diff --git a/tests/test_chatlog.py b/tests/test_chatlog.py
new file mode 100644
index 00000000..88a5f791
--- /dev/null
+++ b/tests/test_chatlog.py
@@ -0,0 +1,137 @@
+"""Tests for ChatLog persistence — issue #1349.
+
+Verifies that ChatLog.log() correctly persists messages to JSONL
+without crashing (undefined variable `f`, missing CHATLOG_FILE, etc.).
+"""
+import json
+import os
+import tempfile
+import threading
+from datetime import datetime
+from pathlib import Path
+from unittest.mock import patch
+
+import pytest
+
+
+# We need to patch CHATLOG_FILE before importing, since it's set at module level
+@pytest.fixture(autouse=True)
+def patch_chatlog_file(tmp_path):
+    """Point CHATLOG_FILE at a temp directory for all tests."""
+    import multi_user_bridge
+    original = multi_user_bridge.CHATLOG_FILE
+    test_file = tmp_path / "chat_history.jsonl"
+    multi_user_bridge.CHATLOG_FILE = test_file
+    yield test_file
+    multi_user_bridge.CHATLOG_FILE = original
+
+
+@pytest.fixture
+def chat_log():
+    from multi_user_bridge import ChatLog
+    return ChatLog(max_per_room=5)
+
+
+class TestChatLog:
+    def test_log_returns_entry(self, chat_log):
+        """log() returns the entry dict with correct fields."""
+        entry = chat_log.log("lobby", "say", "hello world", user_id="u1", username="alice")
+        assert entry["type"] == "say"
+        assert entry["message"] == "hello world"
+        assert entry["room"] == "lobby"
+        assert entry["user_id"] == "u1"
+        assert entry["username"] == "alice"
+        assert "timestamp" in entry
+
+    def test_log_persists_to_jsonl(self, chat_log, patch_chatlog_file):
+        """log() appends a valid JSON line to the chatlog file."""
+        chat_log.log("lobby", "say", "first message")
+        chat_log.log("lobby", "say", "second message")
+
+        lines = patch_chatlog_file.read_text().strip().split("\n")
+        assert len(lines) == 2
+        record = json.loads(lines[0])
+        assert record["message"] == "first message"
+        assert record["room"] == "lobby"
+
+    def test_log_creates_parent_dirs(self, tmp_path, chat_log):
+        """log() creates parent directories if they don't exist."""
+        import multi_user_bridge
+        deep_file = tmp_path / "deep" / "nested" / "chat.jsonl"
+        multi_user_bridge.CHATLOG_FILE = deep_file
+
+        chat_log.log("lobby", "say", "test")
+        assert deep_file.exists()
+
+    def test_log_does_not_crash_on_readonly_dir(self, chat_log, patch_chatlog_file):
+        """log() catches filesystem errors gracefully — no crash."""
+        import multi_user_bridge
+        # Point to an impossible path
+        multi_user_bridge.CHATLOG_FILE = Path("/dev/null/immutable/chat.jsonl")
+
+        # Should NOT raise — exception is caught and printed
+        entry = chat_log.log("lobby", "say", "should not crash")
+        assert entry["message"] == "should not crash"
+
+    def test_rolling_buffer(self, chat_log):
+        """Buffer is capped at max_per_room."""
+        for i in range(10):
+            chat_log.log("lobby", "say", f"msg-{i}")
+
+        history = chat_log.get_history("lobby")
+        assert len(history) == 5  # max_per_room=5
+        assert history[0]["message"] == "msg-5"
+        assert history[-1]["message"] == "msg-9"
+
+    def test_get_history_since_filter(self, chat_log):
+        """get_history() filters by timestamp."""
+        chat_log.log("lobby", "say", "old")
+        # Small delay to get different timestamps
+        import time
+        time.sleep(0.01)
+        chat_log.log("lobby", "say", "new")
+
+        # Get only messages after the first one
+        history = chat_log.get_history("lobby", since=chat_log._history["lobby"][0]["timestamp"])
+        assert len(history) == 1
+        assert history[0]["message"] == "new"
+
+    def test_thread_safety(self, chat_log, patch_chatlog_file):
+        """Concurrent log() calls don't corrupt the buffer or file."""
+        errors = []
+
+        def worker(n):
+            try:
+                for i in range(20):
+                    chat_log.log("lobby", "say", f"thread-{n}-msg-{i}")
+            except Exception as e:
+                errors.append(e)
+
+        threads = [threading.Thread(target=worker, args=(i,)) for i in range(4)]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join()
+
+        assert errors == [], f"Thread errors: {errors}"
+        history = chat_log.get_history("lobby", limit=100)
+        assert len(history) == 5  # max_per_room=5
+
+        # Verify JSONL file has 80 lines (4 threads * 20 messages)
+        lines = patch_chatlog_file.read_text().strip().split("\n")
+        assert len(lines) == 80
+        # All lines should be valid JSON
+        for line in lines:
+            json.loads(line)  # should not raise
+
+    def test_various_message_types(self, chat_log, patch_chatlog_file):
+        """Handles 'say', 'ask', and 'system' message types."""
+        chat_log.log("lobby", "say", "player speaks")
+        chat_log.log("lobby", "ask", "player asks question")
+        chat_log.log("lobby", "system", "system event")
+
+        lines = patch_chatlog_file.read_text().strip().split("\n")
+        assert len(lines) == 3
+        assert json.loads(lines[0])["type"] == "say"
+        assert json.loads(lines[1])["type"] == "ask"
+        assert json.loads(lines[2])["type"] == "system"