diff --git a/.github/workflows/studio.yml b/.github/workflows/studio.yml
index 71f3a28ef6..ec59a68365 100644
--- a/.github/workflows/studio.yml
+++ b/.github/workflows/studio.yml
@@ -121,6 +121,45 @@ jobs:
             CGO_ENABLED=0 GOOS=$os GOARCH=amd64 go build -o /dev/null ./cmd/reasonix-studio-host
           done
 
+  # The ARM64 leg queues on a scarcer runner pool, so a pull request runs it
+  # only when it touches what the leg verifies; a push to studio always does.
+  scope:
+    name: package matrix
+    runs-on: ubuntu-latest
+    outputs:
+      matrix: ${{ steps.pick.outputs.matrix }}
+      arm64: ${{ steps.pick.outputs.arm64 }}
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
+        with:
+          fetch-depth: 0
+
+      - id: pick
+        env:
+          EVENT: ${{ github.event_name }}
+          BASE_SHA: ${{ github.event.pull_request.base.sha }}
+        run: |
+          arm64=true
+          if [ "$EVENT" = pull_request ]; then
+            arm64=false
+            if git diff --name-only "$BASE_SHA" HEAD |
+              grep -qE '^(desktop/electron/|desktop/cmd/studio-manifest/|desktop/go\.(mod|sum)$|cmd/reasonix-studio-host/|internal/platform/(update|appupdate)/|go\.(mod|sum)$|\.github/workflows/studio\.yml$)'; then
+              arm64=true
+            fi
+          fi
+          legs='[
+            {"os":"macos-latest","arch":"arm64","platform":"darwin"},
+            {"os":"ubuntu-22.04","arch":"amd64","platform":"linux"},
+            {"os":"windows-latest","arch":"amd64","platform":"windows","ebarch":"x64"}
+          ]'
+          if [ "$arm64" = true ]; then
+            legs="$(jq -c '. + [{"os":"windows-11-arm","arch":"arm64","platform":"windows","ebarch":"arm64"}]' <<<"$legs")"
+          else
+            legs="$(jq -c . <<<"$legs")"
+          fi
+          echo "arm64=$arm64" >> "$GITHUB_OUTPUT"
+          echo "matrix={\"include\":$legs}" >> "$GITHUB_OUTPUT"
+
   # The packaging contract, on real artifacts rather than on the config that
   # describes them. Unsigned on purpose: signing and notarization are a
   # distribution concern and belong to the release workflow, and treating their
@@ -128,20 +167,12 @@ jobs:
   # cross-compile while only Windows had ever produced a package.
   package:
     name: package (${{ matrix.os }})
+    needs: scope
     runs-on: ${{ matrix.os }}
+    timeout-minutes: 45
     strategy:
       fail-fast: false
-      matrix:
-        include:
-          - os: macos-latest
-            arch: arm64
-            platform: darwin
-          - os: ubuntu-22.04
-            arch: amd64
-            platform: linux
-          - os: windows-latest
-            arch: amd64
-            platform: windows
+      matrix: ${{ fromJSON(needs.scope.outputs.matrix) }}
     defaults:
       run:
         shell: bash
@@ -174,8 +205,13 @@ jobs:
         working-directory: desktop/frontend-next
         run: pnpm install --frozen-lockfile && pnpm build
 
+      # The Windows legs name their architecture instead of taking the
+      # toolchain's, so a runner whose Go or Node is the other architecture
+      # fails here rather than shipping a mislabelled package.
       - name: Build the kernel the shell speaks to
         working-directory: desktop/electron
+        env:
+          REASONIX_GOARCH: ${{ matrix.platform == 'windows' && matrix.arch || '' }}
         run: pnpm install --frozen-lockfile && pnpm run build:host
 
       # The .deb's absolute-path payload. A no-op off Linux, so it is not
@@ -191,7 +227,8 @@ jobs:
         working-directory: desktop/electron
         env:
           CSC_IDENTITY_AUTO_DISCOVERY: "false"
-        run: pnpm run build:dist
+          ELECTRON_BUILDER_7Z_FILTER: ${{ matrix.platform == 'windows' && matrix.arch == 'arm64' && 'BCJ' || '' }}
+        run: pnpm run build:dist ${{ matrix.ebarch && format('--{0}', matrix.ebarch) || '' }}
 
       # The release vocabulary is GOARCH. electron-builder's own is not, and the
       # rename that reconciles them runs here rather than in the manifest.
@@ -207,6 +244,78 @@ jobs:
             exit 1
           fi
 
+      # An emulated x64 program runs on ARM64 Windows, so a package that merely
+      # starts proves nothing about its architecture: every PE image in the
+      # bundle has to be built for the one the package is named for.
