diff --git a/.github/actions/_probe-versioning-fastfail/action.yml b/.github/actions/_probe-versioning-fastfail/action.yml
new file mode 100644
index 0000000..baf4b2b
--- /dev/null
+++ b/.github/actions/_probe-versioning-fastfail/action.yml
@@ -0,0 +1,60 @@
+name: "_probe versioning fast-fail"
+description: >
+  THROWAWAY probe for CI_Toolkit ci-versioning H1 fix. Exercises the fast-fail
+  step's blast radius in isolation: real gh release download of the baseline
+  IncludedDLLs.txt, and the vendored Assert-VersioningPrerequisites.ps1 resolved
+  via GITHUB_ACTION_PATH and invoked with -Force:$force under pwsh 7. Assemblies
+  dir is deliberately empty so every canonical DLL is "missing", which is what
+  drives the force / deletion-only guards. Not for production use.
+
+inputs:
+  force:
+    required: true
+  baseline_installer_repo:
+    required: true
+  changed_list_path:
+    description: Path to the ACMRT change list produced by compute-changed-files.
+    required: true
+
+runs:
+  using: composite
+  steps:
+    - name: Fast-fail on missing versioning-critical DLLs (probe)
+      shell: pwsh
+      env:
+        GH_TOKEN: ${{ github.token }}
+      run: |
+        $baselineRepo = '${{ inputs.baseline_installer_repo }}'
+        $baselineDir  = Join-Path $env:RUNNER_TEMP 'versioning-baseline'
+        New-Item -ItemType Directory -Force -Path $baselineDir | Out-Null
+
+        $downloaded = $false
+        try {
+            $latestTag = gh release list --repo $baselineRepo --limit 1 --json tagName --jq '.[0].tagName' 2>$null
+            if ($LASTEXITCODE -eq 0 -and $latestTag) {
+                gh release download $latestTag --repo $baselineRepo `
+                    --pattern 'IncludedDLLs.txt' `
+                    --dir $baselineDir 2>$null
+                if ($LASTEXITCODE -eq 0 -and (Test-Path (Join-Path $baselineDir 'IncludedDLLs.txt'))) {
+                    $downloaded = $true
+                }
+            }
+        } catch { }
+
+        if (-not $downloaded) {
+            Write-Host "::error::PROBE PRECONDITION FAILED: no IncludedDLLs.txt on $baselineRepo (cannot exercise fast-fail)."
+            exit 1
+        }
+        Write-Host "Baseline: $baselineRepo@$latestTag"
+
+        # Empty assemblies dir => every canonical expected DLL is missing.
+        $emptyAsm = Join-Path $env:RUNNER_TEMP 'empty-assemblies'
+        New-Item -ItemType Directory -Force -Path $emptyAsm | Out-Null
+
+        $force = '${{ inputs.force }}' -eq 'true'
+        & "$env:GITHUB_ACTION_PATH\scripts\Assert-VersioningPrerequisites.ps1" `
+            -BaselineFile    (Join-Path $baselineDir 'IncludedDLLs.txt') `
+            -AssembliesDir   $emptyAsm `
+            -ChangedListPath '${{ inputs.changed_list_path }}' `
+            -Force:$force
+        exit $LASTEXITCODE
diff --git a/.github/actions/_probe-versioning-fastfail/scripts/Assert-VersioningPrerequisites.ps1 b/.github/actions/_probe-versioning-fastfail/scripts/Assert-VersioningPrerequisites.ps1
new file mode 100644
index 0000000..0989b77
--- /dev/null
+++ b/.github/actions/_probe-versioning-fastfail/scripts/Assert-VersioningPrerequisites.ps1
@@ -0,0 +1,60 @@
+[CmdletBinding()]
+param(
+    # IncludedDLLs.txt from the baseline installer's latest published release.
+    [Parameter(Mandatory)][string]$BaselineFile,
+    # Folder produced by dependency resolution + primary build.
+    [Parameter(Mandatory)][string]$AssembliesDir,
+    # ACMRT change list (deletions excluded) written by compute-changed-files.
+    # An existing-but-empty file means every matched change in the PR was a
+    # deletion. Absent (non-PR run) or non-empty means apply the fast-fail.
+    [string]$ChangedListPath = '',
+    # Skip the fast-fail and run versioning regardless. Mirrors BHoMBot's
+    # -force: used when a PR legitimately reduces the produced DLL set.
+    [switch]$Force
+)
+
+$ErrorActionPreference = 'Stop'
+
+# Filter to canonical BHoM DLLs. Third-party shared deps (Newtonsoft.Json, WiX
+# bundles etc.) legitimately vary between builds; the canonical set is what
+# versioning genuinely depends on. Same regex intent as BHoMBot's Versioning.cs:187.
