fix: closes #674

nexus/trajectory_logger.py

@@ -98,11 +98,17 @@ class TrajectoryLogger:
 
         all_cycles = []
         for traj_file in sorted(self.log_dir.glob("trajectory_*.jsonl")):
-            with open(traj_file) as f:
+            with open(traj_file, encoding="utf-8") as f:
                 for line in f:
                     line = line.strip()
-                    if line:
+                    if not line:
+                        continue
+                    try:
                         all_cycles.append(json.loads(line))
+                    except json.JSONDecodeError:
+                        # Skip truncated/corrupt lines so one bad append does not
+                        # poison the entire export.
+                        continue
 
         # Quality filter — only keep cycles where the model actually
         # produced meaningful thought (not just "Nothing has happened")

tests/test_trajectory_logger.py

@@ -0,0 +1,31 @@
+import json
+import sys
+from pathlib import Path
+
+PROJECT_ROOT = Path(__file__).resolve().parent.parent
+if str(PROJECT_ROOT) not in sys.path:
+    sys.path.insert(0, str(PROJECT_ROOT))
+
+from nexus.trajectory_logger import TrajectoryLogger
+
+
+def test_export_for_training_skips_malformed_jsonl_lines(tmp_path):
+    logger = TrajectoryLogger(log_dir=tmp_path, system_prompt="System prompt")
+    logger.log_cycle(
+        perception="A useful perception",
+        thought="This is a sufficiently detailed thought for export.",
+        actions=["World responds with a tactical update."],
+        cycle_ms=42,
+    )
+
+    with logger.log_file.open("a", encoding="utf-8") as handle:
+        handle.write('{"id": "broken"\n')
+
+    output = logger.export_for_training(tmp_path / "training.jsonl")
+
+    exported_lines = output.read_text(encoding="utf-8").strip().splitlines()
+    assert len(exported_lines) == 1
+
+    exported_cycle = json.loads(exported_lines[0])
+    assert exported_cycle["id"].endswith("_cycle_0")
+    assert exported_cycle["message_count"] == 4