diff --git a/docs/usage/results-and-reporting.md b/docs/usage/results-and-reporting.md
index 7750afe0..889dff8a 100644
--- a/docs/usage/results-and-reporting.md
+++ b/docs/usage/results-and-reporting.md
@@ -87,7 +87,9 @@ from rampart.reporting import JsonFileReportSink
 sink = JsonFileReportSink(output_dir=Path(".report"))
 ```
 
-Output: `.report/run_report_2026-04-25T14-30-00.json`
+Output: `.report/run_report_2026-04-25T14-30-00-123.json`
+
+The filename contains a UTC timestamp with millisecond precision. If another report already has the same timestamp, a random UUID is appended to the new filename. Files are created atomically and existing reports are never overwritten. Reports written within the same millisecond have no defined filename order relative to each other.
 
 ### Custom Sinks
 
diff --git a/rampart/reporting/json_file.py b/rampart/reporting/json_file.py
index 5bf576ec..e734eb9c 100644
--- a/rampart/reporting/json_file.py
+++ b/rampart/reporting/json_file.py
@@ -31,6 +31,7 @@ def rampart_sinks():
 import json
 from datetime import UTC, datetime
 from typing import TYPE_CHECKING, Any
+from uuid import uuid4
 
 from rampart.common.text import safe_float, safe_str, safe_str_list
 
@@ -45,8 +46,10 @@ def rampart_sinks():
 class JsonFileReportSink:
     """Writes the test run report to a JSON file.
 
-    Each run produces a timestamped file:
-    ``<output_dir>/run_report_2026-03-19T21-30-00.json``
+    Each run normally produces a file named ``run_report_<timestamp>.json``.
+    The UTC timestamp includes milliseconds. If that filename already exists,
+    a UUID suffix distinguishes the colliding run. Existing files are never
+    overwritten.
 
     Args:
         output_dir (Path): Directory to write report files into.
@@ -62,14 +65,32 @@ async def emit_async(self, *, report: TestRunReport) -> None:
 
         Args:
             report (TestRunReport): The aggregated test run results.
+
+        Raises:
+            FileExistsError: If the random fallback filename also exists, or
+                ``output_dir`` exists and is not a directory.
         """
         self._output_dir.mkdir(parents=True, exist_ok=True)
 
-        timestamp = datetime.now(UTC).strftime("%Y-%m-%dT%H-%M-%S")
+        timestamp = datetime.now(UTC).strftime("%Y-%m-%dT%H-%M-%S-%f")[:-3]
+        data = self._serialize_report(report)
+        content = json.dumps(data, indent=2, default=str)
+
         filepath = self._output_dir / f"run_report_{timestamp}.json"
+        try:
+            report_file = filepath.open("x", encoding="utf-8")
+        except FileExistsError:
+            filepath = self._output_dir / f"run_report_{timestamp}_{uuid4().hex}.json"
 
-        data = self._serialize_report(report)
-        filepath.write_text(json.dumps(data, indent=2, default=str))
+            # Leave the exception handler before opening the fallback so a
+            # second collision reports only the path that actually collided.
+            report_file = None
+
+        if report_file is None:
+            report_file = filepath.open("x", encoding="utf-8")
+
+        with report_file:
+            report_file.write(content)
 
     def _serialize_report(self, report: TestRunReport) -> dict[str, Any]:
         """Convert a TestRunReport to a JSON-serializable dict.
