FIX Error Reject 6: Required tag missing
What this error means
SessionRejectReason 6 indicates that a mandatory tag required by the FIX specification or business rules is missing from the message. The RefTagID (371) identifies which tag was missing. This is one of the most common FIX reject reasons.
Common causes
Common causes include:
1) Message construction omits mandatory field,
2) Field filtering removes required tag,
3) Validation logic incorrectly allows missing tags,
4) FIX version mismatch (different mandatory tags),
5) Business rules require additional tags beyond FIX spec minimums.
Example log snippet
8=FIX.4.4|9=120|35=3|49=EXCHANGE|56=CLIENT|371=11|372=D|373=6|58=Required tag missing: ClOrdID|10=123|
How to fix it
1) Review FIX specification for mandatory tags per message type
2) Ensure all required fields are populated before sending
3) Check field filtering doesn't remove mandatory tags
4) Verify FIX version requirements match counterparty
5) Add missing tag to message construction logic
6) Review RefTagID in reject to identify specific missing tag
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