diff --git a/hermes_cli/auth.py b/hermes_cli/auth.py
index 209f72959..cde67b2b8 100644
--- a/hermes_cli/auth.py
+++ b/hermes_cli/auth.py
@@ -23,6 +23,7 @@ import stat
 import base64
 import hashlib
 import subprocess
+import threading
 import time
 import uuid
 import webbrowser
@@ -44,6 +45,10 @@ try:
     import fcntl
 except Exception:
     fcntl = None
+try:
+    import msvcrt
+except Exception:
+    msvcrt = None
 
 # =============================================================================
 # Constants
@@ -299,31 +304,64 @@ def _auth_lock_path() -> Path:
     return _auth_file_path().with_suffix(".lock")
 
 
+_auth_lock_holder = threading.local()
+
 @contextmanager
 def _auth_store_lock(timeout_seconds: float = AUTH_LOCK_TIMEOUT_SECONDS):
-    """Cross-process advisory lock for auth.json reads+writes."""
+    """Cross-process advisory lock for auth.json reads+writes.  Reentrant."""
+    # Reentrant: if this thread already holds the lock, just yield.
+    if getattr(_auth_lock_holder, "depth", 0) > 0:
+        _auth_lock_holder.depth += 1
+        try:
+            yield
+        finally:
+            _auth_lock_holder.depth -= 1
+        return
+
     lock_path = _auth_lock_path()
     lock_path.parent.mkdir(parents=True, exist_ok=True)
 
-    with lock_path.open("a+") as lock_file:
-        if fcntl is None:
+    if fcntl is None and msvcrt is None:
+        _auth_lock_holder.depth = 1
+        try:
             yield
-            return
+        finally:
+            _auth_lock_holder.depth = 0
+        return
 
+    # On Windows, msvcrt.locking needs the file to have content and the
+    # file pointer at position 0.  Ensure the lock file has at least 1 byte.
+    if msvcrt and (not lock_path.exists() or lock_path.stat().st_size == 0):
+        lock_path.write_text(" ", encoding="utf-8")
+
+    with lock_path.open("r+" if msvcrt else "a+") as lock_file:
         deadline = time.time() + max(1.0, timeout_seconds)
         while True:
             try:
-                fcntl.flock(lock_file.fileno(), fcntl.LOCK_EX | fcntl.LOCK_NB)
+                if fcntl:
+                    fcntl.flock(lock_file.fileno(), fcntl.LOCK_EX | fcntl.LOCK_NB)
+                else:
+                    lock_file.seek(0)
+                    msvcrt.locking(lock_file.fileno(), msvcrt.LK_NBLCK, 1)
                 break
-            except BlockingIOError:
+            except (BlockingIOError, OSError, PermissionError):
                 if time.time() >= deadline:
                     raise TimeoutError("Timed out waiting for auth store lock")
                 time.sleep(0.05)
 
+        _auth_lock_holder.depth = 1
         try:
             yield
         finally:
-            fcntl.flock(lock_file.fileno(), fcntl.LOCK_UN)
+            _auth_lock_holder.depth = 0
+            if fcntl:
+                fcntl.flock(lock_file.fileno(), fcntl.LOCK_UN)
+            elif msvcrt:
+                try:
+                    lock_file.seek(0)
+                    msvcrt.locking(lock_file.fileno(), msvcrt.LK_UNLCK, 1)
+                except (OSError, IOError):
+                    pass
 
 
 def _load_auth_store(auth_file: Optional[Path] = None) -> Dict[str, Any]:
diff --git a/run_agent.py b/run_agent.py
index c1f2623c8..f03d3cb19 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -2689,6 +2689,8 @@ class AIAgent:
             except json.JSONDecodeError as e:
                 logging.warning(f"Unexpected JSON error after validation: {e}")
                 function_args = {}
+            if not isinstance(function_args, dict):
+                function_args = {}
 
             if not self.quiet_mode:
                 args_str = json.dumps(function_args, ensure_ascii=False)
diff --git a/tests/test_display.py b/tests/test_display.py
new file mode 100644
index 000000000..035f4d01c
--- /dev/null
+++ b/tests/test_display.py
@@ -0,0 +1,85 @@
+"""Tests for agent/display.py — build_tool_preview()."""
+
+import pytest
+from agent.display import build_tool_preview
+
+
+class TestBuildToolPreview:
+    """Tests for build_tool_preview defensive handling and normal operation."""
+
+    def test_none_args_returns_none(self):
+        """PR #453: None args should not crash, should return None."""
+        assert build_tool_preview("terminal", None) is None
+
+    def test_empty_dict_returns_none(self):
+        """Empty dict has no keys to preview."""
+        assert build_tool_preview("terminal", {}) is None
+
+    def test_known_tool_with_primary_arg(self):
+        """Known tool with its primary arg should return a preview string."""
+        result = build_tool_preview("terminal", {"command": "ls -la"})
+        assert result is not None
+        assert "ls -la" in result
+
+    def test_web_search_preview(self):
+        result = build_tool_preview("web_search", {"query": "hello world"})
+        assert result is not None
+        assert "hello world" in result
+
+    def test_read_file_preview(self):
+        result = build_tool_preview("read_file", {"path": "/tmp/test.py", "offset": 1})
+        assert result is not None
+        assert "/tmp/test.py" in result
+
+    def test_unknown_tool_with_fallback_key(self):
+        """Unknown tool but with a recognized fallback key should still preview."""
+        result = build_tool_preview("custom_tool", {"query": "test query"})
+        assert result is not None
+        assert "test query" in result
+
+    def test_unknown_tool_no_matching_key(self):
+        """Unknown tool with no recognized keys should return None."""
+        result = build_tool_preview("custom_tool", {"foo": "bar"})
+        assert result is None
+
+    def test_long_value_truncated(self):
+        """Preview should truncate long values."""
+        long_cmd = "a" * 100
+        result = build_tool_preview("terminal", {"command": long_cmd}, max_len=40)
+        assert result is not None
+        assert len(result) <= 43  # max_len + "..."
+
+    def test_process_tool_with_none_args(self):
+        """Process tool special case should also handle None args."""
+        assert build_tool_preview("process", None) is None
+
+    def test_process_tool_normal(self):
+        result = build_tool_preview("process", {"action": "poll", "session_id": "abc123"})
+        assert result is not None
+        assert "poll" in result
+
+    def test_todo_tool_read(self):
+        result = build_tool_preview("todo", {"merge": False})
+        assert result is not None
+        assert "reading" in result
+
+    def test_todo_tool_with_todos(self):
+        result = build_tool_preview("todo", {"todos": [{"id": "1", "content": "test", "status": "pending"}]})
+        assert result is not None
+        assert "1 task" in result
+
+    def test_memory_tool_add(self):
+        result = build_tool_preview("memory", {"action": "add", "target": "user", "content": "test note"})
+        assert result is not None
+        assert "user" in result
+
+    def test_session_search_preview(self):
+        result = build_tool_preview("session_search", {"query": "find something"})
+        assert result is not None
+        assert "find something" in result
+
+    def test_false_like_args_zero(self):
+        """Non-dict falsy values should return None, not crash."""
+        assert build_tool_preview("terminal", 0) is None
+        assert build_tool_preview("terminal", "") is None
+        assert build_tool_preview("terminal", []) is None