fix(result): normalize result artifact timestamps

src/benchflow/_utils/timestamps.py

@@ -0,0 +1,22 @@
+"""Timestamp formatting helpers for persisted artifacts."""
+
+from __future__ import annotations
+
+from datetime import UTC, datetime
+
+
+def artifact_timestamp(value: datetime | None) -> str | None:
+    """Return a timezone-aware ISO 8601 timestamp for JSON artifacts.
+
+    Legacy rollout paths sometimes pass naive ``datetime`` values. Persisting
+    them with ``str(datetime)`` produced a space-separated timestamp without an
+    offset, so downstream consumers could not compare artifacts reliably. Treat
+    naive values as UTC at the artifact boundary and emit a compact ``Z`` suffix.
+    """
+    if value is None:
+        return None
+    if value.tzinfo is None or value.tzinfo.utcoffset(value) is None:
+        value = value.replace(tzinfo=UTC)
+    else:
+        value = value.astimezone(UTC)
+    return value.isoformat().replace("+00:00", "Z")

src/benchflow/hosted_env.py

@@ -33,6 +33,7 @@
     trajectory_summary_from_events,
 )
 from benchflow._utils.reward_events import build_rewards_jsonl_events
+from benchflow._utils.timestamps import artifact_timestamp
 from benchflow.diagnostics import RolloutDiagnostics
 from benchflow.rewards.validation import is_valid_reward_number
 from benchflow.trajectories.types import redact_acp_trajectory_jsonl
@@ -560,9 +561,9 @@ def _write_run_artifacts(
         "agent": config.agent or None,
         "agent_name": "verifiers",
         "model": result.normalized_model or result.model or None,
-        "n_tool_calls": result.total_tool_calls or 0,
-        "n_prompts": len(prompts),
         "agent_result": agent_result,
+        "n_tool_calls": agent_result["n_tool_calls"],
+        "n_prompts": agent_result["n_prompts"],
         "final_metrics": final_metrics_from_agent_result(agent_result),
         "trajectory_summary": trajectory_summary_from_events(
             trajectory,
@@ -583,8 +584,8 @@ def _write_run_artifacts(
         **RolloutDiagnostics().to_result_fields(),
         "partial_trajectory": False,
         "trajectory_source": "hosted_env" if trajectory else None,
-        "started_at": str(started_at),
-        "finished_at": str(finished_at),
+        "started_at": artifact_timestamp(started_at),
+        "finished_at": artifact_timestamp(finished_at),
         "timing": timing,
         "scenes": [],
         "source": source_provenance,
@@ -607,7 +608,7 @@ def _write_run_artifacts(
         "timeout_sec": None,
         "concurrency": config.concurrency,
         "agent_idle_timeout_sec": None,
-        "started_at": str(started_at),
+        "started_at": artifact_timestamp(started_at),
         "agent_env": {},
         "scenes": [],
         "source": source_provenance,

src/benchflow/rollout/_results.py

@@ -28,6 +28,7 @@
 from benchflow._utils.reward_events import build_rewards_jsonl_events
 from benchflow._utils.scoring import classify_error, classify_verifier_error
 from benchflow._utils.source_provenance import artifact_source_provenance
+from benchflow._utils.timestamps import artifact_timestamp
 from benchflow.contracts import (
     BaseUser,
     DocumentNudgeUser,
@@ -194,7 +195,7 @@ def _write_config(
         "timeout_sec": timeout,
         "concurrency": concurrency,
         "agent_idle_timeout_sec": agent_idle_timeout,
-        "started_at": str(started_at),
+        "started_at": artifact_timestamp(started_at),
         "agent_env": recorded_env,
         "scenes": _scene_metadata(scenes or []),
         "loop": loop_block(loop_strategy),
@@ -305,7 +306,13 @@ def _build_rollout_result(
             runtime_skills_dir=None,
             declared_sandbox_skills_dir=None,
         )
-    finished_at = datetime.now()
+    started_tz = (
+        started_at.tzinfo
+        if started_at.tzinfo is not None
+        and started_at.tzinfo.utcoffset(started_at) is not None
+        else None
+    )
+    finished_at = datetime.now(started_tz) if started_tz is not None else datetime.now()
     n_skill_invocations = count_skill_invocations(trajectory)
     error_category = (
         diagnostics.category_for_channel("error") if error is not None else None
@@ -387,10 +394,10 @@ def _build_rollout_result(
         "agent_name": result.agent_name,
         "model": result.model,
         **skill_policy.config_metadata(),
-        "n_tool_calls": result.n_tool_calls,
-        "n_skill_invocations": result.n_skill_invocations,
-        "n_prompts": result.n_prompts,
         "agent_result": agent_result,
+        "n_tool_calls": agent_result["n_tool_calls"],
+        "n_skill_invocations": agent_result["n_skill_invocations"],
+        "n_prompts": agent_result["n_prompts"],
         "final_metrics": final_metrics,
         "trajectory_summary": trajectory_summary,
         "usage_tracking": usage_tracking,
@@ -402,8 +409,8 @@ def _build_rollout_result(
         **diagnostics.to_result_fields(),
         "partial_trajectory": result.partial_trajectory,
         "trajectory_source": result.trajectory_source,
-        "started_at": str(result.started_at),
-        "finished_at": str(result.finished_at),
+        "started_at": artifact_timestamp(result.started_at),
+        "finished_at": artifact_timestamp(result.finished_at),
         "timing": timing,
         "scenes": _scene_metadata(scenes or []),
         "loop": loop or loop_block(None),