+      - name: Every PE image is built for the package's architecture
+        if: matrix.platform == 'windows'
+        run: |
+          dir=$(find dist -maxdepth 1 -type d -name 'win*-unpacked' | head -1)
+          test -n "$dir"
+          node desktop/electron/packaging/pe.js "$dir" "${{ matrix.arch }}"
+
+      # Run on the machine the package is for: the shell's own binary as a
+      # node, the kernel's identity flag, then the installer end to end, per-user
+      # and silent, into a folder of its own.
+      - name: The package starts, installs and uninstalls
+        if: matrix.platform == 'windows'
+        shell: pwsh
+        env:
+          ARCH: ${{ matrix.arch }}
+          REASONIX_HOME: ${{ runner.temp }}/reasonix-home
+        run: |
+          $ErrorActionPreference = 'Stop'
+          $unpacked = Get-ChildItem dist -Directory -Filter 'win*-unpacked' | Select-Object -First 1
+          # A windowed executable returns to the shell at once with no stdout, so
+          # it is started and waited for with its output redirected to a file.
+          $env:ELECTRON_RUN_AS_NODE = '1'
+          $out = Join-Path $env:RUNNER_TEMP 'shell-arch.txt'
+          $node = Start-Process -FilePath "$($unpacked.FullName)/Reasonix Studio.exe" -ArgumentList '-p', 'process.arch' -RedirectStandardOutput $out -Wait -PassThru
+          Remove-Item Env:ELECTRON_RUN_AS_NODE
+          $arch = (Get-Content -Raw $out).Trim()
+          $want = if ($env:ARCH -eq 'amd64') { 'x64' } else { $env:ARCH }
+          if ($node.ExitCode -ne 0 -or $arch -ne $want) { throw "the shell reports arch '$arch' (exit $($node.ExitCode)), want $want" }
+          & "$($unpacked.FullName)/resources/bin/reasonix-studio-host.exe" -instance-id
+          if ($LASTEXITCODE -ne 0) { throw "the kernel exited $LASTEXITCODE" }
+
+          $installer = (Get-ChildItem dist -Filter "ReasonixStudio-windows-$env:ARCH-installer.exe").FullName
+          $target = Join-Path $env:RUNNER_TEMP 'rx-install'
+          $run = Start-Process -FilePath $installer -ArgumentList '/S', "/D=$target" -Wait -PassThru
+          if ($run.ExitCode -ne 0) { throw "the installer exited $($run.ExitCode)" }
+          $clock = [Diagnostics.Stopwatch]::StartNew()
+          $name = [IO.Path]::GetFileNameWithoutExtension($installer)
+          while (-not (Test-Path "$target/Reasonix Studio.exe") -and $clock.Elapsed.TotalSeconds -lt 120) { Start-Sleep -Seconds 2 }
+          Write-Host "application appeared after $([int]$clock.Elapsed.TotalSeconds)s past the installer's exit; installer processes still alive: $(@(Get-Process -Name $name -ErrorAction SilentlyContinue).Count)"
+          if (-not (Test-Path "$target/Reasonix Studio.exe")) {
+            Write-Host "target holds: $((Get-ChildItem -Force $target -ErrorAction SilentlyContinue | ForEach-Object Name) -join ', ')"
+            Write-Host "files under the target: $((Get-ChildItem -Recurse -File $target -ErrorAction SilentlyContinue).Count); resources holds: $((Get-ChildItem -Force "$target/resources" -ErrorAction SilentlyContinue | ForEach-Object Name) -join ', ')"
+            Write-Host "Defender detections: $((Get-MpThreatDetection -ErrorAction SilentlyContinue | ForEach-Object { $_.Resources }) -join '; ')"
+            Write-Host "real-time protection: $((Get-MpComputerStatus -ErrorAction SilentlyContinue).RealTimeProtectionEnabled)"
+            Write-Host "PE files present: $((Get-ChildItem -Recurse -File $target -Include *.exe,*.dll,*.node | ForEach-Object { $_.FullName.Substring($target.Length) }) -join ', ')"
