Fix mixed timestamp timezone parsing

README.md

@@ -120,6 +120,9 @@ Zero dependencies beyond `rich`. No API keys, no network: it reads local files.
 
 ## Design notes
 
+**Timestamps without an offset are treated as UTC.** Explicit offsets are respected when sorting
+runs and calculating durations. Runs with missing or invalid timestamps sort before dated runs.
+
 **Two passes over the transcript, not one.** Results can appear before every use has been seen in
 unusual orderings. A 34MB file is cheap to scan twice compared to getting the pairing subtly wrong.
 

agentrace/parse.py

@@ -17,7 +17,7 @@
 
 import json
 from dataclasses import dataclass, field
-from datetime import datetime
+from datetime import UTC, datetime
 from pathlib import Path
 from typing import Iterator
 
@@ -75,7 +75,9 @@ def _ts(value: str | None) -> datetime | None:
     if not value:
         return None
     try:
-        return datetime.fromisoformat(value.replace("Z", "+00:00"))
+        parsed = datetime.fromisoformat(value.replace("Z", "+00:00"))
+        # Treat missing offsets as UTC, independently of the reader's local timezone.
+        return parsed.replace(tzinfo=UTC) if parsed.tzinfo is None else parsed
     except ValueError:
         return None
 
@@ -159,7 +161,7 @@ def parse_session(path: Path) -> Session:
             )
         )
 
-    runs.sort(key=lambda r: (r.started_at or datetime.min.replace(tzinfo=None), r.tool_use_id))
+    runs.sort(key=lambda r: (r.started_at or datetime.min.replace(tzinfo=UTC), r.tool_use_id))
     return Session(session_id=session_id, path=path, runs=runs)
 
 

tests/test_timestamps.py

@@ -0,0 +1,60 @@
+"""Timestamp ordering and duration regressions for parsed transcripts."""
+
+import json
+from datetime import timedelta
+
+import pytest
+
+from agentrace.parse import parse_session
+
+
+@pytest.mark.parametrize("earlier_timestamp", ["2026-09-08T10:00:00", "2026-09-08T11:00:00+01:00"])
+def test_mixed_timezones_sort_by_instant(tmp_path, earlier_timestamp):
+    transcript = tmp_path / "mixed.jsonl"
+    records = [
+        {
+            "timestamp": timestamp,
+            "message": {"content": [{"type": "tool_use", "name": "Agent", "id": run_id}]},
+        }
+        for run_id, timestamp in [
+            ("later", "2026-09-08T10:01:00Z"),
+            ("earlier", earlier_timestamp),
+        ]
+    ]
+    records.append(
+        {
+            "timestamp": "2026-09-08T10:00:30Z",
+            "message": {
+                "content": [{"type": "tool_result", "tool_use_id": "earlier", "content": "done"}]
+            },
+        }
+    )
+    transcript.write_text("\n".join(json.dumps(record) for record in records))
+
+    runs = parse_session(transcript).runs
+
+    assert [run.tool_use_id for run in runs] == ["earlier", "later"]
+    assert runs[0].duration_s == 30
+    assert runs[0].started_at.utcoffset() is not None
+    assert runs[1].started_at.utcoffset() == timedelta(0)
+
+
+@pytest.mark.parametrize("missing_timestamp", [None, "not-a-timestamp"])
+def test_missing_timestamp_sorts_before_aware_timestamp(tmp_path, missing_timestamp):
+    transcript = tmp_path / "missing.jsonl"
+    records = [
+        {
+            "timestamp": timestamp,
+            "message": {"content": [{"type": "tool_use", "name": "Agent", "id": run_id}]},
+        }
+        for run_id, timestamp in [
+            ("known", "2026-09-08T10:00:00Z"),
+            ("unknown", missing_timestamp),
+        ]
+    ]
+    transcript.write_text("\n".join(json.dumps(record) for record in records))
+
+    runs = parse_session(transcript).runs
+
+    assert [run.tool_use_id for run in runs] == ["unknown", "known"]
+    assert runs[0].duration_s is None