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FIX Reject 371 – Causes, Examples, and How to Fix It

What is Tag 371 (RefTagID)?

Tag 371 (RefTagID) in FIX protocol identifies the specific tag that caused a session-level reject (35=3). When a FIX message is rejected, the receiving party includes tag 371 to pinpoint exactly which field was problematic.

Why FIX Rejects Occur

FIX rejects happen when a message fails validation at the session or application level. Common reasons include:

  • Missing mandatory fields required by the FIX specification
  • Invalid field values (e.g., invalid enum, out-of-range numbers)
  • Incorrect message structure or formatting
  • Business rule violations (e.g., price exceeds limits)
  • Sequence number gaps or duplicates

Common Root Causes

Missing Fields

A NewOrderSingle (35=D) message missing required fields like ClOrdID (11), Side (54), or Symbol (55) will trigger a reject.

8=FIX.4.4|9=123|35=D|34=1|49=SENDER|52=20240101-10:00:00|56=TARGET|10=123|
Missing: 11=ClOrdID, 54=Side, 55=Symbol

Invalid Values

Invalid enum values or out-of-range numbers cause rejects. For example, Side (54) must be 1 (Buy) or 2 (Sell), not 99.

8=FIX.4.4|9=123|35=D|11=ORDER123|34=1|49=SENDER|52=20240101-10:00:00|54=99|55=AAPL|56=TARGET|10=123|
Error: Side=99 is invalid (must be 1 or 2)

Example FIX Logs

Valid NewOrderSingle:

8=FIX.4.4|9=123|35=D|11=ORDER001|34=1|49=SENDER|52=20240101-10:00:00|54=1|55=AAPL|38=100|40=2|44=150.00|56=TARGET|10=123|

Rejected Message (Missing Price for Limit Order):

8=FIX.4.4|9=123|35=D|11=ORDER002|34=2|49=SENDER|52=20240101-10:00:01|54=1|55=AAPL|38=100|40=2|56=TARGET|10=123|
Reject: 35=3|371=44|372=D|58=Price required for Limit order|

Troubleshooting Steps

  1. Check the reject message (35=3) for RefTagID (371) to identify the problematic field
  2. Verify all mandatory fields are present according to FIX specification
  3. Validate enum values against the FIX dictionary
  4. Ensure numeric fields are within acceptable ranges
  5. Review business rules and counterparty-specific requirements
  6. Use FIX Doctor to automatically detect and highlight reject causes

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