diff --git a/tests/test_orphan_cleanup.py b/tests/test_orphan_cleanup.py
new file mode 100644
index 00000000..c0a454ae
--- /dev/null
+++ b/tests/test_orphan_cleanup.py
@@ -0,0 +1,207 @@
+"""Orphan-conversation scan + purge (the ``turnstone-admin orphan-conversations`` verb).
+
+Orphans are conversation rows whose ``workstreams`` row is gone — written by
+historical unregistered paths or by the delete-during-inflight race (a late
+tool-result save re-creating rows after ``delete_workstream``).
+"""
+
+from __future__ import annotations
+
+import argparse
+import hashlib
+
+import pytest
+
+from turnstone.admin import _cmd_orphan_conversations
+
+
+def _orphan(backend, ws_id: str, n: int = 2) -> None:
+    """Persist *n* conversation rows for *ws_id* WITHOUT registering it."""
+    for i in range(n):
+        backend.save_message(ws_id, "user" if i % 2 == 0 else "assistant", f"m{i}")
+
+
+def _blob(backend, payload: bytes, origin: str = "upload") -> str:
+    """Save a content-addressed attachment; each save bumps the refcount."""
+    aid = hashlib.sha256(payload).hexdigest()
+    backend.save_attachment(aid, "f.txt", "text/plain", len(payload), "text", payload, origin)
+    return aid
+
+
+class TestOrphanScan:
+    def test_clean_db_has_no_orphans(self, backend):
+        backend.register_workstream("live1")
+        backend.save_message("live1", "user", "hello")
+        assert backend.list_orphan_conversations() == []
+
+    def test_orphan_reported_with_stats(self, backend):
+        _orphan(backend, "ghost1", n=3)
+        scan = backend.list_orphan_conversations()
+        assert len(scan) == 1
+        entry = scan[0]
+        assert entry["ws_id"] == "ghost1"
+        assert entry["rows"] == 3
+        assert entry["first"] <= entry["last"]
+        assert entry["attachment_refs"] == 0
+
+    def test_scan_counts_attachment_refs(self, backend):
+        _orphan(backend, "ghost2", n=1)
+        msg_id = backend.save_message("ghost2", "user", "with attachment")
+        aid = _blob(backend, b"orphan-bytes")
+        backend.set_message_attachments("ghost2", msg_id, [aid])
+        scan = backend.list_orphan_conversations()
+        assert scan[0]["attachment_refs"] == 1
+
+    def test_scan_is_oldest_first(self, backend):
+        _orphan(backend, "newer")
+        _orphan(backend, "older")
+        # Timestamps are insertion-ordered ISO text; rewrite to force ordering.
+        import sqlalchemy as sa
+
+        from turnstone.core.storage._schema import conversations
+
+        with backend._conn() as conn:
+            conn.execute(
+                sa.update(conversations)
+                .where(conversations.c.ws_id == "older")
+                .values(timestamp="2020-01-01T00:00:00")
+            )
+            conn.commit()
+        scan = backend.list_orphan_conversations()
+        assert [o["ws_id"] for o in scan] == ["older", "newer"]
+
+
+class TestOrphanPurge:
+    def test_purge_deletes_only_orphans(self, backend):
+        backend.register_workstream("live1")
+        backend.save_message("live1", "user", "keep me")
+        _orphan(backend, "ghost1", n=4)
+        result = backend.delete_orphan_conversations(["ghost1"])
+        assert result == {"workstreams": 1, "rows": 4, "released_refs": 0, "skipped": 0}
+        assert backend.list_orphan_conversations() == []
+        assert len(backend.load_messages("live1")) == 1
+
+    def test_purge_skips_reregistered_ws(self, backend):
+        """A ws_id that gained a workstreams row between scan and purge survives."""
+        _orphan(backend, "ghost1", n=2)
+        scan = [o["ws_id"] for o in backend.list_orphan_conversations()]
+        backend.register_workstream("ghost1")
+        result = backend.delete_orphan_conversations(scan)
+        assert result["skipped"] == 1
+        assert result["workstreams"] == 0
+        assert result["rows"] == 0
+        assert len(backend.load_messages("ghost1")) == 2
+
+    def test_purge_releases_refcounts_and_prunes_at_zero(self, backend):
+        _orphan(backend, "ghost1", n=1)
+        msg_id = backend.save_message("ghost1", "user", "img")
+        aid = _blob(backend, b"only-orphan-referenced")
+        backend.set_message_attachments("ghost1", msg_id, [aid])
+        result = backend.delete_orphan_conversations(["ghost1"])
+        assert result["released_refs"] == 1
+        assert backend.get_attachment(aid) is None
+
+    def test_purge_keeps_blob_shared_with_live_ws(self, backend):
+        payload = b"shared-bytes"
+        backend.register_workstream("live1")
+        live_msg = backend.save_message("live1", "user", "live ref")
+        aid_live = _blob(backend, payload)
+        backend.set_message_attachments("live1", live_msg, [aid_live])
+
+        _orphan(backend, "ghost1", n=1)
+        ghost_msg = backend.save_message("ghost1", "user", "ghost ref")
+        aid_ghost = _blob(backend, payload)  # same content hash; refcount -> 2
+        backend.set_message_attachments("ghost1", ghost_msg, [aid_ghost])
+        assert aid_live == aid_ghost
+
+        result = backend.delete_orphan_conversations(["ghost1"])
+        assert result["released_refs"] == 1
+        row = backend.get_attachment(aid_live)
+        assert row is not None
+        assert row["refcount"] == 1
+
+    def test_purge_sweeps_config_rows(self, backend):
+        _orphan(backend, "ghost1", n=1)
+        backend.save_workstream_config("ghost1", {"model": "x"})
+        backend.delete_orphan_conversations(["ghost1"])
+        import sqlalchemy as sa
+
+        from turnstone.core.storage._schema import workstream_config
+
+        with backend._conn() as conn:
+            left = conn.execute(
+                sa.select(sa.func.count()).where(workstream_config.c.ws_id == "ghost1")
+            ).scalar()
+        assert left == 0
+
+    def test_purge_empty_list_is_noop(self, backend):
+        assert backend.delete_orphan_conversations([]) == {
+            "workstreams": 0,
+            "rows": 0,
+            "released_refs": 0,
+            "skipped": 0,
+        }
+
+
+class TestAdminVerb:
+    """The CLI handler over a real (ephemeral) backend."""
+
+    def _args(self, **kw) -> argparse.Namespace:
+        return argparse.Namespace(delete=False, yes=False, **kw)
+
+    def test_scan_reports_and_does_not_delete(self, backend, monkeypatch, capsys):
+        _orphan(backend, "ghost1", n=2)
+        monkeypatch.setattr("turnstone.admin._get_storage", lambda args: backend)
+        _cmd_orphan_conversations(self._args())
+        out = capsys.readouterr().out
+        assert "ghost1" in out
+        assert "--delete" in out
+        assert len(backend.load_messages("ghost1")) == 2
+
+    def test_delete_yes_purges(self, backend, monkeypatch, capsys):
+        _orphan(backend, "ghost1", n=2)
+        monkeypatch.setattr("turnstone.admin._get_storage", lambda args: backend)
+        ns = self._args()
+        ns.delete = True
+        ns.yes = True
+        _cmd_orphan_conversations(ns)
+        out = capsys.readouterr().out
+        assert "Purged 2" in out
+        assert backend.list_orphan_conversations() == []
+
+    def test_delete_confirmation_abort(self, backend, monkeypatch, capsys):
+        _orphan(backend, "ghost1", n=1)
+        monkeypatch.setattr("turnstone.admin._get_storage", lambda args: backend)
+        monkeypatch.setattr("builtins.input", lambda prompt: "n")
+        ns = self._args()
+        ns.delete = True
+        with pytest.raises(SystemExit):
+            _cmd_orphan_conversations(ns)
+        assert len(backend.load_messages("ghost1")) == 1
+
+    def test_delete_summary_reports_partial_skip(self, backend, monkeypatch, capsys):
+        """Mixed batch: summary shows ACTUAL purge counts plus the skipped clause."""
+        _orphan(backend, "ghost1", n=2)
+        _orphan(backend, "ghost2", n=3)
+        real_list = backend.list_orphan_conversations
+
+        def list_then_register():
+            scan = real_list()
+            backend.register_workstream("ghost2")  # wins the scan-to-purge race
+            return scan
+
+        monkeypatch.setattr(backend, "list_orphan_conversations", list_then_register)
+        monkeypatch.setattr("turnstone.admin._get_storage", lambda args: backend)
+        ns = self._args()
+        ns.delete = True
+        ns.yes = True
+        _cmd_orphan_conversations(ns)
+        out = capsys.readouterr().out
+        assert "Purged 2 row(s) across 1 workstream(s)" in out
+        assert "skipped 1 re-registered" in out
+        assert len(backend.load_messages("ghost2")) == 3
+
+    def test_clean_db_message(self, backend, monkeypatch, capsys):
+        monkeypatch.setattr("turnstone.admin._get_storage", lambda args: backend)
+        _cmd_orphan_conversations(self._args())
+        assert "No orphan conversation rows." in capsys.readouterr().out
diff --git a/turnstone/admin.py b/turnstone/admin.py
index a4c5ceba..4249aefb 100644
--- a/turnstone/admin.py
+++ b/turnstone/admin.py
@@ -439,6 +439,44 @@ def _discover_console_url() -> str:
 # ---------------------------------------------------------------------------
 
