GapFillFlag (FIX tag 123)

BooleanFIX 4.4FIX 4.2

GapFillFlag (tag 123) is a Boolean field defined in FIX 4.4 and FIX 4.2. It appears in 1 message type, always optionally. It is an enumerated field with 2 defined values.

At a glance

Tag: 123
Name: GapFillFlag
Datatype: Boolean (char)
Dialects: FIX 4.4 and FIX 4.2
Messages: 1
Valid values: 2

What the specification says

Indicates that the Sequence Reset message is replacing administrative or application messages which will not be resent.

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

A real message using tag 123

SequenceReset · FIX 4.4

Tag	Field	Wire value	Meaning
8	BeginString	FIX.4.4
9	BodyLength	74
35	MsgType	4	Sequence Reset
49	SenderCompID	BOARTEAM
56	TargetCompID	COUNTERPARTY
34	MsgSeqNum	2
52	SendingTime	20240101-12:00:00.000
123	GapFillFlag	N	Sequence Reset, ignore MsgSeqNum
36	NewSeqNo	1
10	CheckSum	174

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	Gap Fill message, MsgSeqNum field valid	YES / GapFillMessage
N	Sequence Reset, ignore MsgSeqNum	NO / SequenceReset

Where tag 123 appears

Requiredness is a property of the message, not of the field: the same tag can be optional in one message and conditionally required in another.

Message	MsgType	FIX 4.4	FIX 4.2	Where in the message
SequenceReset	4	Optional	Optional	directly on the message body

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 123 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 gapFillFlag = message.fields[123]?.raw;// e.g. "Y" — Gap Fill message, MsgSeqNum field valid// The dictionary also carries the labels:const label = fix.dictionary  .fieldByTag(123)  ?.enumValues?.find((e) => e.value === gapFillFlag)  ?.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