test: tighten compression snapshot preservation coverage

CHANGELOG.md

@@ -10,7 +10,7 @@
 
 ### Fixed
 
-- **PR #2285** by @dso2ng (refs #2230) — Context-compression preservation now marks old pre-compression session files with `pre_compression_snapshot: true` and hides those preserved snapshots from the active sidebar list while keeping the JSON files available for lineage traversal. This keeps long-running conversations from accumulating duplicate active sidebar rows after repeated compressions without reusing the manual archive flag for an internal snapshot lifecycle state.
+- **PR #2285** by @dso2ng (refs #2230) - Context-compression preservation now marks old pre-compression session files with `pre_compression_snapshot: true` and hides those preserved snapshots from the active sidebar list while keeping the JSON files available for lineage traversal. This keeps long-running conversations from accumulating duplicate active sidebar rows after repeated compressions without reusing the manual archive flag for an internal snapshot lifecycle state.
 
 ## [v0.51.64] — 2026-05-14 — Release AN (stage-357 — 3-PR small batch — docker_init k8s whoami fallback + PWA manifest session routes (closes #2226) + aux title test coverage)
 

api/streaming.py

@@ -1605,12 +1605,15 @@ def _preserve_pre_compression_snapshot(s, old_sid: str) -> None:
             existing_msgs = len(existing.get('messages') or [])
             existing_snapshot = bool(existing.get('pre_compression_snapshot'))
         except (json.JSONDecodeError, ValueError):
+            # Treat corrupt/malformed old JSON as missing history and rewrite it
+            # from the in-memory pre-compression messages below. That is safer
+            # than leaving an unreadable recovery snapshot behind.
             existing_msgs = -1
             existing_snapshot = False
         if len(s.messages) <= existing_msgs and existing_snapshot:
             return
         if len(s.messages) > existing_msgs:
-            # In-memory messages are newer than the file — save the full old
+            # In-memory messages are newer than the file; save the full old
             # snapshot from the current session object while preserving its
             # pre-existing parent_session_id lineage.
             saved_sid = s.session_id

tests/test_issue2223_compression_no_rename.py

@@ -117,6 +117,10 @@ class TestNoRenameDuringCompression:
         assert old_payload["pre_compression_snapshot"] is True
         assert old_payload["parent_session_id"] == "fork_parent"
         assert len(old_payload["messages"]) == 2
+        index = json.loads((session_dir / "_index.json").read_text(encoding="utf-8"))
+        index_by_id = {entry["session_id"]: entry for entry in index}
+        assert index_by_id["old_sid"]["pre_compression_snapshot"] is True
+        assert "new_sid" not in index_by_id
         assert continuation.session_id == "new_sid"
         assert continuation.parent_session_id == "fork_parent"
         assert not continuation.pre_compression_snapshot