 
+def _cmd_orphan_conversations(args: argparse.Namespace) -> None:
+    """Scan for (and with --delete purge) conversation rows whose workstream is gone."""
+    storage = _get_storage(args)
+    orphans = storage.list_orphan_conversations()
+    if not orphans:
+        print("No orphan conversation rows.")
+        return
+    width = max(len(o["ws_id"]) for o in orphans)
+    print(f"{'ws_id':<{width}}  {'rows':>5}  {'refs':>4}  first       last")
+    for o in orphans:
+        print(
+            f"{o['ws_id']:<{width}}  {o['rows']:>5}  {o['attachment_refs']:>4}  "
+            f"{(o['first'] or '')[:10]}  {(o['last'] or '')[:10]}"
+        )
+    total_rows = sum(o["rows"] for o in orphans)
+    total_refs = sum(o["attachment_refs"] for o in orphans)
+    print(
+        f"\n{len(orphans)} orphan workstream(s), {total_rows} conversation row(s), "
+        f"{total_refs} attachment ref(s)."
+    )
+    if not args.delete:
+        print("Re-run with --delete to purge them.")
+        return
+    if not args.yes:
+        reply = input(f"Delete {total_rows} row(s) across {len(orphans)} workstream(s)? [y/N] ")
+        if reply.strip().lower() not in ("y", "yes"):
+            print("Aborted.", file=sys.stderr)
+            sys.exit(1)
+    result = storage.delete_orphan_conversations([o["ws_id"] for o in orphans])
+    summary = (
+        f"Purged {result['rows']} row(s) across {result['workstreams']} workstream(s); "
+        f"released {result['released_refs']} attachment ref(s)"
+    )
+    if result["skipped"]:
+        summary += f"; skipped {result['skipped']} re-registered workstream(s)"
+    print(summary + ".")
+
+
 def _cmd_rerank_calibrate(args: argparse.Namespace) -> None:
     from turnstone.core.config import get_rerank_instruction
     from turnstone.core.config_store import ConfigStore
@@ -619,6 +657,15 @@ def main() -> None:
         help="Write the calibration onto the model's capabilities",
     )
 
