fix(session): tolerate invalid UTF-8 in saved sessions

corecoder/session.py

@@ -65,7 +65,7 @@ def load_session(session_id: str) -> tuple[list[dict], str] | None:
     try:
         data = json.loads(path.read_text(encoding="utf-8"))
         return data["messages"], data["model"]
-    except (json.JSONDecodeError, KeyError, OSError):
+    except (json.JSONDecodeError, UnicodeDecodeError, KeyError, OSError):
         # a corrupt or truncated session file shouldn't crash resume
         return None
 
@@ -91,7 +91,7 @@ def list_sessions() -> list[dict]:
                 "saved_at": data.get("saved_at", "?"),
                 "preview": preview,
             })
-        except (json.JSONDecodeError, KeyError):
+        except (json.JSONDecodeError, UnicodeDecodeError, KeyError):
             continue
 
     return sessions[:20]  # cap at 20

tests/test_session.py

@@ -1,5 +1,7 @@
+import pytest
+
 from corecoder import session as session_module
-from corecoder.session import load_session, save_session
+from corecoder.session import list_sessions, load_session, save_session
 
 
 def test_default_session_ids_do_not_collide(tmp_path, monkeypatch):
@@ -64,6 +66,33 @@ def test_corrupt_session_file_returns_none(tmp_path, monkeypatch):
     assert load_session("broken") is None
 
 
+@pytest.mark.parametrize("payload", [b"\xff", b'{"messages": [{"content": "\xe4\xbd'])
+def test_invalid_utf8_session_returns_none(tmp_path, monkeypatch, payload):
+    monkeypatch.setattr(session_module, "SESSIONS_DIR", tmp_path)
+    path = tmp_path / "broken.json"
+    path.write_bytes(payload)
+
+    assert load_session("broken") is None
+    assert path.read_bytes() == payload
+
+
+@pytest.mark.parametrize("payload", [b"\xff", b'{"messages": [{"content": "\xe4\xbd'])
+def test_list_sessions_skips_invalid_utf8(tmp_path, monkeypatch, payload):
+    monkeypatch.setattr(session_module, "SESSIONS_DIR", tmp_path)
+    messages = [{"role": "user", "content": "你好"}]
+    sid = save_session(messages, "model-zh", "valid")
+    path = tmp_path / "z-broken.json"
+    path.write_bytes(payload)
+
+    sessions = list_sessions()
+
+    assert len(sessions) == 1
+    assert sessions[0]["id"] == sid
+    assert sessions[0]["preview"] == "你好"
+    assert load_session(sid) == (messages, "model-zh")
+    assert path.read_bytes() == payload
+
+
 def test_session_roundtrips_unicode(tmp_path, monkeypatch):
     monkeypatch.setattr(session_module, "SESSIONS_DIR", tmp_path)