diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 756356bce..7cdc09f8b 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -416,17 +416,40 @@ jobs:
       # false, and the editable-cursor preflight in useScreenRecorder re-opened
       # the same dialog on every press of record — recording was impossible.
       #
-      # Signed with the same runtime and entitlements electron-builder applies,
-      # so a locally signed build and a certificate-signed one differ only in the
-      # identity. Both arches on purpose: 26.8.1 only fell back on arm64, which
-      # left Intel DMGs unsigned for their whole existence.
+      # Entitlements, but deliberately NOT --options runtime.
+      #
+      # This used to match what electron-builder applies to a certificate-signed
+      # build, reasoning that the two should differ only in the identity. That
+      # reasoning shipped an app nobody could open.
+      #
+      # Hardened runtime plus an ad-hoc signature is fatal to Electron: it
+      # validates its helper processes' code signature, that validation needs the
+      # task port, and hardened runtime is what denies it. The app logs
+      #
+      #   codesign_util.cc] task_name_for_pid: (os/kern) failure (5)
+      #
+      # and exits before a window appears. That is what v0.0.1 did —
+      # `flags=0x10002(adhoc,runtime)`. The dev build, which runs perfectly, is
+      # `flags=0x20002(adhoc,linker-signed)`: same ad-hoc, no hardened runtime.
+      #
+      # Hardened runtime is mandatory for notarization and buys nothing without a
+      # real identity, so it belongs on the signed path only. TCC is unaffected —
+      # grants key to a stable signature, which this still produces, not to
+      # hardened runtime.
+      #
+      # Both arches on purpose: 26.8.1 only fell back on arm64, which left Intel
+      # DMGs unsigned for their whole existence.
       - name: Ad-hoc sign the .app
         if: steps.signing.outputs.enabled != 'true'
         run: |
           codesign --force --deep --sign - \
-            --options runtime \
             --entitlements macos.entitlements \
             "${{ steps.find_app.outputs.app_bundle }}"
+          # Fail the build rather than ship another DMG nobody can open.
+          if codesign -d --verbose=2 "${{ steps.find_app.outputs.app_bundle }}" 2>&1 | grep -q 'flags=.*runtime'; then
+            echo "::error::the ad-hoc signature carries hardened runtime; the app will not launch"
+            exit 1
+          fi
 
       # UNCONDITIONAL. Gated on `enabled == 'true'`, this step never ran for the
       # RC builds — the only ones that could be unsigned — so the regression