The version and the database path
FlexSim logs both on the first two lines of every run. Paste them, say which spec profile was active, and attach the log file if you can — the log folder opens from Settings.
Contact
Ask about the ISO 8583 simulator
One address, read by the person who wrote the thing. Bug reports, questions about a host interface FlexSim does not yet describe, or anything the manual leaves unanswered.
The button opens your mail client. If it does not, the address is contact@flexsim8583.dev — copy it by hand.
What to include
A message that can be acted on, first time
Nearly every question about a wrongly built message is answered by the same three facts, and FlexSim prints two of them in its own log on startup. Send them and the reply is an answer rather than a request for more detail.
The hex, both directions
What FlexSim sent and what the host sent back, as hex, plus the data element you believe is wrong. A description of the symptom is rarely enough to locate a framing or encoding fault; the bytes always are.
The spec, or the part that matters
Spec profiles are data, not code, so a new host interface usually needs no new version of the app. Send the message format section and say which fields the existing profiles get wrong.
Redact before you send. A real PAN, a real key or a production capture does not belong in an email, and none of it is needed to diagnose a format problem. Replace the digits and keep the lengths.
Before you write
The manual probably covers it
It is the whole application in order, with screenshots — spec profiles and subfield composition, card data and key derivation, test cases and expected responses, both connection roles, and the reference tools.
Not got it yet?
FlexSim is two files and needs no installer. Unzip it, run it, and it creates its own database on first launch.
Use FlexSim only against systems you are authorised to test