+    p_orph = sub.add_parser(
+        "orphan-conversations",
+        help="Scan (and with --delete purge) conversation rows whose workstream row is gone",
+    )
+    p_orph.add_argument(
+        "--delete", action="store_true", help="Purge the orphans after the scan report"
+    )
+    p_orph.add_argument("--yes", action="store_true", help="Skip the interactive confirmation")
+
     args = parser.parse_args()
     if not args.command:
         parser.print_help()
@@ -638,6 +685,7 @@ def main() -> None:
         "set-node-metadata": _cmd_set_node_metadata,
         "delete-node-metadata": _cmd_delete_node_metadata,
         "export": _cmd_export,
+        "orphan-conversations": _cmd_orphan_conversations,
         "rerank-calibrate": _cmd_rerank_calibrate,
     }
     dispatch[args.command](args)
diff --git a/turnstone/core/storage/_postgresql.py b/turnstone/core/storage/_postgresql.py
index 48e5b6b5..5e1ddc28 100644
--- a/turnstone/core/storage/_postgresql.py
+++ b/turnstone/core/storage/_postgresql.py
@@ -114,17 +114,19 @@ from turnstone.core.storage._utils import (
 from turnstone.core.storage._utils import (
     escape_like as _escape_like,
 )
+from turnstone.core.storage._utils import (
+    find_orphan_conversations,
+    prepare_provider_data_for_save,
+    purge_orphan_conversations,
+    release_attachment_refs,
+    sanitize_text,
+)
 from turnstone.core.storage._utils import (
     normalize_search_terms as _normalize_search_terms,
 )
 from turnstone.core.storage._utils import (
     parse_attachment_refs as _parse_attachment_refs,
 )
-from turnstone.core.storage._utils import (
-    prepare_provider_data_for_save,
-    release_attachment_refs,
-    sanitize_text,
-)
 from turnstone.core.storage._utils import (
     reconstruct_messages as _reconstruct_messages,
 )
@@ -909,6 +911,16 @@ class PostgreSQLBackend:
             conn.commit()
             return result.rowcount > 0
 
