fix(session): bound continuation fallback sidecar reads

api/models.py

@@ -540,6 +540,16 @@ def _find_top_level_json_key(text, key):
     return None
 
 
+def _read_file_head(path: Path, max_prefix_bytes: int = 4096) -> str:
+    """Read at most ``max_prefix_bytes`` bytes from ``path`` and decode UTF-8."""
+    if not isinstance(path, Path):
+        path = Path(path)
+    if max_prefix_bytes <= 0:
+        return ''
+    with path.open('rb') as fp:
+        return fp.read(max_prefix_bytes).decode('utf-8', errors='ignore')
+
+
 def _read_metadata_json_prefix(path, max_prefix_bytes=65536):
     """Read only the metadata portion before the top-level messages array."""
     buf = ''
@@ -2129,9 +2139,7 @@ def _has_compression_continuation(session) -> bool:
             if path.name.startswith('_') or path.stem == sid:
                 continue
             try:
-                head = path.read_text(encoding='utf-8', errors='ignore')[:4096]
-            except TypeError:
-                head = path.read_text(encoding='utf-8')[:4096]
+                head = _read_file_head(path, max_prefix_bytes=4096)
             except OSError:
                 continue
             if needle in head:

tests/test_session_switch_performance.py

@@ -0,0 +1,80 @@
+import api.models as models
+
+
+def test_compression_continuation_fallback_reads_only_file_head(monkeypatch, tmp_path):
+    session_dir = tmp_path / "sessions"
+    session_dir.mkdir()
+    index_file = tmp_path / "_index.json"
+    index_file.write_text("[]", encoding="utf-8")
+
+    monkeypatch.setattr(models, "SESSION_DIR", session_dir)
+    monkeypatch.setattr(models, "SESSION_INDEX_FILE", index_file)
+    models.SESSIONS.clear()
+
+    parent_sid = "parent"
+    candidate = session_dir / "child.json"
+    candidate.write_text("x" * 8192 + f'"parent_session_id": "{parent_sid}"', encoding="utf-8")
+
+    opened_for_read = []
+    read_text_calls = []
+
+    original_path_open = models.Path.open
+    original_path_read_text = models.Path.read_text
+
+    class _TrackingHandle:
+        def __init__(self, path, inner):
+            self._path = str(path)
+            self._inner = inner
+
+        def read(self, size=-1):
+            opened_for_read.append((self._path, size))
+            return self._inner.read(size)
+
+        def __enter__(self):
+            return self
+
+        def __exit__(self, exc_type, exc, tb):
+            return self._inner.__exit__(exc_type, exc, tb)
+
+        def __getattr__(self, name):
+            return getattr(self._inner, name)
+
+    def tracking_open(path, *args, **kwargs):
+        return _TrackingHandle(path, original_path_open(path, *args, **kwargs))
+
+    def tracking_read_text(self, *args, **kwargs):
+        read_text_calls.append(str(self))
+        return original_path_read_text(self, *args, **kwargs)
+
+    monkeypatch.setattr(models.Path, "open", tracking_open)
+    monkeypatch.setattr(models.Path, "read_text", tracking_read_text)
+
+    session = models.Session(session_id=parent_sid)
+
+    assert models._has_compression_continuation(session) is False
+
+    assert str(index_file) in read_text_calls
+    assert all(path != str(candidate) for path in read_text_calls)
+
+    candidate_reads = [size for path, size in opened_for_read if path == str(candidate)]
+    assert candidate_reads, "fallback should read the candidate sidecar"
+    assert all(size != -1 and size <= 4096 for size in candidate_reads)
+
+
+def test_compression_continuation_prefix_match_stays_true(monkeypatch, tmp_path):
+    session_dir = tmp_path / "sessions"
+    session_dir.mkdir()
+    index_file = tmp_path / "_index.json"
+    index_file.write_text("[]", encoding="utf-8")
+
+    monkeypatch.setattr(models, "SESSION_DIR", session_dir)
+    monkeypatch.setattr(models, "SESSION_INDEX_FILE", index_file)
+    models.SESSIONS.clear()
+
+    parent_sid = "parent"
+    candidate = session_dir / "child.json"
+    candidate.write_text('{"session_id":"child", "parent_session_id": "parent",', encoding="utf-8")
+
+    session = models.Session(session_id=parent_sid)
+
+    assert models._has_compression_continuation(session) is True