diff --git a/.claude/CLAUDE.md b/.claude/CLAUDE.md
index 77a3bc3c0e..318e7d3e32 100644
--- a/.claude/CLAUDE.md
+++ b/.claude/CLAUDE.md
@@ -99,8 +99,17 @@ hlslkit-generate-defines --log CommunityShaders.log
 
 # Scan for buffer conflicts across features
 hlslkit-buffer-scan --features-dir features/
+
+# Prove a shader refactor changed no behavior (compiles base ref vs working tree,
+# compares DXBC across VR x HDR_OUTPUT permutations; exit 0 identical / 2 differs)
+pwsh tools/verify-shader-refactor.ps1 package/Shaders/Foo.hlsl   # bash: tools/verify-shader-refactor.sh
 ```
 
+When refactoring an existing shader (especially the decompile-transcription shaders like
+`ISTemporalAA.hlsl`), use `tools/verify-shader-refactor.ps1` to prove the change is
+behavior-preserving: identical compiled bytecode means a provable no-op. See
+`docs/development/shader-workflow.md` for details.
+
 ### Custom CMake Targets
 
 **Package and Deployment Targets**:
diff --git a/docs/development/shader-workflow.md b/docs/development/shader-workflow.md
index f32233a73e..bf58da9431 100644
--- a/docs/development/shader-workflow.md
+++ b/docs/development/shader-workflow.md
@@ -8,8 +8,30 @@ cmake --build build/ALL --target COPY_SHADERS
 
 # Full deployment (DLL + tests + shaders)
 cmake --build build/ALL --target DEPLOY_ALL
+
+# Prove an HLSL refactor changed no behavior (compares compiled DXBC vs a git ref)
+pwsh tools/verify-shader-refactor.ps1 package/Shaders/Foo.hlsl   # or tools/verify-shader-refactor.sh
 ```
 
+## Verifying refactors
+
+`tools/verify-shader-refactor.ps1` (bash wrapper: `tools/verify-shader-refactor.sh`)
+compiles a shader from a base git ref and from the working tree across the
+`VR` × `HDR_OUTPUT` permutations, then compares the compiled bytecode. The base
+ref's whole include tree is materialized (via `git archive`), so the base compiles
+against base-ref `.hlsli` headers and the working tree against working headers — a
+refactor that also edits a shared header is compared correctly, not masked:
+
+-   **IDENTICAL** SHA-256 of the `.cso` ⇒ the refactor is a provable no-op (fxc emits
+    no timestamps without `/Zi`, so identical source ⇒ identical bytes).
+-   **DIFFERS** ⇒ it dumps and diffs the `/Fc` assembly so a legitimate-but-non-identical
+    change can be reviewed.
+
+Exit codes: `0` all identical, `2` some differ, `1` compile error. Defaults to comparing
+the working tree against `merge-base(HEAD, origin/dev)`; pass `-BaseRef <ref>` to override.
+Requires `fxc.exe` from the Windows SDK. The permutation sweep is strong evidence, not the
+full `shader-validation.yaml` matrix — pass `-Permutations` for exotic define combos.
+
 ## Overview
 
 Two deployment targets for different workflows:
diff --git a/tools/verify-shader-refactor.ps1 b/tools/verify-shader-refactor.ps1
new file mode 100644
index 0000000000..c18954f977
--- /dev/null
+++ b/tools/verify-shader-refactor.ps1
@@ -0,0 +1,208 @@
+<#
+.SYNOPSIS
+    Prove an HLSL refactor is behavior-preserving by comparing compiled bytecode.
+
+.DESCRIPTION
+    Compiles a shader from a base git revision and from the current working tree
+    across a set of preprocessor permutations, then compares the resulting DXBC.
+    The base ref's entire include tree (-IncludeDir) is materialized via git archive,
+    so the base compiles against base-ref headers and the working tree against working
+    headers -- a refactor that also edits a shared .hlsli is therefore compared correctly.