+    def list_orphan_conversations(self) -> list[dict[str, Any]]:
+        with self._conn() as conn:
+            return find_orphan_conversations(conn)
+
+    def delete_orphan_conversations(self, ws_ids: list[str]) -> dict[str, int]:
+        with self._conn() as conn:
+            result = purge_orphan_conversations(conn, ws_ids)
+            conn.commit()
+            return result
+
     # -- Workstream attachments (content-addressed, refcounted) ----------------
 
     def save_attachment(
diff --git a/turnstone/core/storage/_protocol.py b/turnstone/core/storage/_protocol.py
index 7b054dd8..a3381d87 100644
--- a/turnstone/core/storage/_protocol.py
+++ b/turnstone/core/storage/_protocol.py
@@ -633,6 +633,27 @@ class StorageBackend(Protocol):
         """Delete a workstream and all its conversations + config."""
         ...
 
+    def list_orphan_conversations(self) -> list[dict[str, Any]]:
+        """Conversation ws_ids with no ``workstreams`` row.
+
+        One dict per orphan workstream — keys ``ws_id``, ``rows``, ``first``,
+        ``last`` (ISO text timestamps), ``attachment_refs`` — ordered
+        oldest-first.  Read-only; feeds the ``turnstone-admin
+        orphan-conversations`` maintenance verb.
+        """
+        ...
+
+    def delete_orphan_conversations(self, ws_ids: list[str]) -> dict[str, int]:
+        """Purge conversation rows for the *ws_ids* that are STILL orphaned.
+
+        Re-verifies against ``workstreams`` in-transaction (a re-registered
+        ws_id is skipped, never deleted), releases the deleted rows'
+        attachment refcounts, and sweeps matching ``workstream_config`` /
+        ``workstream_overrides`` rows.  Returns counts keyed ``workstreams``,
+        ``rows``, ``released_refs``, ``skipped``.
+        """
+        ...
+
     def list_workstreams(
         self,
         node_id: str | None = None,
diff --git a/turnstone/core/storage/_sqlite.py b/turnstone/core/storage/_sqlite.py
index a913d5e5..a60540df 100644
--- a/turnstone/core/storage/_sqlite.py
+++ b/turnstone/core/storage/_sqlite.py
@@ -114,17 +114,19 @@ from turnstone.core.storage._utils import (
 from turnstone.core.storage._utils import (
     escape_like as _escape_like,
 )
+from turnstone.core.storage._utils import (
+    find_orphan_conversations,
+    prepare_provider_data_for_save,
+    purge_orphan_conversations,
+    release_attachment_refs,
+    sanitize_text,
+)
 from turnstone.core.storage._utils import (
     normalize_search_terms as _normalize_search_terms,
 )
 from turnstone.core.storage._utils import (
     parse_attachment_refs as _parse_attachment_refs,
 )
-from turnstone.core.storage._utils import (
-    prepare_provider_data_for_save,
-    release_attachment_refs,
-    sanitize_text,
-)
 from turnstone.core.storage._utils import (
     reconstruct_messages as _reconstruct_messages,
 )
@@ -1055,6 +1057,16 @@ class SQLiteBackend:
             conn.commit()
             return result.rowcount > 0
 
+    def list_orphan_conversations(self) -> list[dict[str, Any]]:
+        with self._conn() as conn:
+            return find_orphan_conversations(conn)
+
+    def delete_orphan_conversations(self, ws_ids: list[str]) -> dict[str, int]:
+        with self._conn() as conn:
+            result = purge_orphan_conversations(conn, ws_ids)
+            conn.commit()
+            return result
+
     # -- Workstream attachments (content-addressed, refcounted) ----------------
 
     def save_attachment(
diff --git a/turnstone/core/storage/_utils.py b/turnstone/core/storage/_utils.py
index 3b258455..744f8cc9 100644
--- a/turnstone/core/storage/_utils.py
+++ b/turnstone/core/storage/_utils.py
@@ -12,7 +12,13 @@ import sqlalchemy as sa
 
 from turnstone.core.attachments import unreadable_placeholder
 from turnstone.core.log import get_logger
-from turnstone.core.storage._schema import workstream_attachments
+from turnstone.core.storage._schema import (
+    conversations,
+    workstream_attachments,
+    workstream_config,
+    workstream_overrides,
+    workstreams,
+)
 from turnstone.core.trajectory import (
     AttachmentRef,
     ContentBlock,
@@ -173,6 +179,100 @@ def release_attachment_refs(conn: Any, attachment_ids: list[str]) -> None:
     )
 
