diff --git a/.github/actions/ci-versioning/action.yml b/.github/actions/ci-versioning/action.yml
index b454836..a7ecddd 100644
--- a/.github/actions/ci-versioning/action.yml
+++ b/.github/actions/ci-versioning/action.yml
@@ -557,6 +557,47 @@ runs:
         }
         Write-Host "::notice title=Versioning::Found versioning datasets for: $($versions.Name -join ', ')"
 
+    # Precondition, not a policy choice. The runner is handed
+    # --subject-assemblies "<workspace>\Build" and restricts attribution to the
+    # namespaces the assemblies there declare. If that directory is absent it falls
+    # back to attributing every failure across the whole dependency closure, which
+    # reports other repositories' defects against this one: measured at 1,056 of 1,056
+    # on a real pull request. The runner does warn, but on stderr, and a warning does
+    # not stop the check producing a verdict it has no basis for.
+    #
+    # Asserted here rather than left to the runner because this is a property of the
+    # build, and the build is this action's job. Same shape as the dataset guard above.
+    #
+    # Not the vacuous-green question. "Nothing to check" is a legitimate green;
+    # "the thing to check was never built" is a broken precondition. This is the second.
+    #
+    # Two causes when it fires, and they need different fixes. Either the projects declare
+    # <OutputPath>..\Build\ only inside PropertyGroups conditioned on Debug or Test, so a
+    # Release build does not apply it; or they declare no <OutputPath> at all and take the
+    # SDK default. Both send output to bin\<config>\<tfm>\ and leave Build\ absent.
+    #
+    # Neither is a fault in those repositories. Nothing ever verified this directory: the
+    # convention was enforced by a linter that only rewrote <OutputPath> lines already
+    # present, and no check read Build\ until this one.
+    - name: Validate subject build output
+      if: steps.changed.outputs.count != '0'
+      shell: pwsh
+      run: |
+        $subjectDir = "${{ github.workspace }}\Build"
+
+        if (-not (Test-Path $subjectDir)) {
+          Write-Host "::error title=Versioning::Subject build output missing at $subjectDir. This repository's own assemblies were not built to the directory the check attributes against, so no failure could be attributed to it. Two usual causes: the projects declare <OutputPath>..\Build\ only under Debug or Test conditions, which a Release build does not apply; or they declare no <OutputPath> at all and take the SDK default. Either way the output went to bin\Release\<tfm>\ instead."
+          exit 1
+        }
+
+        $dlls = @(Get-ChildItem $subjectDir -Filter *.dll -Recurse -ErrorAction SilentlyContinue)
+        if ($dlls.Count -eq 0) {
+          Write-Host "::error title=Versioning::Subject build output at $subjectDir contains no assemblies. The directory exists but nothing was built into it, so no failure could be attributed to this repository."
+          exit 1
+        }
+
+        Write-Host "::notice title=Versioning::Subject build output: $($dlls.Count) assembl(ies) in $subjectDir."
+
     - name: Prepare VersioningRunner
       id: runner
       if: steps.changed.outputs.count != '0'
diff --git a/.github/scripts/tests/ci-versioning-action.Tests.ps1 b/.github/scripts/tests/ci-versioning-action.Tests.ps1
new file mode 100644
index 0000000..460f5ca
--- /dev/null
+++ b/.github/scripts/tests/ci-versioning-action.Tests.ps1
@@ -0,0 +1,64 @@
+# ci-versioning-action.Tests.ps1 — structural assertions over ci-versioning's action.yml.
+#
+# Same shape as the sibling files for ci-serialisation and ci-compliance: properties of the
+# action as a whole that a reader cannot check from any one place in it.
+#
+# The property here is that the check asserts its own preconditions. The runner is handed a
+# subject build directory and narrows attribution to the assemblies in it; if that directory is
+# missing it silently widens to the whole dependency closure and reports other repositories'
+# defects against this one. The runner warns, but on stderr, and a warning does not stop a
+# verdict being produced on no basis.
+#
+# Run locally:  pwsh -Command "Invoke-Pester .github/scripts/tests -Output Detailed"
+# Run in CI:    lint-workflows.yml, the powershell-tests job.
+
+BeforeAll {
+    $repoRoot = (Resolve-Path (Join-Path $PSScriptRoot '../../..')).Path
+    $script:actionPath = Join-Path $repoRoot '.github/actions/ci-versioning/action.yml'
+    $script:lines = Get-Content $actionPath
+    $script:text  = $lines -join "`n"
+}
+
+Describe 'ci-versioning action.yml' {
+
+    Context 'preconditions are asserted before the runner is invoked' {
+
+        # The datasets guard already existed and is the pattern being followed. Asserted so
+        # that if it is ever removed, the reasoning below stops resting on something absent.
+        It 'still guards the datasets directory' {
+            $text | Should -Match 'Datasets directory missing at'
+        }
+
+        It 'guards the subject build output the same way' {
+            $text | Should -Match 'Subject build output missing at' -Because 'an absent subject directory makes the check attribute every failure across the whole closure'
+        }
+
+        It 'also rejects a subject directory that exists but is empty' {
+            $text | Should -Match 'contains no assemblies' -Because 'a present but empty directory yields an empty subject set, which reports nothing and passes'
+        }
+
+        It 'fails rather than warns, because the check cannot do its job without it' {
+            $guard = $text -split '- name: Validate subject build output' | Select-Object -Last 1
+            $guard = ($guard -split '- name: ')[0]
+            $guard | Should -Match 'exit 1'
+            $guard | Should -Not -Match '::warning'
+        }
+
+        # Ordering matters and is not obvious from either step alone: the guard is worthless
+        # after the thing it protects has already run.
+        It 'runs before the versioning tests, not after' {
+            $guardIdx  = ($lines | Select-String -Pattern '- name: Validate subject build output' | Select-Object -First 1).LineNumber
+            $runnerIdx = ($lines | Select-String -Pattern '- name: Run versioning tests'          | Select-Object -First 1).LineNumber
+            $guardIdx | Should -BeLessThan $runnerIdx
+        }
+
+        # Substring rather than regex: the path contains backslashes and braces, and a guard
+        # that checked a different directory from the one the runner reads would pass a
+        # loosely-written pattern while protecting nothing.
+        It 'checks the same directory the runner is pointed at' {
+            $subjectPath = '"${{ github.workspace }}\Build"'
+            $text.Contains('$subjectDir = ' + $subjectPath)          | Should -BeTrue -Because 'the guard must read the path the runner is given'
+            $text.Contains('--subject-assemblies ' + $subjectPath)   | Should -BeTrue -Because 'if the runner argument changes, this guard stops protecting it'
+        }
+    }
+}