diff --git a/tests/pm/test_security_consumers.py b/tests/pm/test_security_consumers.py
index 1fa8441764..bc9cf77639 100644
--- a/tests/pm/test_security_consumers.py
+++ b/tests/pm/test_security_consumers.py
@@ -314,6 +314,8 @@ def test_tirith_failed_cold_scans_make_one_attempt_then_explicit_can_retry(consu
         real_get(handler)
     monkeypatch.setattr(RangeHandler, "do_GET", record)
     tirith.check_command_security("echo hello")
+    for thread in tirith._install_threads.values():
+        thread.join(10)
     first_attempt = list(requests)
     assert first_attempt
     tirith.check_command_security("echo hello")
diff --git a/tests/tools/test_tirith_security.py b/tests/tools/test_tirith_security.py
index c99fbdf991..56ea702016 100644
--- a/tests/tools/test_tirith_security.py
+++ b/tests/tools/test_tirith_security.py
@@ -348,8 +348,11 @@ class TestPmInstall:
         ensure.side_effect = lambda *_a, **_k: setattr(
             installed, "return_value", MagicMock(binary="/pm/tirith"))
 
-        assert _tirith_mod._resolve_tirith_path("tirith") == "/pm/tirith"
+        assert _tirith_mod._resolve_tirith_path("tirith") == "tirith"
+        for thread in _tirith_mod._install_threads.values():
+            thread.join(5)
         ensure.assert_called_once_with("tirith")
+        assert _tirith_mod._resolve_tirith_path("tirith") == "/pm/tirith"
 
     def test_failed_install_is_not_retried(self, pm_tirith):
         """After a failed install, subsequent resolves fall back without retrying."""
@@ -357,6 +360,8 @@ class TestPmInstall:
         ensure.side_effect = RuntimeError("download failed")
 
         assert _tirith_mod._resolve_tirith_path("tirith") == "tirith"
+        for thread in _tirith_mod._install_threads.values():
+            thread.join(5)
         assert _tirith_mod._resolve_tirith_path("tirith") == "tirith"
         assert ensure.call_count == 1
 
diff --git a/tools/tirith_security.py b/tools/tirith_security.py
index 5dd3ce5083..e90a744b74 100644
--- a/tools/tirith_security.py
+++ b/tools/tirith_security.py
@@ -64,7 +64,7 @@ _circuit_open_at: float = 0.0
 _breaker_lock = threading.Lock()
 
 # Warn-once: spawn/path warnings sit in the hot path and would otherwise repeat once per
-# terminal command while tirith is unavailable (e.g. install thread still running).
+# terminal command while tirith is unavailable.
 _warned_messages: set[str] = set()
 _warned_lock = threading.Lock()
 
@@ -144,6 +144,11 @@ def _claim_install_attempt() -> bool:
         return True
 
 
+def _install_in_flight() -> threading.Thread | None:
+    thread = _install_threads.get(hermes_home_key())
+    return thread if thread is not None and thread.is_alive() else None
+
+
 def is_platform_supported() -> bool:
     """Whether PM has a managed Tirith build for this host."""
     import pm
@@ -174,15 +179,8 @@ def _resolve_tirith_path(configured_path: str) -> str:
     if configured_path == "tirith":
         import pm
 
-        if not pm.lazy_installs_allowed() or not _claim_install_attempt():
-            return os.path.expanduser(configured_path)
-        try:
-            pm.ensure("tirith")
-            selected = pm.installed_package("tirith")
-            if selected and selected.binary:
-                return str(selected.binary)
-        except Exception as exc:
-            _warn_once("tirith_install", "tirith install unavailable: %s", exc)
+        if pm.lazy_installs_allowed():
+            _start_background_install(log_failures=True)
     return os.path.expanduser(configured_path)
 
 
@@ -196,6 +194,17 @@ def _background_install(*, log_failures: bool) -> None:
         log("tirith install failed: %s", exc)
 
 
+def _start_background_install(*, log_failures: bool) -> None:
+    if _claim_install_attempt():
+        context = copy_context()
+        thread = threading.Thread(
+            target=context.run, args=(_background_install,),
+            kwargs={"log_failures": log_failures}, daemon=True,
+        )
+        _install_threads[hermes_home_key()] = thread
+        thread.start()
+
+
 def ensure_installed(*, log_failures: bool = True, explicit: bool = False):
     """Opt-in startup is non-blocking. Explicit setup waits and reports errors.
 
@@ -218,14 +227,7 @@ def ensure_installed(*, log_failures: bool = True, explicit: bool = False):
         return found
     if not is_platform_supported() or not pm.lazy_installs_allowed():
         return None
-    if _claim_install_attempt():
-        context = copy_context()
-        thread = threading.Thread(
-            target=context.run, args=(_background_install,),
-            kwargs={"log_failures": log_failures}, daemon=True,
-        )
-        _install_threads[hermes_home_key()] = thread
-        thread.start()
+    _start_background_install(log_failures=log_failures)
     return None
 
 
@@ -241,8 +243,7 @@ def missing_is_expected() -> bool:
     configured = _load_security_config()["tirith_path"]
     if configured != "tirith":
         return False
-    thread = _install_threads.get(hermes_home_key())
-    if thread is not None and thread.is_alive():
+    if _install_in_flight():
         return True
     return _local_tirith(configured) is not None or not pm.lazy_installs_allowed()
 
@@ -310,6 +311,11 @@ def check_command_security(command: str) -> dict:
     if tirith_path is None:
         _warn_once("tirith_path_none", "tirith path resolved to None; scanning disabled")
         return _fail(fail_open, "tirith path unavailable", "tirith path unavailable (fail-closed)")
+    if tirith_path == "tirith" and (install := _install_in_flight()):
+        if fail_open:
+            return _verdict("allow", "tirith installing")
+        install.join()
+        tirith_path = _resolve_tirith_path(cfg["tirith_path"])
     try:
         result = subprocess.run(
             [tirith_path, "check", "--json", "--non-interactive", "--shell", "posix", "--", command],