tests/test_hosted_env_rollout_contract.py

@@ -101,7 +101,7 @@ def _classify(result) -> str:
 
 
 def test_hosted_env_writes_contract_result_json(tmp_path, monkeypatch):
-    """result.json carries the rollout-contract field set, not a custom shape."""
+    """Guards issue #528 and PR #419: hosted result.json keeps the contract."""
     result = _run(tmp_path, monkeypatch)
     payload = json.loads((result.run_dir / "result.json").read_text())
 
@@ -151,6 +151,11 @@ def test_hosted_env_writes_contract_result_json(tmp_path, monkeypatch):
     }
     assert payload["trajectory_summary"]["steps"] >= 0
     assert payload["trajectory_summary"]["tool_call_steps"] >= 0
+    assert payload["n_tool_calls"] == agent_result["n_tool_calls"]
+    assert payload["n_prompts"] == agent_result["n_prompts"]
+    assert payload["started_at"].endswith("Z")
+    assert payload["finished_at"].endswith("Z")
+    assert "T" in payload["started_at"]
 
 
 def test_hosted_env_writes_rewards_jsonl(tmp_path, monkeypatch):

tests/test_verify.py

@@ -4,7 +4,7 @@
 import contextlib
 import json
 import logging
-from datetime import datetime
+from datetime import datetime, timedelta, timezone
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
@@ -136,6 +136,65 @@ def test_token_total_round_trips_into_agent_result(self, tmp_path):
         assert data["final_metrics"]["total_completion_tokens"] == 5993
         assert "total_tokens" not in data
 
+    def test_result_timestamps_are_timezone_aware_iso8601(self, tmp_path):
+        """Guards issue #528: result.json timestamps are ISO 8601 UTC strings."""
+        from benchflow.rollout import _build_rollout_result
+
+        rollout_dir = tmp_path / "trial"
+        rollout_dir.mkdir()
+        _build_rollout_result(
+            rollout_dir,
+            task_name="t1",
+            rollout_name="trial-1",
+            agent="test",
+            agent_name="openhands",
+            model="m",
+            n_tool_calls=0,
+            prompts=[],
+            error=None,
+            verifier_error=None,
+            trajectory=[],
+            partial_trajectory=False,
+            rewards={"reward": 1.0},
+            started_at=datetime(2026, 3, 24, 10, 0),
+            timing={},
+        )
+
+        data = json.loads((rollout_dir / "result.json").read_text())
+        assert data["started_at"] == "2026-03-24T10:00:00Z"
+        assert data["finished_at"].endswith("Z")
+        assert " " not in data["finished_at"]
+        assert datetime.fromisoformat(data["finished_at"].replace("Z", "+00:00"))
+
+    def test_result_timestamps_normalize_offsets_to_utc(self, tmp_path):
+        """Guards issue #528: aware result timestamps serialize in UTC."""
+        from benchflow.rollout import _build_rollout_result
+
+        rollout_dir = tmp_path / "trial"
+        rollout_dir.mkdir()
+        _build_rollout_result(
+            rollout_dir,
+            task_name="t1",
+            rollout_name="trial-1",
+            agent="test",
+            agent_name="openhands",
+            model="m",
+            n_tool_calls=0,
+            prompts=[],
+            error=None,
+            verifier_error=None,
+            trajectory=[],
+            partial_trajectory=False,
+            rewards={"reward": 1.0},
+            started_at=datetime(
+                2026, 3, 24, 3, 0, tzinfo=timezone(timedelta(hours=-7))
+            ),
+            timing={},
+        )
+
+        data = json.loads((rollout_dir / "result.json").read_text())
+        assert data["started_at"] == "2026-03-24T10:00:00Z"
+
 
 class TestSdkVerify:
     @pytest.fixture