From 3fb300b592a4c4cd8571bec1eae90ded81d97046 Mon Sep 17 00:00:00 2001
From: Dominique Broeglin <dominique.broeglin@microsoft.com>
Date: Sun, 5 Jul 2026 16:16:57 +0200
Subject: [PATCH] feat: warn on skipped malformed JSON lines in session logs

---
 src/copilot_experiments/sessionlog.py | 13 +++++++++++++
 tests/test_sessionlog.py              | 26 ++++++++++++++++++++++++--
 2 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/src/copilot_experiments/sessionlog.py b/src/copilot_experiments/sessionlog.py
index d31ecc2..60b3711 100644
--- a/src/copilot_experiments/sessionlog.py
+++ b/src/copilot_experiments/sessionlog.py
@@ -8,9 +8,13 @@
 from pathlib import Path
 from typing import Any
 
+from rich.console import Console
+
 from . import pricing
 from .models import Metrics, ModelMetric, TokenEconomics
 
+err = Console(stderr=True)
+
 
 def session_state_root(base: Path | None = None) -> Path:
     """Directory where the Copilot CLI stores per-session state."""
@@ -27,6 +31,7 @@ def load_events(path: Path) -> list[dict[str, Any]]:
     if not path.exists():
         return []
     events: list[dict[str, Any]] = []
+    skipped_lines = 0
     for line in path.read_text(encoding="utf-8").splitlines():
         line = line.strip()
         if not line:
@@ -34,7 +39,15 @@ def load_events(path: Path) -> list[dict[str, Any]]:
         try:
             events.append(json.loads(line))
         except json.JSONDecodeError:
+            skipped_lines += 1
             continue
+
+    if skipped_lines > 0:
+        err.print(
+            f"[yellow]Warning: Skipped {skipped_lines} malformed JSON line(s) "
+            f"in {path.name}[/yellow]"
+        )
+
     return events
 
 
diff --git a/tests/test_sessionlog.py b/tests/test_sessionlog.py
index ea49b1b..a44d7ee 100644
--- a/tests/test_sessionlog.py
+++ b/tests/test_sessionlog.py
@@ -2,7 +2,9 @@
 
 from __future__ import annotations
 
-from copilot_experiments.sessionlog import parse_metrics
+from pathlib import Path
+
+from copilot_experiments.sessionlog import load_events, parse_metrics
 
 
 def _events():
@@ -61,7 +63,7 @@ def test_parse_metrics_empty():
 # --------------------------------------------------------------------------- #
 
 
-def test_load_events_skips_blank_and_invalid_lines(tmp_path):
+def test_load_events_skips_blank_and_invalid_lines(tmp_path, capsys):
     from copilot_experiments.sessionlog import load_events
 
     path = tmp_path / "events.jsonl"
@@ -72,6 +74,9 @@ def test_load_events_skips_blank_and_invalid_lines(tmp_path):
     events = load_events(path)
     assert [e["type"] for e in events] == ["session.start", "user.message"]
 
+    out, err = capsys.readouterr()
+    assert "Skipped 1 malformed JSON line(s)" in err
+
 
 def test_load_events_missing_file_returns_empty(tmp_path):
     from copilot_experiments.sessionlog import load_events
@@ -121,3 +126,20 @@ def test_compaction_start_tracks_peak_without_shutdown():
     econ = extract_economics(events)
     assert econ.total_tokens is None  # no shutdown -> no authoritative totals
     assert econ.peak_context_tokens == 9500  # 1000 + 8000 + 500
+
+
+def test_load_events_with_malformed_lines(tmp_path: Path, capsys):
+    events_file = tmp_path / "events.jsonl"
+    events_file.write_text(
+        '{"type": "session.start"}\n'
+        '{"type": "malformed"\n'  # bad json
+        '{"type": "session.end"}\n',
+        encoding="utf-8",
+    )
+    events = load_events(events_file)
+    assert len(events) == 2
+    assert events[0]["type"] == "session.start"
+    assert events[1]["type"] == "session.end"
+
+    out, err = capsys.readouterr()
+    assert "Skipped 1 malformed JSON line(s)" in err