UnderlyingLastPx (FIX tag 651)

PriceFIX 4.4

UnderlyingLastPx (tag 651) is a Price field defined in FIX 4.4. It appears in 1 message type, always optionally.

At a glance

Tag: 651
Name: UnderlyingLastPx
Datatype: Price (float)
Dialects: FIX 4.4
Messages: 1

What the specification says

The calculated or traded price for the underlying instrument that corresponds to a derivative. Used for transactions that include the cash instrument and the derivative.

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

A real message using tag 651

ExecutionReport · FIX 4.4

Tag	Field	Wire value	Meaning
8	BeginString	FIX.4.4
9	BodyLength	145
35	MsgType	8	Execution Report
49	SenderCompID	BOARTEAM
56	TargetCompID	COUNTERPARTY
34	MsgSeqNum	2
52	SendingTime	20240101-12:00:00.000
37	OrderID	ORD-10042
17	ExecID	EXEC-88117
150	ExecType	0	New
39	OrdStatus	0	New
54	Side	1	Buy
651	UnderlyingLastPx	1.09210
151	LeavesQty	0
14	CumQty	1000000
6	AvgPx	1.09215
10	CheckSum	081

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.

Where tag 651 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	Requiredness	Where in the message
ExecutionReport	8	Optional	directly on the message body

Wire format

A decimal price. Precision is a venue convention, not a protocol rule, so compare the raw string when exactness matters. @boarteam/fix parses the value as a number and reports parse/invalid-float when it does not lex as one; the raw string is kept so precision and trailing zeros are never lost.

Read tag 651 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 field = message.fields[651];field?.name;   // "UnderlyingLastPx"field?.raw;    // the verbatim wire stringfield?.value;  // coerced to a number

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