# MRPC

> Terminal route error (`/{mt4|mt5}/{id}/…`): the terminal processed the request but the operation failed.

Source: https://tickwire.io/docs/errors/mrpc/

HTTP 500. Unknown: it may have executed. Resolve it with the same idempotency key before acting.

**Likely causes:**

- Unknown symbol (`reason: symbol_not_found`).
- The terminal returned an error (`reason: backend_error`, MT code in `code`).

**What to do:** Check `reason`. Fix the symbol, or retry a read. Repeat a trading call only with the same Idempotency-Key.

**Retry:** Only with the same `Idempotency-Key` (or `clientId`). The first attempt may have executed; never resend a trade blindly.

Listed under [Compatibility error names](https://tickwire.io/docs/errors/#compatibility-error-names) in [Errors](https://tickwire.io/docs/errors/). Send the error's `requestId` to support and we can find the request.