diff --git a/tests/unit/test_trace_events.py b/tests/unit/test_trace_events.py
new file mode 100644
index 0000000..521f70e
--- /dev/null
+++ b/tests/unit/test_trace_events.py
@@ -0,0 +1,134 @@
+from __future__ import annotations
+
+import unittest
+from datetime import datetime
+
+from pydantic import ValidationError
+
+from contribarena.trace import TraceEvent
+from contribarena.trace.events import TraceEvent as TraceEventDirect
+
+
+class TraceEventDefaultsTest(unittest.TestCase):
+    def test_required_fields_with_minimal_construction(self) -> None:
+        event = TraceEvent(run_id="run-1", state="run_started", event="run.started")
+
+        self.assertEqual("run-1", event.run_id)
+        self.assertEqual("run_started", event.state)
+        self.assertEqual("run.started", event.event)
+
+    def test_payload_defaults_to_empty_dict(self) -> None:
+        event = TraceEvent(run_id="r", state="s", event="e")
+
+        self.assertEqual({}, event.payload)
+
+    def test_payload_default_factory_returns_independent_dicts(self) -> None:
+        first = TraceEvent(run_id="r1", state="s", event="e")
+        second = TraceEvent(run_id="r2", state="s", event="e")
+
+        first.payload["k"] = "v"
+
+        self.assertEqual({}, second.payload)
+
+    def test_ts_default_is_iso_parseable(self) -> None:
+        event = TraceEvent(run_id="r", state="s", event="e")
+
+        parsed = datetime.fromisoformat(event.ts)
+        self.assertIsNotNone(parsed.tzinfo)
+
+    def test_ts_default_is_monotonic_non_decreasing(self) -> None:
+        first = TraceEvent(run_id="r", state="s", event="e")
+        second = TraceEvent(run_id="r", state="s", event="e")
+
+        # Both default timestamps should be valid ISO strings, and the
+        # second construction must not produce an earlier ts than the first.
+        first_dt = datetime.fromisoformat(first.ts)
+        second_dt = datetime.fromisoformat(second.ts)
+        self.assertLessEqual(first_dt, second_dt)
+
+
+class TraceEventExplicitValuesTest(unittest.TestCase):
+    def test_explicit_ts_is_preserved(self) -> None:
+        event = TraceEvent(
+            ts="2026-01-02T03:04:05+00:00",
+            run_id="r",
+            state="s",
+            event="e",
+        )
+
+        self.assertEqual("2026-01-02T03:04:05+00:00", event.ts)
+
+    def test_explicit_payload_is_preserved(self) -> None:
+        payload = {"ok": True, "count": 3, "nested": {"k": "v"}}
+        event = TraceEvent(run_id="r", state="s", event="e", payload=payload)
+
+        self.assertEqual(payload, event.payload)
+
+    def test_payload_supports_mixed_value_types(self) -> None:
+        payload = {
+            "str": "x",
+            "int": 1,
+            "float": 1.5,
+            "bool": False,
+            "none": None,
+            "list": [1, 2, 3],
+        }
+        event = TraceEvent(run_id="r", state="s", event="e", payload=payload)
+
+        self.assertEqual(payload, event.payload)
+
+
+class TraceEventValidationTest(unittest.TestCase):
+    def test_missing_run_id_raises(self) -> None:
+        with self.assertRaises(ValidationError):
+            TraceEvent(state="s", event="e")  # type: ignore[call-arg]
+
+    def test_missing_state_raises(self) -> None:
+        with self.assertRaises(ValidationError):
+            TraceEvent(run_id="r", event="e")  # type: ignore[call-arg]
+
+    def test_missing_event_raises(self) -> None:
+        with self.assertRaises(ValidationError):
+            TraceEvent(run_id="r", state="s")  # type: ignore[call-arg]
+
+    def test_non_dict_payload_rejected(self) -> None:
+        with self.assertRaises(ValidationError):
+            TraceEvent(run_id="r", state="s", event="e", payload=["not", "a", "dict"])  # type: ignore[arg-type]
+
+
+class TraceEventSerializationTest(unittest.TestCase):
+    def test_model_dump_round_trips_payload(self) -> None:
+        event = TraceEvent(
+            run_id="r",
+            state="s",
+            event="e",
+            payload={"k": "v"},
+        )
+
+        dumped = event.model_dump(mode="json")
+
+        self.assertEqual("r", dumped["run_id"])
+        self.assertEqual("s", dumped["state"])
+        self.assertEqual("e", dumped["event"])
+        self.assertEqual({"k": "v"}, dumped["payload"])
+        self.assertIn("ts", dumped)
+
+    def test_model_validate_round_trip(self) -> None:
+        original = TraceEvent(run_id="r", state="s", event="e", payload={"k": 1})
+
+        rebuilt = TraceEvent.model_validate(original.model_dump(mode="json"))
+
+        self.assertEqual(original.run_id, rebuilt.run_id)
+        self.assertEqual(original.state, rebuilt.state)
+        self.assertEqual(original.event, rebuilt.event)
+        self.assertEqual(original.payload, rebuilt.payload)
+        self.assertEqual(original.ts, rebuilt.ts)
+
+
+class TraceEventImportTest(unittest.TestCase):
+    def test_reexport_from_package_is_same_class(self) -> None:
+        self.assertIs(TraceEvent, TraceEventDirect)
+
+
+if __name__ == "__main__":
+    unittest.main()