+            Write-Host "Defender events: $((Get-WinEvent -FilterHashtable @{LogName='Microsoft-Windows-Windows Defender/Operational'; Id=1116,1117,1121,1122} -MaxEvents 5 -ErrorAction SilentlyContinue | ForEach-Object { $_.Message -replace '\s+',' ' }) -join ' || ')"
+            Set-MpPreference -DisableRealtimeMonitoring $true
+            $again = Join-Path $env:RUNNER_TEMP 'rx-install-nodefender'
+            Start-Process -FilePath $installer -ArgumentList '/S', "/D=$again" -Wait | Out-Null
+            Start-Sleep -Seconds 20
+            Write-Host "without real-time scanning the application is present: $(Test-Path "$again/Reasonix Studio.exe")"
+            throw 'the installer reported success and installed nothing'
+          }
+          node desktop/electron/packaging/pe.js $target $env:ARCH
+          if ($LASTEXITCODE -ne 0) { throw 'the installed tree is not the package architecture' }
+
+          $run = Start-Process -FilePath "$target/Uninstall Reasonix Studio.exe" -ArgumentList '/S', "_?=$target" -Wait -PassThru
+          if ($run.ExitCode -ne 0) { throw "the uninstaller exited $($run.ExitCode)" }
+          if (Test-Path "$target/Reasonix Studio.exe") { throw 'the uninstaller left the application behind' }
+
+      - name: Keep the arm64 installer for the x64 refusal check
+        if: matrix.platform == 'windows' && matrix.arch == 'arm64'
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: studio-arm64-installer-ci
+          path: dist/ReasonixStudio-windows-arm64-installer.exe
+          if-no-files-found: error
+          retention-days: 1
+
       # The update channel is the signed manifest. A second answer to which
       # version is current is not a convenience. The .blockmap is the same
       # channel's differential index and carries the release prefix, so it also
@@ -301,3 +410,27 @@ jobs:
             echo "::error::the manifest has no entry for ${{ matrix.platform }}-${{ matrix.arch }}"
             exit 1
           fi
+
+  # The arm64 installer carries one application and no other. Run anywhere but
+  # ARM64 Windows it has to say so and stop, instead of exiting 0 with nothing
+  # installed.
+  arm64-installer-refuses-x64:
+    name: arm64 installer refuses an x64 machine
+    needs: [scope, package]
+    if: needs.scope.outputs.arm64 == 'true'
+    runs-on: windows-latest
+    steps:
+      - uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
+        with:
+          name: studio-arm64-installer-ci
+          path: installer
+
+      - name: Run it silently
+        shell: pwsh
+        run: |
+          $ErrorActionPreference = 'Stop'
+          $target = Join-Path $env:RUNNER_TEMP 'rx-refused'
+          $run = Start-Process -FilePath installer/ReasonixStudio-windows-arm64-installer.exe -ArgumentList '/S', "/D=$target" -Wait -PassThru
+          Write-Host "exit code $($run.ExitCode)"
+          if ($run.ExitCode -eq 0) { throw 'the arm64 installer exited 0 on an x64 machine' }
+          if (@(Get-ChildItem -Recurse -Force $target -ErrorAction SilentlyContinue).Count -ne 0) { throw 'the arm64 installer wrote into the target on an x64 machine' }
diff --git a/desktop/electron/assets/installer.nsh b/desktop/electron/assets/installer.nsh
index 57f76531d5..3e91fef848 100644
--- a/desktop/electron/assets/installer.nsh
+++ b/desktop/electron/assets/installer.nsh
@@ -99,6 +99,17 @@
 # full of someone's work gets named in the first place. Refuse with an exit
 # code, because that is all a script driving this can read.
 !macro customInit
+  # The arm64 installer carries only an arm64 application: on any other
+  # machine electron-builder's extraction picks no archive and installs nothing
+  # while reporting success.
