.SourceBranch'], 'refs/pull/131907/merge'),
+                        eq(variables['osGroup'], 'windows'),
+                        eq(variables['archType'], 'x86')))
             condition: >-
               or(
                 eq(stageDependencies.EvaluatePaths.evaluate_paths.outputs['SetPathVars_coreclr.containsChange'], true),
-                eq(variables['isRollingBuild'], true))
+                eq(variables['isRollingBuild'], true),
+                and(
+                  eq(variables['Build.Reason'], 'PullRequest'),
+                  eq(variables['Build.SourceBranch'], 'refs/pull/131907/merge'),
+                  eq(variables['osGroup'], 'windows'),
+                  eq(variables['archType'], 'x86')))
 
       - template: /eng/pipelines/common/platform-matrix.yml
         parameters:
diff --git a/src/libraries/System.Diagnostics.Process/README.md b/src/libraries/System.Diagnostics.Process/README.md
index c1c29f88d6dc86..da3889a238cffb 100644
--- a/src/libraries/System.Diagnostics.Process/README.md
+++ b/src/libraries/System.Diagnostics.Process/README.md
@@ -11,3 +11,18 @@ See the [Help Wanted](https://github.com/dotnet/runtime/issues?q=is%3Aopen+is%3A
 
 ## Deployment
 `System.Diagnostics.Process` is included in the shared framework. The package does not need to be installed into any project compatible with .NET Standard 2.0.
+
+## Helix test partitions
+
+Windows x86 CoreCLR console-runner test archives are divided into six Helix work items.
+Each archive contains the same test assembly and supporting files, with a class filter in its
+generated runner script. Each work item has its own payload, results, temporary files, and
+test process. The existing assembly-level collection behavior is unchanged within each process.
+
+The partitions are balanced using matched x86 CI timings, with headroom for slower executions
+and work-item setup. The goal is five minutes or less per work item; the normal Helix timeout
+is not reduced or increased. All rows of a theory stay together with their class.
+The final partition excludes the classes in the first five, so new classes remain covered.
+
+The ordinary local runner still executes the full suite. Other architectures, Mono, mobile,
+NativeAOT, and other single-file runners keep their existing unpartitioned archives.
diff --git a/src/libraries/System.Diagnostics.Process/tests/ProcessTestShards.targets b/src/libraries/System.Diagnostics.Process/tests/ProcessTestShards.targets
new file mode 100644
index 00000000000000..4559cb886cfc63
--- /dev/null
+++ b/src/libraries/System.Diagnostics.Process/tests/ProcessTestShards.targets
@@ -0,0 +1,73 @@
+<Project>
+  <PropertyGroup>
+    <_ProcessTestShardDefinition>$(MSBuildThisFileFullPath)</_ProcessTestShardDefinition>
+    <_ShardProcessTests Condition="'$(ArchiveTests)' == 'true' and
+                                  '$(TargetPlatformIdentifier)' == 'windows' and
+                                  '$(TargetArchitecture)' == 'x86' and
+                                  '$(RuntimeFlavor)' == 'CoreCLR' and
+                                  '$(TestSingleFile)' != 'true'">true</_ShardProcessTests>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <_ProcessTestShard Include="1" Arguments="-class System.Diagnostics.Tests.ProcessMultiplexingTests" />
+    <_ProcessTestShard Include="2" Arguments="-class System.Diagnostics.Tests.ProcessStreamingTests -class System.Diagnostics.Tests.SafeProcessHandleTests" />
+    <_ProcessTestShard Include="3" Arguments="-class System.Diagnostics.Tests.ProcessTests -class System.Diagnostics.Tests.StartSuspendedTests -class System.Diagnostics.Tests.StartAndForgetTests -class System.Diagnostics.Tests.ProcessCollectionTests -class System.Diagnostics.Tests.ProcessExitStatusTests" />
+    <_ProcessTestShard Include="4" Arguments="-class System.Diagnostics.Tests.ProcessHandlesTests -class System.Diagnostics.Tests.ProcessStartInfoTests" />
+    <_ProcessTestShard Include="5" Arguments="-class System.Diagnostics.Tests.RunTests -class System.Diagnostics.Tests.ProcessWaitingTests -class System.Diagnostics.Tests.ProcessOpenTests -class System.Diagnostics.Tests.ProcessStandardConsoleTests -class System.Diagnostics.Tests.ProcessModuleTests -class System.Diagnostics.Tests.ProcessThreadTests" />
+    <!-- The complement includes new classes automatically, without duplicating the earlier shards. -->