+$canonicalRegex = '(?i)(oM|_Engine|_Adapter)\.dll$|(?i)Revit_.+20\d{2}\.dll$'
+
+$expected = Get-Content $BaselineFile |
+    ForEach-Object { [System.IO.Path]::GetFileName($_.Trim()) } |
+    Where-Object { $_ -match $canonicalRegex } |
+    Sort-Object -Unique
+
+$present = @(Get-ChildItem $AssembliesDir -Filter '*.dll' -ErrorAction SilentlyContinue |
+             ForEach-Object { $_.Name })
+
+$missing = @($expected | Where-Object { $_ -notin $present })
+
+if ($missing.Count -eq 0) {
+    Write-Host "::notice::Versioning prerequisites present ($($expected.Count) canonical DLLs matched from baseline)."
+    exit 0
+}
+
+$missingList = $missing -join ', '
+
+if ($Force) {
+    Write-Host "::warning::Versioning prerequisites missing ($missingList), but force=true, so running versioning anyway."
+    exit 0
+}
+
+# Deletion-only guard: mirrors BHoMBot Versioning.cs:209 (csFiles.Any(File.Exists)).
+# A PR that only removes source legitimately produces fewer DLLs, so a missing
+# DLL is expected: run versioning rather than blocking it here. The check applies
+# only when the change list exists (PR run) and is empty (all matched changes
+# were deletions); an absent list (non-PR run) or a non-empty list falls through
+# to the fast-fail.
+if ($ChangedListPath -and (Test-Path $ChangedListPath)) {
+    $nonDeleted = @(Get-Content $ChangedListPath | Where-Object { $_.Trim() })
+    if ($nonDeleted.Count -eq 0) {
+        Write-Host "::warning::Versioning prerequisites missing ($missingList), but this PR only deletes source files, so fewer DLLs is expected. Skipping fast-fail and running versioning."
+        exit 0
+    }
+}
+
+Write-Host "::error title=Versioning prerequisites missing::The following DLLs are expected (per the last published baseline release) but were not produced by dependency resolution + primary build: $missingList. Fix the upstream failure before re-running versioning."
+exit 1
diff --git a/.github/workflows/_probe-versioning-fastfail.yml b/.github/workflows/_probe-versioning-fastfail.yml
new file mode 100644
index 0000000..5779403
--- /dev/null
+++ b/.github/workflows/_probe-versioning-fastfail.yml
@@ -0,0 +1,77 @@
+name: _probe versioning fast-fail
+
+# THROWAWAY validation harness for the CI_Toolkit ci-versioning H1 fix.
+# Runs on PRs that touch this branch's probe files. Delete after validating.
+
+on:
+  pull_request:
+    paths:
+      - '.github/actions/_probe-versioning-fastfail/**'
+      - '.github/workflows/_probe-versioning-fastfail.yml'
+      - 'probe-src/**'
+
+permissions:
+  contents: read
+
+jobs:
+  probe:
+    runs-on: windows-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - name: missing-fail          # canonical DLLs missing, no force, real source change -> fail
+            force: 'false'
+            empty: 'false'
+            expect: failure
+          - name: force-skip            # force bypass -> succeed
+            force: 'true'
+            empty: 'false'
+            expect: success
+          - name: deletion-only-skip    # empty ACMRT list (deletion-only PR) -> succeed
+            force: 'false'
+            empty: 'true'
+            expect: success
+    steps:
+      - name: Checkout PR branch
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          fetch-depth: 0
+
+      - name: Compute changed files
+        uses: BuroHappoldEngineeringAdmin/CI_Toolkit/.github/actions/compute-changed-files@develop
+        with:
+          patterns: '*.cs'
+          skip_if_not_pr: 'false'
+          output_file: changed_versioning_files.txt
+
+      - name: Show / tamper change list
+        shell: bash
+        run: |
+          echo "--- changed_versioning_files.txt (as produced) ---"
+          cat changed_versioning_files.txt || echo "(absent)"
+          if [ "${{ matrix.empty }}" = "true" ]; then
+            : > changed_versioning_files.txt
+            echo "--- emptied to simulate a deletion-only PR ---"
+          fi
+
+      - name: Run fast-fail probe
+        id: probe
+        continue-on-error: true
+        uses: ./.github/actions/_probe-versioning-fastfail
+        with:
+          force: ${{ matrix.force }}
+          baseline_installer_repo: BuroHappoldEngineeringSandbox/BHoM_Installer
+          changed_list_path: ${{ github.workspace }}/changed_versioning_files.txt
+
+      - name: Assert outcome
+        shell: bash
+        run: |
+          got='${{ steps.probe.outcome }}'
+          want='${{ matrix.expect }}'
+          echo "leg=${{ matrix.name }} force=${{ matrix.force }} empty=${{ matrix.empty }} outcome=$got expected=$want"
+          if [ "$got" != "$want" ]; then
+            echo "::error::leg ${{ matrix.name }}: probe outcome '$got' != expected '$want'"
+            exit 1
+          fi
+          echo "leg ${{ matrix.name }}: OK"
diff --git a/probe-src/Dummy.cs b/probe-src/Dummy.cs
new file mode 100644
index 0000000..5606393
--- /dev/null
+++ b/probe-src/Dummy.cs
@@ -0,0 +1 @@
+namespace Probe { public class Dummy { public int V => 1; } }