_session_message_content_key (the state.db reconciliation key in
api/models.py) normalized whitespace only, while the streaming-side
identity _message_identity strips the workspace prefix for user turns.
WebUI sends the model a workspace-prefixed user_message
([Workspace::v1: /path]\n<text>) while the visible/optimistic bubble and
sidecar row carry the bare <text>. The mismatch made a prefixed state.db
row and a bare sidecar row key differently, so state_db_delta_after_context
failed to align them, treated the state.db copy as new, and appended a
duplicate user turn. The agent-side merge then concatenated the two
adjacent user rows into a permanent composite -- the post-restart
stale-user-prepend bug.
Fix: strip the workspace prefix for role=='user' in the reconciliation
key, reusing the same _strip_workspace_prefix helper the streaming side
uses (lazy import to avoid the api.streaming -> api.models cycle) so the
two dedup layers can't drift again. Assistant/tool keys are unchanged and
prefix-free user messages key identically (idempotent).
Fixes#5339.
starship-s