+  !ifdef APP_ARM64
+    ${IfNot} ${IsNativeARM64}
+      DetailPrint "This installer is for Windows on ARM64 and cannot run on this machine"
+      MessageBox MB_ICONSTOP "This Reasonix Studio installer is built for Windows on ARM64.$\r$\n$\r$\nDownload the x64 installer for this machine." /SD IDOK
+      SetErrorLevel 3
+      Abort
+    ${EndIf}
+  !endif
   Push $R9
   !insertmacro instDirTakesNothingElse $R9
   ${If} $R9 == 0
diff --git a/desktop/electron/packaging/build-host.js b/desktop/electron/packaging/build-host.js
index dc90e10b25..5c70a67291 100644
--- a/desktop/electron/packaging/build-host.js
+++ b/desktop/electron/packaging/build-host.js
@@ -45,22 +45,35 @@ function go(args, env) {
 // statically so the helper needs no redistributable on the person's machine.
 const VS_INSTALLER = path.join(process.env["ProgramFiles(x86)"] || "C:\\Program Files (x86)", "Microsoft Visual Studio", "Installer");
 
-function msvcVars() {
+// The helper is built for the architecture the kernel is, with that
+// architecture's own tools: the x64 toolset on an x64 machine, the ARM64 one on
+// an ARM64 machine (vcvarsarm64 is the native environment there).
+const WINDOWS_TOOLCHAIN = {
+  amd64: { component: "Microsoft.VisualStudio.Component.VC.Tools.x86.x64", vars: "vcvars64.bat", label: "x64" },
+  arm64: { component: "Microsoft.VisualStudio.Component.VC.Tools.ARM64", vars: "vcvarsarm64.bat", label: "ARM64" },
+};
+
+function msvcVars(toolchain) {
   const vswhere = path.join(VS_INSTALLER, "vswhere.exe");
   if (!fs.existsSync(vswhere)) return "";
-  const found = spawnSync(vswhere, ["-latest", "-products", "*", "-requires", "Microsoft.VisualStudio.Component.VC.Tools.x86.x64", "-property", "installationPath"], { encoding: "utf8" });
+  const found = spawnSync(vswhere, ["-latest", "-products", "*", "-requires", toolchain.component, "-property", "installationPath"], { encoding: "utf8" });
   const root = (found.stdout || "").trim().split(/\r?\n/)[0];
-  const vars = root ? path.join(root, "VC", "Auxiliary", "Build", "vcvars64.bat") : "";
+  const vars = root ? path.join(root, "VC", "Auxiliary", "Build", toolchain.vars) : "";
   return vars && fs.existsSync(vars) ? vars : "";
 }
 
-function buildWindowsHelper() {
-  const vars = msvcVars();
+function buildWindowsHelper(goarch) {
+  const toolchain = WINDOWS_TOOLCHAIN[goarch];
+  if (!toolchain) {
+    console.error(`no MSVC toolchain is declared for windows/${goarch}`);
+    process.exit(1);
+  }
+  const vars = msvcVars(toolchain);
   if (!vars) {
     // A release without the helper would ship computer use switched off with
     // nothing saying why, so CI refuses; a dev build only loses the feature.
     if (process.env.CI) {
-      console.error("MSVC with the x64 C++ tools is required to build reasonix-computer-helper.exe");
+      console.error(`MSVC with the ${toolchain.label} C++ tools is required to build reasonix-computer-helper.exe`);
       process.exit(1);
     }
     console.warn("MSVC not found: building without reasonix-computer-helper.exe, so computer use is unavailable in this build");
@@ -91,9 +104,12 @@ function buildWindowsHelper() {
 if (process.platform !== "darwin") {
   // go build names the binary after the package and adds .exe where it belongs.
   // Without cgo, because with it the Linux kernel inherits the runner's glibc.
