FIX Error Reject 5: Incorrect value
What this error means
SessionRejectReason 5 (Invalid tag number) 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, custom tags not supported by the counterparty, or when there's a typo in the tag number.
Common causes
Common causes include:
1) Using FIX 4.4 tags with a FIX 4.2 counterparty,
2) Custom tags not supported by the receiving party,
3) Typo in tag number during message construction,
4) Tag deprecated in current FIX version,
5) Version mismatch between parties (BeginString mismatch).
Example log snippet
8=FIX.4.4|9=120|35=3|49=EXCHANGE|56=CLIENT|34=5|52=20231116-10:30:00|371=999|372=D|373=5|58=Invalid tag number|10=123|
How to fix it
1) Verify FIX version compatibility (check BeginString matches counterparty expectations)
2) Check that all tags are valid for your FIX version
3) Remove unsupported custom tags or coordinate with counterparty
4) Verify tag numbers are correct (no typos)
5) Align FIX versions with counterparty - both parties should use the same FIX version
6) Review FIX specification for your version to ensure tag compatibility
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