diff --git a/tests-js/update-ui.test.mjs b/tests-js/update-ui.test.mjs
index a55434c385..3906cb7fd0 100644
--- a/tests-js/update-ui.test.mjs
+++ b/tests-js/update-ui.test.mjs
@@ -74,7 +74,8 @@ test('checks the live Desktop bridge against the staged target before opening Ab
   const f = fixture({ statusOverride: { supported: true, branch: 'main', currentSha: 'b'.repeat(40), targetSha: sha, behind: 1, dirty: false } })
   await updateUi.assertStagedBranch(f.page, sha, f.log)
   expect(f.log).toHaveBeenCalledWith(expect.stringContaining('"targetSha"'))
-  const current = fixture({ statusOverride: { supported: true, branch: 'main', currentSha: 'b'.repeat(40), targetSha: sha, behind: 1, updateAvailable: true } })
+  // The current checker's hosted shape: it cannot count staged commits via GitHub compare.
+  const current = fixture({ statusOverride: { supported: true, branch: 'main', currentSha: 'b'.repeat(40), targetSha: sha, behind: null, commits: [], updateAvailable: true } })
   await updateUi.assertStagedBranch(current.page, sha, current.log)
   const racing = fixture()
   let checks = 0
@@ -90,6 +91,7 @@ test('checks the live Desktop bridge against the staged target before opening Ab
     { supported: true, branch: 'main', targetSha: sha, behind: 0 },
     { supported: true, branch: 'main', currentSha: sha, targetSha: sha, behind: 1 },
     { supported: true, branch: 'main', targetSha: sha, behind: 1, updateAvailable: false },
+    { supported: true, branch: 'main', currentSha: 'b'.repeat(40), targetSha: sha, behind: null },
   ]) {
     const refused = fixture({ statusOverride })
     await expect(updateUi.assertStagedBranch(refused.page, sha, refused.log)).rejects.toThrow(/refusing to click/)
diff --git a/tests/install/e2e-assets/source-driver.sh b/tests/install/e2e-assets/source-driver.sh
index 7067b18d00..3bda813bcc 100644
--- a/tests/install/e2e-assets/source-driver.sh
+++ b/tests/install/e2e-assets/source-driver.sh
@@ -20,6 +20,21 @@ source_hermes() {
   printf '%s\n' "$command"
 }
 
+# v2026.6.19's install.sh (also run by its DMG bootstrap) writes .install_method
+# into the checkout without ignoring it, and that release's Desktop checker
+# reports git status --porcelain verbatim. Teach only this disposable clone that
+# the installer's own marker is not a source edit; keep the marker for
+# install-method detection and refuse any other dirty state.
+accept_installer_marker() {
+  local root="$1" status
+  status="$(git -C "$root" status --porcelain --untracked-files=all)" || return 1
+  if [ "$status" = '?? .install_method' ] && [ "$(cat "$root/.install_method")" = git ]; then
+    printf '\n/.install_method\n' >> "$(git -C "$root" rev-parse --absolute-git-dir)/info/exclude"
+    status="$(git -C "$root" status --porcelain --untracked-files=all)" || return 1
+  fi
+  [ -z "$status" ] || { printf 'installed source has changes other than the installer marker:\n%s\n' "$status" >&2; return 1; }
+}
+
 # Hand out a command to DRIVE the next ordinary startup, even when the
 # published launcher is not there yet.
 #
diff --git a/tests/install/e2e-assets/update-ui.cjs b/tests/install/e2e-assets/update-ui.cjs
index 62e2e5e8bb..e80856f98b 100644
--- a/tests/install/e2e-assets/update-ui.cjs
+++ b/tests/install/e2e-assets/update-ui.cjs
@@ -90,12 +90,16 @@ async function assertStagedBranch(page, expectedSha, log) {
     await page.waitForTimeout(1_000)
   }
   // Historical Desktop status has no updateAvailable field: its About/overlay
-  // offers the button when behind > 0. Never accept an explicit false from a
-  // newer checker, a dirty source tree, or merely a matching remote tip.
+  // offers the button when behind > 0. A newer checker states updateAvailable
+  // and leaves behind null when it cannot count (GitHub compare does not know a
+  // staged commit). Never accept an explicit false, a dirty source tree, or
+  // merely a matching remote tip.