-  go(["build", "-ldflags", LDFLAGS, "-o", OUT + path.sep, PKG], { CGO_ENABLED: "0" });
+  // REASONIX_GOARCH names the architecture to build for; unset, Go's own is
+  // used, which is the runner's.
+  const goarch = (process.env.REASONIX_GOARCH || "").trim();
+  go(["build", "-ldflags", LDFLAGS, "-o", OUT + path.sep, PKG], goarch ? { CGO_ENABLED: "0", GOARCH: goarch } : { CGO_ENABLED: "0" });
   console.log("built reasonix-studio-host");
-  if (process.platform === "win32") buildWindowsHelper();
+  if (process.platform === "win32") buildWindowsHelper(goarch || (process.arch === "arm64" ? "arm64" : "amd64"));
   process.exit(0);
 }
 
diff --git a/desktop/electron/packaging/pe.js b/desktop/electron/packaging/pe.js
new file mode 100644
index 0000000000..6c82d11408
--- /dev/null
+++ b/desktop/electron/packaging/pe.js
@@ -0,0 +1,77 @@
+"use strict";
+const fs = require("node:fs");
+const path = require("node:path");
+
+const MACHINE = { amd64: 0x8664, arm64: 0xaa64 };
+// Written by the NSIS installer rather than built by us, and 32-bit on every
+// architecture; the release pins it by content (windows-signing-lib.ps1).
+const INSTALLER_OWNED = { "resources/elevate.exe": 0x14c, "Uninstall Reasonix Studio.exe": 0x14c };
+const EXTENSIONS = new Set([".exe", ".dll", ".node"]);
+
+// The machine field of a PE image's COFF header. An emulated x64 binary and a
+// native one are both valid programs on ARM64 Windows, so the architecture a
+// package claims has to be read from the files themselves.
+function peMachine(bytes) {
+  if (bytes.length < 0x40 || bytes.readUInt16LE(0) !== 0x5a4d) throw new Error("not a PE image");
+  const header = bytes.readUInt32LE(0x3c);
+  if (header + 6 > bytes.length || bytes.readUInt32LE(header) !== 0x00004550) throw new Error("no PE signature");
+  return bytes.readUInt16LE(header + 4);
+}
+
+function isPeCandidate(file) {
+  if (EXTENSIONS.has(path.extname(file).toLowerCase())) return true;
+  const fd = fs.openSync(file, "r");
+  try {
+    const head = Buffer.alloc(2);
+    return fs.readSync(fd, head, 0, 2, 0) === 2 && head.readUInt16LE(0) === 0x5a4d;
+  } finally {
+    fs.closeSync(fd);
+  }
+}
+
+function walk(dir, into) {
+  for (const entry of fs.readdirSync(dir, { withFileTypes: true })) {
+    const full = path.join(dir, entry.name);
+    if (entry.isDirectory()) walk(full, into);
+    else if (entry.isFile() && isPeCandidate(full)) into.push(full);
+  }
+  return into;
+}
+
+// Every PE image under root with the machine it was built for.
+function machines(root) {
+  return walk(root, []).sort().map((file) => {
+    const name = path.relative(root, file).split(path.sep).join("/");
+    try {
+      return { file: name, machine: peMachine(fs.readFileSync(file)) };
+    } catch (err) {
+      return { file: name, machine: null, error: err.message };
+    }
+  });
+}
+
+// The images under root that are not built for arch; none means the tree is
+// uniformly that architecture. An empty tree is refused, not passed.
