diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index ee4a42da564..7b182a4d98c 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -75,7 +75,16 @@ jobs:
       run: |
         ./dev.sh test
       working-directory: src
-      if: matrix.runtime == 'arm64' || matrix.runtime == 'amd64'
+      # Compares against matrix.runs_on (bare arch names: amd64/arm64/ppc64le/s390x), not
+      # matrix.runtime (always 'linux-*'-prefixed) -- the two were previously confused here,
+      # which meant this condition never evaluated true for any matrix leg and L0 silently
+      # never ran in this workflow. linux-x64/linux-arm64 build/run natively on matching
+      # self-hosted hardware with no patches applied, so they're safe to test directly here.
+      # linux-arm (32-bit armv7l, cross-targeted even though it builds on arm64 hardware),
+      # ppc64le, and s390x are intentionally left out: they require the arm/ppc64le/s390x
+      # patches to be applied first and their 32-bit/cross-arch test-tooling behavior under
+      # dotnet test is not yet validated.
+      if: matrix.runtime == 'linux-x64' || matrix.runtime == 'linux-arm64'
 
     # Create runner package tar.gz/zip
     - name: Package Release
diff --git a/src/Test/L0/Worker/StepHostL0.cs b/src/Test/L0/Worker/StepHostL0.cs
index bac7d41d90a..6030320101f 100644
--- a/src/Test/L0/Worker/StepHostL0.cs
+++ b/src/Test/L0/Worker/StepHostL0.cs
@@ -55,6 +55,11 @@ public async Task DetermineNodeRuntimeVersionInContainerAsync()
             }
         }
 
+        // Alpine-container Node support is only implemented for X64 (see StepHost.CheckPlatformForAlpineContainer,
+        // which throws NotSupportedException for any other Constants.Runner.PlatformArchitecture). That value is
+        // baked in at compile time via this same X64/ARM/ARM64 preprocessor symbol, so these tests can only pass
+        // when the Test assembly itself is built targeting X64.
+#if X64
         [Fact]
         [Trait("Level", "L0")]
         [Trait("Category", "Worker")]
@@ -108,6 +113,7 @@ public async Task DetermineNode20RuntimeVersionInAlpineContainerAsync()
                 Assert.Equal("node20_alpine", nodeVersion);
             }
         }
+#endif
 
         [Fact]
         [Trait("Level", "L0")]
@@ -163,6 +169,9 @@ public async Task DetermineNode20RuntimeVersionInUnknowContainerAsync()
             }
         }
 
+        // See the comment above DetermineNodeRuntimeVersionInAlpineContainerAsync: Alpine-container Node support
+        // is X64-only in production code, so this test is only valid for X64-targeted Test assembly builds.
+#if X64
         [Fact]
         [Trait("Level", "L0")]
         [Trait("Category", "Worker")]
@@ -189,6 +198,7 @@ public async Task DetermineNode24RuntimeVersionInAlpineContainerAsync()
                 Assert.Equal("node24_alpine", nodeVersion);
             }
         }
+#endif
 
         [Fact]
         [Trait("Level", "L0")]