diff --git a/eng/pipelines/runtime-diagnostics.yml b/eng/pipelines/runtime-diagnostics.yml
index cd7b5a81f1f527..d394167a81efd0 100644
--- a/eng/pipelines/runtime-diagnostics.yml
+++ b/eng/pipelines/runtime-diagnostics.yml
@@ -17,6 +17,19 @@ parameters:
     - linux_arm
     - osx_arm64
     - osx_x64
+# osx_arm64 / osx_x64 are intentionally excluded from x-plat coverage.
+# Their dump payloads are large enough that downloading every platform's dumps
+# onto a single Helix host exceeds available disk space.
+- name: cdacXPlatDumpPlatforms
+  displayName: cDAC X-Plat Dump Platforms
+  type: object
+  default:
+    - windows_x64
+    - windows_x86
+    - linux_x64
+    - windows_arm64
+    - linux_arm64
+    - linux_arm
 - name: cdacDumpTestMode
   displayName: cDAC Dump Test Mode
   type: string
@@ -304,7 +317,7 @@ extends:
           parameters:
             jobTemplate: /eng/pipelines/common/global-build-job.yml
             buildConfig: release
-            platforms: ${{ parameters.cdacDumpPlatforms }}
+            platforms: ${{ parameters.cdacXPlatDumpPlatforms }}
             shouldContinueOnError: true
             jobParameters:
               nameSuffix: CdacXPlatDumpGen
@@ -351,7 +364,7 @@ extends:
           parameters:
             jobTemplate: /eng/pipelines/common/global-build-job.yml
             buildConfig: release
-            platforms: ${{ parameters.cdacDumpPlatforms }}
+            platforms: ${{ parameters.cdacXPlatDumpPlatforms }}
             shouldContinueOnError: true
             jobParameters:
               nameSuffix: CdacXPlatDumpTest
@@ -363,7 +376,7 @@ extends:
                   buildDebuggees: false
                   skipDebuggeeCopy: true
               # Download dump artifacts from all source platforms
-              - ${{ each platform in parameters.cdacDumpPlatforms }}:
+              - ${{ each platform in parameters.cdacXPlatDumpPlatforms }}:
                 - task: DownloadPipelineArtifact@2
                   inputs:
                     artifactName: CdacDumps_${{ platform }}
@@ -371,7 +384,7 @@ extends:
                   displayName: 'Download dumps from ${{ platform }}'
               # Extract dump tars into the Helix payload
               - pwsh: |
-                  $platforms = "${{ join(';', parameters.cdacDumpPlatforms) }}".Split(';')
+                  $platforms = "${{ join(';', parameters.cdacXPlatDumpPlatforms) }}".Split(';')
                   $payloadDumpsDir = "$(Build.SourcesDirectory)/artifacts/helixPayload/cdac/dumps"
                   foreach ($platform in $platforms) {
                       $downloadDir = "$(Build.SourcesDirectory)/artifacts/xplatDumps/$platform"
@@ -396,7 +409,7 @@ extends:
                     _Creator: dotnet-bot
                     SYSTEM_ACCESSTOKEN: $(System.AccessToken)
                     NUGET_PACKAGES: $(Build.SourcesDirectory)$(dir).packages
-                    SourcePlatforms: ${{ join(';', parameters.cdacDumpPlatforms) }}
+                    SourcePlatforms: ${{ join(';', parameters.cdacXPlatDumpPlatforms) }}
               - pwsh: |
                   if ("$(Agent.JobStatus)" -ne "Succeeded") {
                     Write-Error "One or more cDAC x-plat dump test failures were detected. Failing the job."
diff --git a/src/native/managed/cdac/tests/DumpTests/cdac-dump-xplat-test-helix.proj b/src/native/managed/cdac/tests/DumpTests/cdac-dump-xplat-test-helix.proj
index a8e869fca5bde3..03ac8d812e31c9 100644
--- a/src/native/managed/cdac/tests/DumpTests/cdac-dump-xplat-test-helix.proj
+++ b/src/native/managed/cdac/tests/DumpTests/cdac-dump-xplat-test-helix.proj
@@ -8,9 +8,11 @@
     CDAC_DUMP_ROOT pointing to that platform's dump subdirectory. This gives each
     platform independent exit code tracking and test result reporting.
 
-    The dumps are sent as per-work-item payloads (one platform per work item) to
-    avoid exceeding the 2 GB MemoryStream/ZipArchive limit that occurs when all
-    platforms' dumps are zipped into a single payload.
+    The dumps are pre-zipped per platform with the MSBuild ZipDirectory task and
+    sent as a <PayloadArchive>. This avoids the Helix SDK's <PayloadDirectory>
+    code path which builds the zip in a MemoryStream and so caps the payload at
+    int.MaxValue (~2 GB). ZipDirectory writes the archive directly to a
+    FileStream, so per-platform dump payloads can grow well past that limit.
 
     The test DLLs are sent as a correlation payload (shared across work items).
 
@@ -84,12 +86,18 @@
     Create one Helix work item per source platform. Target batching (Outputs)
     iterates once per _SourcePlatform. Each work item gets its own dump payload
     (per-platform subdirectory) and uses shared test DLLs from the correlation payload.
+
+    The per-platform dump directory is pre-zipped to a <PayloadArchive> instead of
+    using <PayloadDirectory>. The Helix SDK's directory payload zips through a
+    MemoryStream (capped at ~2 GB); ZipDirectory + PayloadArchive streams to a
+    FileStream and has no such limit.
   -->
   <Target Name="_CreateHelixWorkItems"
           Outputs="%(_SourcePlatform.Identity)"
           BeforeTargets="CoreTest">
     <PropertyGroup>
       <_HelixCommandFile>$(DumpPayloadBase)/%(_SourcePlatform.Identity)/HelixCommand.txt</_HelixCommandFile>
+      <_DumpPayloadArchive>$(DumpPayloadBase)/%(_SourcePlatform.Identity).zip</_DumpPayloadArchive>
     </PropertyGroup>
 
     <ItemGroup>
@@ -108,9 +116,20 @@
 
     <WriteLinesToFile File="$(_HelixCommandFile)" Lines="@(_HelixCommandLines)" Overwrite="true" />
 
+    <!--
+      Pre-zip the per-platform dump directory so the Helix SDK can stream it
+      from disk via ArchivePayload instead of zipping it in-memory.
+      CompressionLevel=Fastest keeps build time low; dumps don't compress well anyway.
+    -->
+    <ZipDirectory
+        SourceDirectory="$(DumpPayloadBase)/%(_SourcePlatform.Identity)"
+        DestinationFile="$(_DumpPayloadArchive)"
+        Overwrite="true"
+        CompressionLevel="Fastest" />
+
     <ItemGroup>
       <HelixWorkItem Include="CdacXPlatDumpTests_host_$(TargetOS)_$(TargetArchitecture)_dumps_%(_SourcePlatform.Identity)">
-        <PayloadDirectory>$(DumpPayloadBase)/%(_SourcePlatform.Identity)</PayloadDirectory>
+        <PayloadArchive>$(_DumpPayloadArchive)</PayloadArchive>
         <Command>$([System.IO.File]::ReadAllText('$(_HelixCommandFile)'))</Command>
         <Timeout>$(WorkItemTimeout)</Timeout>
       </HelixWorkItem>