+
+    Tier 1 (this script): identical SHA-256 of the compiled .cso == provably identical
+    GPU program. fxc emits no timestamps without /Zi, so same source -> same bytes.
+
+    Tier 2 (this script, on mismatch): dumps /Fc assembly for both revisions and lists
+    the differing lines (base/work markers), so a legitimate-but-non-identical refactor
+    (e.g. register reorder) can be eyeballed.
+
+    A refactor that is Tier-1 IDENTICAL on the swept permutations needs no further proof.
+    Note: the default sweep (VR x HDR_OUTPUT) is strong evidence, not the full build
+    matrix from shader-validation.yaml. Pass -Permutations for exotic define combos.
+
+.PARAMETER Shader
+    Path to the .hlsl file (repo-relative or absolute).
+
+.PARAMETER BaseRef
+    Git ref to treat as "before". Default: merge-base of HEAD and origin/dev.
+
+.PARAMETER IncludeDir
+    Shader include root passed to fxc /I. Default: package/Shaders.
+
+.PARAMETER Permutations
+    Optional explicit permutation list; each entry is a space-separated define set,
+    e.g. -Permutations "PSHADER","PSHADER VR". Overrides the auto sweep.
+
+.PARAMETER Entry
+    Shader entry point. Default: main.
+
+.PARAMETER Profile
+    fxc target profile. Default: auto (cs_5_0 for *CS.hlsl, else ps_5_0).
+
+.EXAMPLE
+    pwsh tools/verify-shader-refactor.ps1 package/Shaders/ISTemporalAA.hlsl
+
+.EXAMPLE
+    pwsh tools/verify-shader-refactor.ps1 package/Shaders/Foo.hlsl -BaseRef HEAD~1
+#>
+[CmdletBinding()]
+param(
+    [Parameter(Mandatory = $true, Position = 0)]
+    [string]$Shader,
+    [string]$BaseRef,
+    [string]$IncludeDir = "package/Shaders",
+    [string[]]$Permutations,
+    [string]$Entry = "main",
+    [string]$Profile,
+    [string]$Fxc
+)
+
+# Continue (not Stop): native git calls write warnings to stderr that would otherwise
+# abort the run; control flow keys off explicit $LASTEXITCODE checks and `throw`.
+$ErrorActionPreference = "Continue"
+
+function Resolve-Fxc {
+    if ($Fxc -and (Test-Path $Fxc)) { return $Fxc }
+    $cmd = Get-Command fxc.exe -ErrorAction SilentlyContinue
+    if ($cmd) { return $cmd.Source }
+    $roots = @("${env:ProgramFiles(x86)}\Windows Kits\10\bin", "${env:ProgramFiles}\Windows Kits\10\bin")
+    $found = foreach ($r in $roots) {
+        if (Test-Path $r) {
+            Get-ChildItem -Path $r -Recurse -Filter fxc.exe -ErrorAction SilentlyContinue |
+                Where-Object { $_.FullName -match "x64" }
+        }
+    }
+    $pick = $found | Sort-Object FullName -Descending | Select-Object -First 1
+    if (-not $pick) { throw "fxc.exe not found. Install the Windows 10/11 SDK or pass -Fxc." }
+    return $pick.FullName
+}
+
+# Resolve repo root so the script works from any cwd.
+$repoRoot = (git rev-parse --show-toplevel 2>$null)
+if (-not $repoRoot) { throw "Not inside a git repository." }
+Push-Location $repoRoot
+$work = $null
+try {
+    $fxcPath = Resolve-Fxc
+
+    # Normalize the shader path to repo-relative (forward slashes) for git.
+    # (Path.GetRelativePath is unavailable in Windows PowerShell 5.1 / .NET Framework.)
+    $shaderFull = (Resolve-Path $Shader).Path
+    $rootFull = (Resolve-Path $repoRoot).Path
+    if (-not $shaderFull.StartsWith($rootFull, [StringComparison]::OrdinalIgnoreCase)) {
+        throw "Shader '$Shader' resolves outside the repo root '$rootFull'."
