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SaveAgentBuilderDraft read the chat session, then upserted agent_builder_draft as a separate statement, with no lock between them. DeleteChatSession takes LockChatSessionForDelete for its whole transaction, so a save could pass its checks, block on nothing, and land its INSERT after the delete committed. agent_builder_draft carries no chat_session FK (repo rule), so nothing rejected that write. The surviving row held the configuration the user had just confirmed discarding. It is invisible to the UI — the drafts list joins through chat_session — and no prune can reach it: DeleteAgentBuilderDraft and the runtime teardown both key off a session that no longer exists, leaving only the workspace teardown. The client autosaves on an 800ms debounce and the conversation is addressable by URL, so a second tab can autosave at any moment while this one discards. Add LockChatSessionForDraftWrite, the same row and lock mode the delete and runtime-bind paths take, and run the upsert in a transaction that acquires it first and re-reads the session under it. Existence and status are the only two things a concurrent writer can change, and both are now decided inside the lock; workspace, creator and carrier are immutable for a session and stay on the cheap unlocked read. Either ordering is now correct: the save commits first and the delete prunes it, or the delete commits first and the save returns 404. The same lock closes the archive variant, where the last autosave after "create agent" could write a draft onto an already read-only session. Both regression tests drive the interleaving deterministically — hold the session row, prove the save blocks, then commit — and fail on the pre-fix handler with a 204 that writes the orphan. Co-authored-by: multica-agent <github@multica.ai>