diff --git a/run_agent.py b/run_agent.py
index e91376b03..ba214b715 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -100,6 +100,7 @@ from agent.trajectory import (
     convert_scratchpad_to_think, has_incomplete_scratchpad,
     save_trajectory as _save_trajectory_to_file,
 )
+from utils import atomic_json_write
 
 HONCHO_TOOL_NAMES = {
     "honcho_context",
@@ -1398,8 +1399,12 @@ class AIAgent:
                 "messages": cleaned,
             }
 
-            with open(self.session_log_file, "w", encoding="utf-8") as f:
-                json.dump(entry, f, indent=2, ensure_ascii=False, default=str)
+            atomic_json_write(
+                self.session_log_file,
+                entry,
+                indent=2,
+                default=str,
+            )
 
         except Exception as e:
             if self.verbose_logging:
diff --git a/tests/conftest.py b/tests/conftest.py
index 9469ee45f..9c9f9a44e 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,5 +1,6 @@
 """Shared fixtures for the hermes-agent test suite."""
 
+import asyncio
 import os
 import signal
 import sys
@@ -59,6 +60,39 @@ def mock_config():
 def _timeout_handler(signum, frame):
     raise TimeoutError("Test exceeded 30 second timeout")
 
+@pytest.fixture(autouse=True)
+def _ensure_current_event_loop(request):
+    """Provide a default event loop for sync tests that call get_event_loop().
+
+    Python 3.11+ no longer guarantees a current loop for plain synchronous tests.
+    A number of gateway tests still use asyncio.get_event_loop().run_until_complete(...).
+    Ensure they always have a usable loop without interfering with pytest-asyncio's
+    own loop management for @pytest.mark.asyncio tests.
+    """
+    if request.node.get_closest_marker("asyncio") is not None:
+        yield
+        return
+
+    try:
+        loop = asyncio.get_event_loop_policy().get_event_loop()
+    except RuntimeError:
+        loop = None
+
+    created = loop is None or loop.is_closed()
+    if created:
+        loop = asyncio.new_event_loop()
+        asyncio.set_event_loop(loop)
+
+    try:
+        yield
+    finally:
+        if created and loop is not None:
+            try:
+                loop.close()
+            finally:
+                asyncio.set_event_loop(None)
+
+
 @pytest.fixture(autouse=True)
 def _enforce_test_timeout():
     """Kill any individual test that takes longer than 30 seconds."""
diff --git a/tests/test_atomic_json_write.py b/tests/test_atomic_json_write.py
index 681b7d8a8..cb8b2d6d0 100644
--- a/tests/test_atomic_json_write.py
+++ b/tests/test_atomic_json_write.py
@@ -97,6 +97,17 @@ class TestAtomicJsonWrite:
         text = target.read_text()
         assert '    "a"' in text  # 4-space indent
 
+    def test_accepts_json_dump_default_hook(self, tmp_path):
+        class CustomValue:
+            def __str__(self):
+                return "custom-value"
+
+        target = tmp_path / "custom_default.json"
+        atomic_json_write(target, {"value": CustomValue()}, default=str)
+
+        result = json.loads(target.read_text(encoding="utf-8"))
+        assert result == {"value": "custom-value"}
+
     def test_unicode_content(self, tmp_path):
         target = tmp_path / "unicode.json"
         data = {"emoji": "🎉", "japanese": "日本語"}
diff --git a/tests/test_run_agent.py b/tests/test_run_agent.py
index 1fcb191e7..15a0d5fba 100644
--- a/tests/test_run_agent.py
+++ b/tests/test_run_agent.py
@@ -1928,6 +1928,24 @@ class TestSafeWriter:
         assert inner.getvalue() == "test"
 
 
+class TestSaveSessionLogAtomicWrite:
+    def test_uses_shared_atomic_json_helper(self, agent, tmp_path):
+        agent.session_log_file = tmp_path / "session.json"
+        messages = [{"role": "user", "content": "hello"}]
+
+        with patch("run_agent.atomic_json_write", create=True) as mock_atomic_write:
+            agent._save_session_log(messages)
+
+        mock_atomic_write.assert_called_once()
+        call_args = mock_atomic_write.call_args
+        assert call_args.args[0] == agent.session_log_file
+        payload = call_args.args[1]
+        assert payload["session_id"] == agent.session_id
+        assert payload["messages"] == messages
+        assert call_args.kwargs["indent"] == 2
+        assert call_args.kwargs["default"] is str
+
+
 # ===================================================================
 # Anthropic adapter integration fixes
 # ===================================================================
diff --git a/utils.py b/utils.py
index 1b99d60fe..762bcb84f 100644
--- a/utils.py
+++ b/utils.py
@@ -9,17 +9,25 @@ from typing import Any, Union
 import yaml
 
 
-def atomic_json_write(path: Union[str, Path], data: Any, *, indent: int = 2) -> None:
+def atomic_json_write(
+    path: Union[str, Path],
+    data: Any,
+    *,
+    indent: int = 2,
+    **dump_kwargs: Any,
+) -> None:
     """Write JSON data to a file atomically.
 
     Uses temp file + fsync + os.replace to ensure the target file is never
-    left in a partially-written state.  If the process crashes mid-write,
+    left in a partially-written state. If the process crashes mid-write,
     the previous version of the file remains intact.
 
     Args:
         path: Target file path (will be created or overwritten).
         data: JSON-serializable data to write.
         indent: JSON indentation (default 2).
+        **dump_kwargs: Additional keyword args forwarded to json.dump(), such
+            as default=str for non-native types.
     """
     path = Path(path)
     path.parent.mkdir(parents=True, exist_ok=True)
@@ -31,7 +39,13 @@ def atomic_json_write(path: Union[str, Path], data: Any, *, indent: int = 2) ->
     )
     try:
         with os.fdopen(fd, "w", encoding="utf-8") as f:
-            json.dump(data, f, indent=indent, ensure_ascii=False)
+            json.dump(
+                data,
+                f,
+                indent=indent,
+                ensure_ascii=False,
+                **dump_kwargs,
+            )
             f.flush()
             os.fsync(f.fileno())
         os.replace(tmp_path, path)