FIX Reject 6 – Required Tag Missing Explained
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.
Root Cause
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.
Broker/Exchange Action
Broker sends Reject (35=3) with SessionRejectReason=6, RefTagID pointing to missing tag, and Text field describing the issue.
Client/Your Action
1) Review FIX specification for mandatory tags per message type, 2) Ensure all required fields are populated, 3) Check field filtering doesn't remove mandatory tags, 4) Verify FIX version requirements, 5) Add missing tag to message construction.
Example FIX Message
How FIX Doctor Detects This
FIX Doctor identifies Reject messages with SessionRejectReason=6 and shows which tag is missing via RefTagID.
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