diff --git a/tests/unit/reporting/test_json_file.py b/tests/unit/reporting/test_json_file.py
index ded7f08c..e162dc76 100644
--- a/tests/unit/reporting/test_json_file.py
+++ b/tests/unit/reporting/test_json_file.py
@@ -6,8 +6,11 @@
 from __future__ import annotations
 
 import json
+from datetime import UTC, datetime
 from pathlib import Path
 from typing import Any
+from unittest.mock import patch
+from uuid import UUID
 
 import pytest
 
@@ -366,6 +369,93 @@ async def test_emitted_file_contains_metadata_async(self, tmp_path: Path) -> Non
             "page_url": "https://example.com/chat",
         }
 
+    async def test_same_timestamp_preserves_every_report_async(
+        self,
+        tmp_path: Path,
+    ) -> None:
+        sink = JsonFileReportSink(output_dir=tmp_path)
+        fixed = datetime(2026, 8, 27, 12, 0, 0, 123456, tzinfo=UTC)
+
+        with patch("rampart.reporting.json_file.datetime") as clock:
+            clock.now.return_value = fixed
+            for run in range(3):
+                await sink.emit_async(report=TestRunReport(metadata={"run": run}))
+            clock.now.assert_called_with(UTC)
+
+        files = list(tmp_path.glob("run_report_*.json"))
+        assert len(files) == 3
+        assert {
+            json.loads(path.read_text(encoding="utf-8"))["metadata"]["run"]
+            for path in files
+        } == {0, 1, 2}
+
+        concise = tmp_path / "run_report_2026-08-27T12-00-00-123.json"
+        assert json.loads(concise.read_text(encoding="utf-8"))["metadata"] == {
+            "run": 0,
+        }
+        colliding_files = [path for path in files if path != concise]
+        assert len(colliding_files) == 2
+        for path in colliding_files:
+            assert path.name.startswith("run_report_2026-08-27T12-00-00-123_")
+            identifier = path.stem.rsplit("_", 1)[1]
+            assert len(identifier) == 32
+            assert UUID(hex=identifier).version == 4
+
+    async def test_existing_report_is_not_replaced_async(self, tmp_path: Path) -> None:
+        original = tmp_path / "run_report_2026-08-27T12-00-00-000.json"
+        original.write_text("keep me", encoding="utf-8")
+        sink = JsonFileReportSink(output_dir=tmp_path)
+        fixed = datetime(2026, 8, 27, 12, 0, 0, tzinfo=UTC)
+
+        with patch("rampart.reporting.json_file.datetime") as clock:
+            clock.now.return_value = fixed
+            await sink.emit_async(report=TestRunReport(metadata={"run": "new"}))
+
+        assert original.read_text(encoding="utf-8") == "keep me"
+        new_files = list(tmp_path.glob("run_report_2026-08-27T12-00-00-000_*.json"))
+        assert len(new_files) == 1
+        assert json.loads(new_files[0].read_text(encoding="utf-8"))["metadata"] == {
+            "run": "new",
+        }
+
+    async def test_uuid_collision_does_not_overwrite_existing_report_async(
+        self,
+        tmp_path: Path,
+    ) -> None:
+        identifier = UUID("a3f18c92-654d-4b75-ad15-687d383d951b")
+        timestamp_file = tmp_path / "run_report_2026-08-27T12-00-00-000.json"
+        timestamp_file.write_text("keep timestamp", encoding="utf-8")
+        uuid_file = (
+            tmp_path / f"run_report_2026-08-27T12-00-00-000_{identifier.hex}.json"
+        )
+        uuid_file.write_text("keep uuid", encoding="utf-8")
+        sink = JsonFileReportSink(output_dir=tmp_path)
+        fixed = datetime(2026, 8, 27, 12, 0, 0, tzinfo=UTC)
+
+        with (
+            patch("rampart.reporting.json_file.datetime") as clock,
+            patch("rampart.reporting.json_file.uuid4", return_value=identifier),
+        ):
+            clock.now.return_value = fixed
+            with pytest.raises(FileExistsError, match=identifier.hex):
+                await sink.emit_async(report=TestRunReport())
+
+        assert timestamp_file.read_text(encoding="utf-8") == "keep timestamp"
+        assert uuid_file.read_text(encoding="utf-8") == "keep uuid"
+        assert set(tmp_path.glob("run_report_*.json")) == {timestamp_file, uuid_file}
+
+    async def test_serialization_failure_does_not_create_a_file_async(
+        self,
+        tmp_path: Path,
+    ) -> None:
+        sink = JsonFileReportSink(output_dir=tmp_path)
+        report = TestRunReport(metadata={"bad": {("tuple", "key"): "value"}})
+
+        with pytest.raises(TypeError, match="keys must be"):
+            await sink.emit_async(report=report)
+
+        assert list(tmp_path.glob("run_report_*.json")) == []
+
 
 class TestReportMetadata:
     """Run-level TestRunReport.metadata is projected into the JSON output."""