+function foreign(root, arch) {
+  const want = MACHINE[arch];
+  if (want === undefined) throw new Error(`unknown architecture ${arch}`);
+  const found = machines(root);
+  if (found.length === 0) throw new Error(`no PE images under ${root}`);
+  return found.filter((f) => f.machine !== (INSTALLER_OWNED[f.file] ?? want));
+}
+
+module.exports = { peMachine, machines, foreign, MACHINE };
+
+if (require.main === module) {
+  const [root, arch] = process.argv.slice(2);
+  if (!root || !arch) {
+    console.error("usage: node packaging/pe.js <dir> <amd64|arm64>");
+    process.exit(2);
+  }
+  for (const f of machines(root)) console.log(`${f.machine === null ? f.error : "0x" + f.machine.toString(16)} ${f.file}`);
+  const wrong = foreign(root, arch);
+  for (const f of wrong) {
+    console.error(f.error ? `::error::${f.file} cannot be read as a PE image: ${f.error}` : `::error::${f.file} is machine 0x${f.machine.toString(16)}, not ${arch}`);
+  }
+  process.exit(wrong.length === 0 ? 0 : 1);
+}
diff --git a/desktop/electron/test/unit.mjs b/desktop/electron/test/unit.mjs
index d6681ea769..f1524ec22b 100644
--- a/desktop/electron/test/unit.mjs
+++ b/desktop/electron/test/unit.mjs
@@ -1396,3 +1396,48 @@ test("the tray has a file for every Windows scale, at exactly 16 * scale pixels"
   assert.equal(trayAsset(5).pixels, 48);
   assert.equal(trayAsset(NaN).pixels, 16);
 });
+
+function pe(machine) {
+  const b = Buffer.alloc(0x100);
+  b.writeUInt16LE(0x5a4d, 0);
+  b.writeUInt32LE(0x80, 0x3c);
+  b.writeUInt32LE(0x00004550, 0x80);
+  b.writeUInt16LE(machine, 0x84);
+  return b;
+}
+
+test("the PE check reads each image's own machine and refuses a tree of another architecture", () => {
+  const { peMachine, machines, foreign } = require("../packaging/pe.js");
+  assert.equal(peMachine(pe(0xaa64)), 0xaa64);
+  assert.throws(() => peMachine(Buffer.from("#!/bin/sh\n".repeat(10))), /not a PE/);
+  assert.throws(() => peMachine(pe(0x8664).subarray(0, 0x70)), /no PE signature/);
+
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), "reasonix-pe-"));
+  try {
+    assert.throws(() => foreign(dir, "arm64"), /no PE images/);
+    fs.mkdirSync(path.join(dir, "resources", "bin"), { recursive: true });
+    fs.writeFileSync(path.join(dir, "app.exe"), pe(0xaa64));
+    fs.writeFileSync(path.join(dir, "resources", "bin", "host.exe"), pe(0xaa64));
+    fs.writeFileSync(path.join(dir, "locales.pak"), "not an image");
+    fs.writeFileSync(path.join(dir, "noext"), pe(0xaa64));
+    assert.equal(machines(dir).length, 3);
+    assert.deepEqual(foreign(dir, "arm64"), []);
+    assert.deepEqual(foreign(dir, "amd64").map((f) => f.file), ["app.exe", "noext", "resources/bin/host.exe"]);
+    fs.writeFileSync(path.join(dir, "ffmpeg.dll"), pe(0x8664));
+    assert.deepEqual(foreign(dir, "arm64").map((f) => f.file), ["ffmpeg.dll"]);
+    assert.throws(() => foreign(dir, "x64"), /unknown architecture/);
+    fs.writeFileSync(path.join(dir, "broken.dll"), "text, not an image");
+    const broken = foreign(dir, "arm64").find((f) => f.file === "broken.dll");
+    assert.match(broken.error, /not a PE/);
+    fs.rmSync(path.join(dir, "broken.dll"));
+    fs.writeFileSync(path.join(dir, "ffmpeg.dll"), pe(0xaa64));
+    fs.writeFileSync(path.join(dir, "resources", "elevate.exe"), pe(0x14c));
+    assert.deepEqual(foreign(dir, "arm64"), []);
+    fs.writeFileSync(path.join(dir, "Uninstall Reasonix Studio.exe"), pe(0x14c));
+    assert.deepEqual(foreign(dir, "arm64"), []);
+    fs.writeFileSync(path.join(dir, "resources", "elevate.exe"), pe(0xaa64));
+    assert.deepEqual(foreign(dir, "arm64").map((f) => f.file), ["resources/elevate.exe"]);
+  } finally {
+    fs.rmSync(dir, { recursive: true, force: true });
+  }
+});