+    }
+    $relPath = $shaderFull.Substring($rootFull.Length).TrimStart('\', '/').Replace('\', '/')
+
+    if (-not $BaseRef) {
+        $BaseRef = (git merge-base HEAD origin/dev 2>$null)
+        if (-not $BaseRef) { $BaseRef = "HEAD" }
+    }
+
+    if (-not $Profile) {
+        $Profile = if ($relPath -match 'CS\.hlsl$') { "cs_5_0" } else { "ps_5_0" }
+    }
+    $stageDefine = switch -Wildcard ($Profile) {
+        "cs_*" { "CSHADER" }
+        "vs_*" { "VSHADER" }
+        default { "PSHADER" }
+    }
+
+    if (-not $Permutations -or $Permutations.Count -eq 0) {
+        $Permutations = @(
+            "$stageDefine",
+            "$stageDefine VR",
+            "$stageDefine HDR_OUTPUT",
+            "$stageDefine VR HDR_OUTPUT"
+        )
+    }
+
+    # Materialize the base revision's FULL include tree (not just the target shader) so a
+    # refactor that also touches a shared .hlsli is compared correctly: base compiles against
+    # base-ref headers, work compiles against working-tree headers. git archive -> tar (via a
+    # file, never a PS pipeline, which would corrupt the binary tar).
+    $work = Join-Path ([IO.Path]::GetTempPath()) ("shaderverify_" + [Guid]::NewGuid().ToString("N"))
+    $baseRoot = Join-Path $work "base"
+    New-Item -ItemType Directory -Force $baseRoot | Out-Null
+    $tar = Join-Path $work "base.tar"
+    git archive --format=tar -o $tar $BaseRef -- $IncludeDir $relPath 2>$null
+    if ($LASTEXITCODE -ne 0 -or -not (Test-Path $tar)) {
+        throw "git archive failed for '$BaseRef' (paths: $IncludeDir, $relPath)."
+    }
+    tar -xf $tar -C $baseRoot
+    if ($LASTEXITCODE -ne 0) { throw "Failed to extract base archive." }
+    $baseFile = Join-Path $baseRoot $relPath
+    $baseInclude = Join-Path $baseRoot $IncludeDir
+    if (-not (Test-Path $baseFile)) { throw "'$relPath' not found at '$BaseRef'." }
+
+    function Compile([string]$src, [string]$incDir, [string]$defs, [string]$outFile, [switch]$Asm) {
+        # Preserve explicit-valued defines (e.g. SHADOWFILTER=0); only bare names get =1.
+        $defArgs = @()
+        foreach ($d in ($defs -split '\s+' | Where-Object { $_ })) {
+            $defArgs += "/D"
+            $defArgs += $(if ($d -like '*=*') { $d } else { "$d=1" })
+        }
+        $fmt = if ($Asm) { "/Fc" } else { "/Fo" }
+        $out = & $fxcPath /nologo /T $Profile /E $Entry @defArgs /I $incDir $src $fmt $outFile 2>&1
+        return @{ Code = $LASTEXITCODE; Out = $out }
+    }
+
+    Write-Host "Shader   : $relPath"
+    Write-Host "Base ref : $BaseRef  (full include tree materialized)"
+    Write-Host "Profile  : $Profile  (entry $Entry)"
+    Write-Host "Include  : $IncludeDir"
+    Write-Host ("-" * 60)
+
+    $allIdentical = $true
+    $anyError = $false
+
+    foreach ($perm in $Permutations) {
+        $tag = $perm
+        $baseCso = Join-Path $work "base.cso"
+        $workCso = Join-Path $work "work.cso"
+        $rb = Compile $baseFile $baseInclude $perm $baseCso
+        $rw = Compile $shaderFull $IncludeDir $perm $workCso
+
+        if ($rb.Code -ne 0 -or $rw.Code -ne 0) {
+            $anyError = $true
