FIX Reject 5 – Invalid Tag Number Explained
SessionRejectReason 5 indicates that a tag number in the message is not recognized or is invalid for the FIX version being used. This typically happens when using tags from a newer FIX version or custom tags not supported by the counterparty.
Root Cause
1) Using FIX 4.4 tags with FIX 4.2 counterparty, 2) Custom tags not supported, 3) Typo in tag number, 4) Tag deprecated in current FIX version, 5) Version mismatch between parties.
Broker/Exchange Action
Broker sends Reject (35=3) with SessionRejectReason=5 and RefTagID pointing to the invalid tag. Message is rejected.
Client/Your Action
1) Verify FIX version compatibility (BeginString), 2) Check tag is valid for your FIX version, 3) Remove unsupported custom tags, 4) Verify tag numbers are correct, 5) Align FIX versions with counterparty.
Example FIX Message
How FIX Doctor Detects This
FIX Doctor identifies Reject messages with SessionRejectReason=5 and flags the invalid tag number in the error summary.
Related Errors
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