diff --git a/scripts/desktop-update/posix.sh b/scripts/desktop-update/posix.sh
index f9182787e7..b183521a68 100755
--- a/scripts/desktop-update/posix.sh
+++ b/scripts/desktop-update/posix.sh
@@ -245,6 +245,10 @@ start_ui() {
   fi
   { [ -f "$html" ] && [ -n "$py" ] && [ -n "$browser" ]; } || { log "shim: no renderer; skipping UI"; return; }
 
+  UI_PROFILE_DIR="$(mktemp -d "${TMPDIR:-/tmp}/hermes-update-ui-XXXXXXXX")" || {
+    log "shim: could not allocate a browser profile; skipping UI"
+    return
+  }
   publish_stage ""
   # The Desktop's final teardown targets the updater process group.  Put both
   # UI processes in their own sessions so neither the HTTP server nor a Chrome
@@ -267,7 +271,6 @@ start_ui() {
   [ -n "$port" ] || { kill -9 "$UI_SERVER_PID" 2>/dev/null; UI_SERVER_PID=""; return; }
 
   # Throwaway profile: new window/process we own; user's browser untouched.
-  UI_PROFILE_DIR="${TMPDIR:-/tmp}/hermes-update-ui-$$"
   "$py" -c 'import os, signal, sys; os.setsid(); signal.signal(signal.SIGTERM, signal.SIG_DFL); os.execv(sys.argv[1], sys.argv[1:])' \
     "$browser" --app="http://127.0.0.1:$port/" --user-data-dir="$UI_PROFILE_DIR" \
     --no-first-run --no-default-browser-check --window-size=280,320 >/dev/null 2>&1 &
diff --git a/tests/test_desktop_update_shim_profile_cleanup.py b/tests/test_desktop_update_shim_profile_cleanup.py
index b4de7ff2a5..2c5b93e43d 100644
--- a/tests/test_desktop_update_shim_profile_cleanup.py
+++ b/tests/test_desktop_update_shim_profile_cleanup.py
@@ -1,4 +1,4 @@
-"""The update hand-off cleans only the browser profile it launched."""
+"""The update hand-off cleans only an atomically claimed browser profile."""
 import os
 from pathlib import Path
 import subprocess
@@ -7,8 +7,8 @@ import pytest
 
 
 @pytest.mark.linux_only
-@pytest.mark.parametrize("failed", [False, True])
-def test_shim_removes_owned_profile_on_exit(tmp_path, failed):
+@pytest.mark.parametrize("outcome", ["success", "error", "allocation-failed"])
+def test_shim_removes_only_its_owned_profile(tmp_path, outcome):
     bin_dir = tmp_path / "bin"
     bin_dir.mkdir()
     browser = bin_dir / "google-chrome"
@@ -20,6 +20,10 @@ def test_shim_removes_owned_profile_on_exit(tmp_path, failed):
         'while :; do sleep 0.1; done\n', encoding="utf-8",
     )
     browser.chmod(0o755)
+    if outcome == "allocation-failed":
+        allocator = bin_dir / "mktemp"
+        allocator.write_text("#!/bin/sh\nexit 1\n", encoding="utf-8")
+        allocator.chmod(0o755)
     config = tmp_path / ".config"
     config.mkdir()
     (config / "mimeapps.list").write_text(
@@ -27,21 +31,29 @@ def test_shim_removes_owned_profile_on_exit(tmp_path, failed):
         'x-scheme-handler/https=google-chrome.desktop\ntext/html=google-chrome.desktop\n',
         encoding="utf-8",
     )
-    sibling = tmp_path / "hermes-update-ui-unrelated"
-    sibling.mkdir()
-    (sibling / "keep").write_text("keep", encoding="utf-8")
     install = tmp_path / "hermes-agent"
     install.mkdir()
     env = {**os.environ, "HOME": str(tmp_path), "HERMES_HOME": str(tmp_path),
            "XDG_CONFIG_HOME": str(config), "TMPDIR": str(tmp_path),
            "PATH": f"{bin_dir}:/usr/bin:/bin", "HERMES_SELFTEST_HOLD_SECONDS": "1",
-           "HERMES_UPDATE_SHIM_GRACE_SECONDS": "1", "HERMES_SELFTEST_FAIL": "1" if failed else ""}
-    result = subprocess.run(
-        ["bash", str(Path(__file__).resolve().parents[1] / "scripts/desktop-update/posix.sh"),
+           "HERMES_UPDATE_SHIM_GRACE_SECONDS": "1", "HERMES_SELFTEST_FAIL": "1" if outcome == "error" else ""}
+    process = subprocess.Popen(
+        ["bash", "-c", 'while [ ! -f "$HOME/start" ]; do sleep .05; done; exec bash "$@"', "probe",
+         str(Path(__file__).resolve().parents[1] / "scripts/desktop-update/posix.sh"),
          "--install-root", str(install), "--self-test-ui"],
-        env=env, capture_output=True, text=True, timeout=30,
+        env=env, stdout=subprocess.PIPE, stderr=subprocess.PIPE, text=True,
     )
-    assert result.returncode == (1 if failed else 0), result.stdout + result.stderr
-    owned = Path((tmp_path / "launched-profile").read_text(encoding="utf-8"))
-    assert not owned.exists()
-    assert (sibling / "keep").read_text(encoding="utf-8") == "keep"
+    collision = tmp_path / f"hermes-update-ui-{process.pid}"
+    collision.mkdir()
+    (collision / "keep").write_text("keep", encoding="utf-8")
+    (tmp_path / "start").touch()
+    stdout, stderr = process.communicate(timeout=30)
+    assert process.returncode == (1 if outcome == "error" else 0), stdout + stderr
+    launched = tmp_path / "launched-profile"
+    if outcome == "allocation-failed":
+        assert not launched.exists()
+    else:
+        owned = Path(launched.read_text(encoding="utf-8"))
+        assert owned != collision
+        assert not owned.exists()
+    assert (collision / "keep").read_text(encoding="utf-8") == "keep"