diff --git a/coworker/conversations.py b/coworker/conversations.py
index 300ca9cc9..2ecb96add 100644
--- a/coworker/conversations.py
+++ b/coworker/conversations.py
@@ -533,12 +533,18 @@ def canonicalize_workspaces(self) -> None:
             self._conn.commit()
 
     def delete(self, session_id: str) -> bool:
+        # Resolve (and validate) the path BEFORE any DB or filesystem mutation so a hostile
+        # id can't unlink a .jsonl outside conv_dir — path.unlink() below runs regardless of
+        # the DB rowcount, which made it an arbitrary-delete primitive.
+        try:
+            path = self._file(session_id)
+        except ValueError:
+            return False
         with self._lock:
             cur = self._conn.execute(
                 "DELETE FROM sessions WHERE session_id = ?", (session_id,)
             )
             self._conn.commit()
-        path = self._file(session_id)
         if path.exists():
             path.unlink()
         return cur.rowcount > 0
diff --git a/tests/test_conversation_store_paths.py b/tests/test_conversation_store_paths.py
new file mode 100644
index 000000000..9c6d9b802
--- /dev/null
+++ b/tests/test_conversation_store_paths.py
@@ -0,0 +1,57 @@
+"""ConversationStore transcript paths must stay inside conv_dir.
+
+A client-minted session_id flows straight into ConversationStore._file() from the REST/WS
+routes. Starlette's {session_id} param excludes "/" but not "\\" — a path separator on Windows
+— so "..%5C..%5C.." would escape conv_dir and delete()'s unconditional unlink becomes an
+arbitrary-delete primitive. These tests pin the store-level guard (platform-independent).
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from coworker.conversations import ConversationStore
+from coworker.sessions import SessionRecord
+
+HOSTILE_IDS = [
+    "../secret",  # posix separator escapes on Linux/macOS
+    "..\\..\\secret",  # backslash escapes on Windows
+    "sub/evil",  # any embedded separator
+    "a.b",  # "." is disallowed — keeps f"{sid}.jsonl" a single filename
+    "",
+    "x" * 129,  # over the length bound
+]
+
+
+@pytest.mark.parametrize("sid", HOSTILE_IDS)
+def test_file_rejects_ids_that_could_escape_conv_dir(tmp_path, sid):
+    store = ConversationStore(tmp_path)
+    with pytest.raises(ValueError):
+        store._file(sid)
+
+
+def test_delete_with_traversal_id_touches_nothing_and_returns_false(tmp_path):
+    store = ConversationStore(tmp_path)
+    # A transcript-looking file one level above conv_dir (tmp_path/secret.jsonl).
+    secret = tmp_path / "secret.jsonl"
+    secret.write_text("keep me", encoding="utf-8")
+
+    # conv_dir is tmp_path/conversations, so "../secret" resolves to the file above — the
+    # exact arbitrary-delete the unconditional unlink allowed before the fix.
+    assert store.delete("../secret") is False
+    assert secret.exists()  # not unlinked
+
+
+def test_valid_session_id_still_round_trips(tmp_path):
+    store = ConversationStore(tmp_path)
+    store.save(
+        SessionRecord(
+            session_id="__task__task-abc123",
+            workspace=str(tmp_path),
+            model="m",
+            mode="interactive",
+        )
+    )
+    assert store.load("__task__task-abc123") is not None
+    assert store.delete("__task__task-abc123") is True
+    assert store.load("__task__task-abc123") is None