[Bug]: Deleting a session whose memory is not open creates its storage first, only to delete it #1728

Describe the bug

Deleting a session whose episodic memory is not open builds the whole memory first, and building it creates the session's storage when it is missing. So deleting a session that was never written to creates a vector store collection and a segment store partition, only to delete them.

As of 82b6c6f58 (main), paths under packages/server/src/memmachine_server/:

• episodic_memory/episodic_memory_manager.py:288, delete_episodic_session: when the instance is not cached, it calls episodic_memory_params_from_config and builds an EpisodicMemory to call delete_session_episodes on it.
• episodic_memory/long_term_memory/service_locator.py:92, _event_params: on the event backend, this opens the session's vector store collection, creating it when absent (create_collection, line 125), and opens or creates its segment store partition (open_or_create_partition, line 140).

Deletion therefore depends on creation succeeding. Any failure to create the storage (a backend error, a timeout) fails the deletion, even when there was nothing to delete.

Steps to reproduce

1. Create a project or session, and do not add any memories to it.
2. Delete it.
3. The event backend creates the session's vector store collection and segment store partition, then deletes them (visible as a create followed by a delete in the backends).

Expected behavior

Deleting a session deletes whatever storage it has, by key, and creates none.

Notes

• [session storage 1/2] Create a session's storage with the session, never on a request #1622 (open) makes this change: "Deleting a session with no open instance deletes its partitions by key, so a session whose storage was never fully created can still be deleted."
• Related: A search or add on a nonexistent project creates it (session row, partition and collection), so deletion is not final and the search 404 path is unreachable #1576 (a search or add on a nonexistent project creates it).
• With the collection registry of [vector store scale-out] Mint an incarnation per collection life in a SQL-arbitrated registry, so any process may serve any Qdrant or Milvus collection #1631, a collection whose creation was abandoned stays pending. Through this path, the session holding it then cannot be deleted at all.

Environment

• MemMachine: main at 82b6c6f58

🤖 Written by Claude Code (Claude Opus 5.5) on behalf of @edwinyyyu.