+  const offered = status.updateAvailable === undefined
+    ? Number.isInteger(status.behind) && status.behind > 0
+    : status.updateAvailable === true
   if (status.supported !== true || status.error || status.dirty === true ||
       status.branch !== 'main' || status.targetSha !== expectedSha ||
-      !Number.isInteger(status.behind) || status.behind <= 0 ||
-      status.currentSha === expectedSha || status.updateAvailable === false) {
+      status.currentSha === expectedSha || !offered) {
     throw new Error('Desktop source check did not offer staged Git main; refusing to click an unrelated update')
   }
 }
diff --git a/tests/install/installer-script-e2e.sh b/tests/install/installer-script-e2e.sh
index a00d127eb8..9f90322bc4 100755
--- a/tests/install/installer-script-e2e.sh
+++ b/tests/install/installer-script-e2e.sh
@@ -452,18 +452,7 @@ case "$UPDATE_METHOD" in
     # fixup) runs for real in the installed code.
     EXPECT_DESKTOP=present
     HERMES="$(source_hermes "$INSTALL_DIR")" || fail "no installed desktop command"
-    # v2026.6.19's installer writes this marker into its checkout without
-    # ignoring it. Its Electron checker reports git status --porcelain verbatim;
-    # teach only this disposable clone that the installer's own state is not
-    # a source edit. Keep the marker for install-method detection and refuse
-    # any other dirty state rather than masking it for the GUI update.
-    marker_status="$(git -C "$INSTALL_DIR" status --porcelain --untracked-files=all)"
-    if [ "$marker_status" = '?? .install_method' ] &&
-       [ "$(cat "$INSTALL_DIR/.install_method")" = git ]; then
-      printf '\n/.install_method\n' >> "$(git -C "$INSTALL_DIR" rev-parse --absolute-git-dir)/info/exclude"
-      ok "ignored only the installer's generated .install_method in local Git exclude"
-    fi
-    [ -z "$(git -C "$INSTALL_DIR" status --porcelain --untracked-files=all)" ] \
+    accept_installer_marker "$INSTALL_DIR" \
       || fail "installed source has changes other than the generated install marker"
     ASSETS="$REPO_ROOT/tests/install/e2e-assets"
     SPEC="$WORK_ROOT/launch-spec.json"
diff --git a/tests/install/macos-desktop-e2e.sh b/tests/install/macos-desktop-e2e.sh
index f3970ccaa9..4a096a05b1 100755
--- a/tests/install/macos-desktop-e2e.sh
+++ b/tests/install/macos-desktop-e2e.sh
@@ -271,6 +271,8 @@ run_playwright_update() {
   # $1: spec file to launch from.
   local spec="$1"
   local rc=0
+  accept_installer_marker "$INSTALL_DIR" \
+    || fail "installed source has changes other than the generated install marker"
   (cd "$WORK_ROOT" && "$HERMES_E2E_NODE" "$ASSETS/launch-from-spec.mjs" \
     --spec "$spec" \
     --old-sha "$OLD_SHA" --chat-out "$LOG_DIR/update-window" --mock-url "$HERMES_E2E_MOCK_URL" \
diff --git a/tests/scripts/test_source_driver.py b/tests/scripts/test_source_driver.py
index 224fee9e7d..1978a30a69 100644
--- a/tests/scripts/test_source_driver.py
+++ b/tests/scripts/test_source_driver.py
@@ -47,6 +47,31 @@ if source_hermes "$INSTALL_DIR"; then exit 92; fi
     assert result.stdout.splitlines() == ["published:literal argument", "legacy"]
 
 
+@pytest.mark.platforms("posix")
+def test_installer_marker_is_the_only_dirty_state_a_driver_accepts(tmp_path):
+    root = tmp_path / "installed source"
+    root.mkdir()
+    git = ["git", "-C", str(root), "-c", "user.name=t", "-c", "user.email=t@t"]
+    subprocess.run([*git, "init", "-q"], check=True)
+    (root / "tracked").write_text("v1\n", encoding="utf-8")
+    subprocess.run([*git, "add", "tracked"], check=True)
+    subprocess.run([*git, "commit", "-qm", "base"], check=True)
+    (root / ".install_method").write_text("git\n", encoding="utf-8")
+    env = dict(os.environ, INSTALL_DIR=str(root), ASSETS=str(ASSETS), HOME=str(tmp_path))
+
+    def accept():
+        return subprocess.run(["bash", "-euc", 'source "$ASSETS/source-driver.sh"; accept_installer_marker "$INSTALL_DIR"'],
+                              env=env, capture_output=True, text=True, timeout=30)
+
+    assert accept().returncode == 0
+    assert (root / ".install_method").read_text(encoding="utf-8") == "git\n"
+    assert subprocess.run([*git, "status", "--porcelain", "--untracked-files=all"],
+                          capture_output=True, text=True, check=True).stdout == ""
+    (root / "tracked").write_text("edited\n", encoding="utf-8")
+    refused = accept()
+    assert refused.returncode != 0 and "M tracked" in refused.stderr
+
+
 @pytest.mark.platforms("windows", "posix")
 def test_powershell_selects_exact_exe_or_cmd_and_legacy_fallback(tmp_path):
     # These are file-selection rules, not Windows execution emulation. Run