PossDupFlag (FIX tag 43)

BooleanFIX 4.4FIX 4.2session envelope

PossDupFlag (tag 43) is a Boolean field defined in FIX 4.4 and FIX 4.2. It belongs to the session envelope: every FIX 4.4 message carries it, so it identifies the session rather than describing the business event. It is an enumerated field with 2 defined values.

At a glance

Tag: 43
Name: PossDupFlag
Datatype: Boolean (char)
Dialects: FIX 4.4 and FIX 4.2
Messages: 93
Valid values: 2

What the specification says

Indicates possible retransmission of message with this sequence number

Descriptions are quoted from the FIX Orchestra sources under the Apache 2.0 licence.

A real message using tag 43

NewOrderSingle · FIX 4.4

Tag	Field	Wire value	Meaning
8	BeginString	FIX.4.4
9	BodyLength	116
35	MsgType	D	Order – Single
49	SenderCompID	BOARTEAM
56	TargetCompID	COUNTERPARTY
34	MsgSeqNum	2
43	PossDupFlag	N	Original transmission
52	SendingTime	20240101-12:00:00.000
11	ClOrdID	ORD-10042
54	Side	1	Buy
60	TransactTime	20240101-12:00:00.000
40	OrdType	1	Market
10	CheckSum	232

Edit and decode it yourself

Shown with | separators for readability. On the wire FIX uses SOH (0x01), an invisible control byte — and because BodyLength and CheckSum are computed over the actual bytes, the two forms have different checksums. Both are valid and both decode here.

Open in the full decoder →

Generated from the FIX 4.4 dictionary and verified at build time: it parses and validates with no issues. Open the decoder to try your own message.

Valid values

A value outside this list is not a parse error — the message still decodes — but validate() reports it as validate/value-not-in-enum.

Value	Meaning	Symbolic name
Y	Possible duplicate	YES / PossibleDuplicate
N	Original transmission	NO / OriginalTransmission

Where tag 43 appears

Session envelope field

Tag 43 appears in every one of the 93 FIX 4.4 message types, because it is part of the standard header or trailer. Listing them all would say nothing useful — it is present on anything you will ever decode.

Wire format

Y or N. Not 1/0, and not true/false. @boarteam/fix keeps the value as a string and, for enumerated fields, reports validate/value-not-in-enum when it is not a defined value.

Read tag 43 in TypeScript

decode.tsts

import { createFixEngine } from "@boarteam/fix";import { dictionary } from "@boarteam/fix-dict-fix44";const fix = createFixEngine(dictionary);const { message, issues } = fix.parse(raw);const possDupFlag = message.fields[43]?.raw;// e.g. "Y" — Possible duplicate// The dictionary also carries the labels:const label = fix.dictionary  .fieldByTag(43)  ?.enumValues?.find((e) => e.value === possDupFlag)  ?.description;

Decode this in your own code

The same engine that produced the decoded example above is an Apache-2.0 npm package with zero runtime dependencies. It runs in Node and in the browser, and parse() returns problems as data instead of throwing.

npm i @boarteam/fix @boarteam/fix-dict-fix44

Try the decoder Source on GitHub