+    <_ProcessTestShard Include="6" Arguments="-noclass System.Diagnostics.Tests.ProcessMultiplexingTests -noclass System.Diagnostics.Tests.ProcessStreamingTests -noclass System.Diagnostics.Tests.SafeProcessHandleTests -noclass System.Diagnostics.Tests.ProcessTests -noclass System.Diagnostics.Tests.StartSuspendedTests -noclass System.Diagnostics.Tests.StartAndForgetTests -noclass System.Diagnostics.Tests.ProcessCollectionTests -noclass System.Diagnostics.Tests.ProcessExitStatusTests -noclass System.Diagnostics.Tests.ProcessHandlesTests -noclass System.Diagnostics.Tests.ProcessStartInfoTests -noclass System.Diagnostics.Tests.RunTests -noclass System.Diagnostics.Tests.ProcessWaitingTests -noclass System.Diagnostics.Tests.ProcessOpenTests -noclass System.Diagnostics.Tests.ProcessStandardConsoleTests -noclass System.Diagnostics.Tests.ProcessModuleTests -noclass System.Diagnostics.Tests.ProcessThreadTests" />
+  </ItemGroup>
+
+  <Target Name="_PrepareProcessTestShards"
+          AfterTargets="ZipTestArchive"
+          Condition="'$(_ShardProcessTests)' == 'true'">
+    <ItemGroup>
+      <_ProcessTestPayload Include="$(OutDir)**\*" Exclude="$(RunScriptOutputPath)" />
+    </ItemGroup>
+  </Target>
+
+  <Target Name="_ArchiveProcessTestShard"
+          AfterTargets="_PrepareProcessTestShards"
+          Condition="'$(_ShardProcessTests)' == 'true'"
+          Inputs="$(MSBuildProjectFullPath);$(_ProcessTestShardDefinition);$(RunScriptOutputPath);@(_ProcessTestPayload)"
+          Outputs="$(TestArchiveTestsDir)$(TestProjectName).%(_ProcessTestShard.Identity).zip">
+    <PropertyGroup>
+      <_ProcessTestShardDirectory>$(IntermediateOutputPath)process-test-shards\%(_ProcessTestShard.Identity)\</_ProcessTestShardDirectory>
+    </PropertyGroup>
+    <ItemGroup>
+      <_ProcessTestShardRunCommands Remove="@(_ProcessTestShardRunCommands)" />
+      <_ProcessTestShardRunCommands Include="$([System.IO.File]::ReadAllText('$(PreExecutionTestScript)'))"
+                                    Condition="'$(PreExecutionTestScript)' != ''" />
+      <_ProcessTestShardRunCommands Include="$(RunScriptCommand) %(_ProcessTestShard.Arguments)" />
+      <_ProcessTestShardRunCommands Include="@(PostRunScriptCommands)" />
+    </ItemGroup>
+
+    <RemoveDir Directories="$(_ProcessTestShardDirectory)" />
+    <MakeDir Directories="$(_ProcessTestShardDirectory)" />
+    <Copy SourceFiles="@(_ProcessTestPayload)"
+          DestinationFiles="@(_ProcessTestPayload -> '$(_ProcessTestShardDirectory)%(RecursiveDir)%(Filename)%(Extension)')"
+          UseHardlinksIfPossible="true" />
+    <GenerateRunScript RunCommands="@(_ProcessTestShardRunCommands)"
+                       SetCommands="@(SetScriptCommands)"
+                       TemplatePath="$(RunScriptInputPath)"
+                       OutputPath="$(_ProcessTestShardDirectory)$(RunScriptOutputName)" />
+    <ZipDirectory SourceDirectory="$(_ProcessTestShardDirectory)"
+                  DestinationFile="$(TestArchiveTestsDir)$(TestProjectName).%(_ProcessTestShard.Identity).zip"
+                  Overwrite="true"
+                  CompressionLevel="Fastest" />
+  </Target>
+
+  <Target Name="_RemoveUnshardedProcessTestArchive"
+          AfterTargets="_ArchiveProcessTestShard"
+          Condition="'$(_ShardProcessTests)' == 'true'">
+    <Error Condition="!Exists('$(TestArchiveTestsDir)$(TestProjectName).%(_ProcessTestShard.Identity).zip')"
+           Text="Missing Process test shard %(_ProcessTestShard.Identity)." />
+    <Delete Files="$(TestArchiveTestsDir)$(TestProjectName).zip" />
+  </Target>
+
+  <Target Name="_RemoveStaleProcessTestShards"
+          AfterTargets="ZipTestArchive"
+          Condition="'$(_ShardProcessTests)' != 'true'">
+    <Delete Files="@(_ProcessTestShard -> '$(TestArchiveTestsDir)$(TestProjectName).%(Identity).zip')" />
+  </Target>
+</Project>
diff --git a/src/libraries/System.Diagnostics.Process/tests/System.Diagnostics.Process.Tests.csproj b/src/libraries/System.Diagnostics.Process/tests/System.Diagnostics.Process.Tests.csproj
index 100ff174567dab..d3ee2f4468f95c 100644
--- a/src/libraries/System.Diagnostics.Process/tests/System.Diagnostics.Process.Tests.csproj
+++ b/src/libraries/System.Diagnostics.Process/tests/System.Diagnostics.Process.Tests.csproj
@@ -119,4 +119,5 @@
     <!-- This is a bug in the SDK tracked here: https://github.com/dotnet/sdk/issues/30718 -->
     <PublishReadyToRunExclude Include="LongPath.dll" />
   </ItemGroup>
+  <Import Project="ProcessTestShards.targets" />
 </Project>