Heal before resend

Checklist before you blast the same ClOrdID again:

1. Read 58 Text and 373/380 reason codes
2. Fix the field in 371 or profile mismatch (symbol, side, TIF)
3. Confirm session still up (heartbeats flowing)
4. Decide: same 11 (idempotent retry) vs new 11 (new intent)

Mistake	Fix
Bad checksum / 9	Rebuild message, never hand-edit 10
Wrong 55 format	ROE / venue spec
Sending D before A	Wait for Logon complete

4H codegen catches many typos at compile time — runtime rejects are often profile drift or venue rules.