change

backend/app/services/storage.py

@@ -72,6 +72,14 @@ class WorkspaceStore:
     def conversation_path(self, conversation_id: str) -> Path:
         return self.conversation_dir(conversation_id) / "conversation.json"
 
+    def write_tool_call_diagnostic(self, conversation_id: str, payload: dict[str, Any]) -> str:
+        """Persist one failed model tool call without creating a CAD revision."""
+        conversation = safe_conversation_id(conversation_id)
+        relative = Path("diagnostics") / f"tool_call_{secrets.token_hex(8)}.json"
+        path = self.conversation_dir(conversation) / relative
+        write_json(path, payload)
+        return (Path(conversation) / relative).as_posix()
+
     def ensure_conversation(
         self,
         conversation_id: str | None,