+            $which = if ($rb.Code -ne 0) { "BASE" } else { "WORK" }
+            Write-Host "[$tag] COMPILE-ERROR ($which)" -ForegroundColor Red
+            ($(if ($rb.Code -ne 0) { $rb.Out } else { $rw.Out }) | Where-Object { $_ -match 'error|warning' } | Select-Object -First 6) |
+                ForEach-Object { Write-Host "    $_" }
+            continue
+        }
+
+        $hb = (Get-FileHash $baseCso -Algorithm SHA256).Hash
+        $hw = (Get-FileHash $workCso -Algorithm SHA256).Hash
+        if ($hb -eq $hw) {
+            Write-Host "[$tag] IDENTICAL" -ForegroundColor Green
+        } else {
+            $allIdentical = $false
+            Write-Host "[$tag] DIFFERS  base=$($hb.Substring(0,12)) work=$($hw.Substring(0,12))" -ForegroundColor Yellow
+            # Tier 2: assembly diff for inspection (Compare-Object avoids git's CRLF/exit noise).
+            $baseAsm = Join-Path $work "base.asm"; $workAsm = Join-Path $work "work.asm"
+            Compile $baseFile $baseInclude $perm $baseAsm -Asm | Out-Null
+            Compile $shaderFull $IncludeDir $perm $workAsm -Asm | Out-Null
+            $d = Compare-Object (Get-Content $baseAsm) (Get-Content $workAsm)
+            if ($d) {
+                $d | Select-Object -First 40 | ForEach-Object {
+                    $mark = if ($_.SideIndicator -eq '=>') { 'work' } else { 'base' }
+                    Write-Host ("    [{0}] {1}" -f $mark, $_.InputObject)
+                }
+                if (@($d).Count -gt 40) { Write-Host ("    ... (+{0} more asm lines)" -f (@($d).Count - 40)) }
+            }
+        }
+    }
+
+    Write-Host ("-" * 60)
+    if ($anyError) { Write-Host "RESULT: compile error" -ForegroundColor Red; $exit = 1 }
+    elseif ($allIdentical) { Write-Host "RESULT: behavior-preserving (all permutations identical)" -ForegroundColor Green; $exit = 0 }
+    else { Write-Host "RESULT: bytecode differs - inspect asm diff above" -ForegroundColor Yellow; $exit = 2 }
+
+    exit $exit
+}
+finally {
+    # Runs on normal exit and on throw, so the temp dir never leaks.
+    if ($work -and (Test-Path $work)) { Remove-Item -Recurse -Force $work -ErrorAction SilentlyContinue }
+    Pop-Location
+}
diff --git a/tools/verify-shader-refactor.sh b/tools/verify-shader-refactor.sh
new file mode 100644
index 0000000000..6f87588956
--- /dev/null
+++ b/tools/verify-shader-refactor.sh
@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+# Thin wrapper around verify-shader-refactor.ps1 for bash/WSL/git-bash users.
+# fxc.exe is Windows-only and MSYS mangles its /switches, so the real work lives
+# in PowerShell. This just forwards arguments verbatim.
+#
+# Usage: tools/verify-shader-refactor.sh package/Shaders/Foo.hlsl [-BaseRef HEAD~1] ...
+set -euo pipefail
+
+here="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+ps1="$here/verify-shader-refactor.ps1"
+
+if command -v pwsh >/dev/null 2>&1; then
+	exec pwsh -NoProfile -ExecutionPolicy Bypass -File "$ps1" "$@"
+elif command -v powershell.exe >/dev/null 2>&1; then
+	exec powershell.exe -NoProfile -ExecutionPolicy Bypass -File "$ps1" "$@"
+else
+	echo "Need PowerShell (pwsh or powershell.exe) on PATH." >&2
+	exit 1
+fi