 
+def find_orphan_conversations(conn: Any) -> list[dict[str, Any]]:
+    """Conversation ws_ids that have no ``workstreams`` row, with row stats.
+
+    Orphans come from writers that persisted without a registered workstream:
+    historically the pre-unification CLI/server paths, and the
+    delete-during-inflight race (a late tool-result save re-creating rows
+    after ``delete_workstream``).  Read-only; ordered oldest-first.  Each
+    entry carries the attachment-ref count so a purge's refcount release is
+    visible before it happens.
+    """
+    rows = conn.execute(
+        sa.select(
+            conversations.c.ws_id,
+            sa.func.count().label("row_count"),
+            sa.func.min(conversations.c.timestamp).label("first"),
+            sa.func.max(conversations.c.timestamp).label("last"),
+        )
+        .select_from(
+            conversations.outerjoin(workstreams, conversations.c.ws_id == workstreams.c.ws_id)
+        )
+        .where(workstreams.c.ws_id.is_(None))
+        .group_by(conversations.c.ws_id)
+        .order_by(sa.func.min(conversations.c.timestamp))
+    ).fetchall()
+    orphans: list[dict[str, Any]] = []
+    for ws_id, row_count, first, last in rows:
+        referenced = conn.execute(
+            sa.select(conversations.c.attachments).where(
+                sa.and_(
+                    conversations.c.ws_id == ws_id,
+                    conversations.c.attachments.is_not(None),
+                )
+            )
+        ).fetchall()
+        ref_count = sum(len(parse_attachment_refs(refs)) for (refs,) in referenced)
+        orphans.append(
+            {
+                "ws_id": ws_id,
+                "rows": int(row_count),
+                "first": first,
+                "last": last,
+                "attachment_refs": ref_count,
+            }
+        )
+    return orphans
+
+
+def purge_orphan_conversations(conn: Any, ws_ids: list[str]) -> dict[str, int]:
+    """Delete conversation rows for the *ws_ids* that are STILL orphans.
+
+    Orphan-ness is re-verified here, inside the caller's transaction — a
+    ws_id that gained a ``workstreams`` row between scan and purge is counted
+    in ``skipped`` and left untouched, so a stale scan can never delete a
+    live workstream's history.  Mirrors ``delete_workstream``'s cascade for
+    rows with no owning workstream: release the deleted rows' attachment
+    refcounts, then sweep the matching ``workstream_config`` /
+    ``workstream_overrides`` rows.  Caller owns commit.
+    """
+    if not ws_ids:
+        return {"workstreams": 0, "rows": 0, "released_refs": 0, "skipped": 0}
+    registered = {
+        row[0]
+        for row in conn.execute(
+            sa.select(workstreams.c.ws_id).where(workstreams.c.ws_id.in_(ws_ids))
+        ).fetchall()
+    }
+    targets = [w for w in ws_ids if w not in registered]
+    if not targets:
+        return {"workstreams": 0, "rows": 0, "released_refs": 0, "skipped": len(registered)}
+    referenced = conn.execute(
+        sa.select(conversations.c.attachments).where(
+            sa.and_(
+                conversations.c.ws_id.in_(targets),
+                conversations.c.attachments.is_not(None),
+            )
+        )
+    ).fetchall()
+    ref_ids: list[str] = []
+    for (refs,) in referenced:
+        ref_ids.extend(parse_attachment_refs(refs))
+    release_attachment_refs(conn, ref_ids)
+    deleted = conn.execute(
+        sa.delete(conversations).where(conversations.c.ws_id.in_(targets))
+    ).rowcount
+    conn.execute(sa.delete(workstream_config).where(workstream_config.c.ws_id.in_(targets)))
+    conn.execute(sa.delete(workstream_overrides).where(workstream_overrides.c.ws_id.in_(targets)))
+    return {
+        "workstreams": len(targets),
+        "rows": int(deleted or 0),
+        "released_refs": len(ref_ids),
+        "skipped": len(registered),
+    }
+
+
 def attachment_to_content_part(att: dict[str, Any]) -> dict[str, Any] | None:
     """Convert a stored attachment row into an OpenAI-style content part.