backtest_not_found

HTTP 404. The backtest_id does not name one of your backtests. Deleted and failed backtests stop resolving.

What to do: Poll the id returned by POST /v1/backtests; treat a 404 after a delete as expected.

Every error is an RFC 9457 problem document - match on code (stable forever), show detail to humans, and quote instance when reporting an occurrence to support.