Nasdaq BX Adds New FIX Protocol for Smoother Stock Trading
Published Date: 9/10/2025
Notice
Summary
Nasdaq BX is rolling out a new way to send stock orders called the CORE FIX protocol, making trading smoother and more flexible for brokers and traders. They’re also updating some rules to match how certain order types actually work today. These changes kicked in right away on August 26, 2025, with no extra fees, aiming to keep the market fast and fair.
Analyzed Economic Effects
6 provisions identified: 4 benefits, 1 costs, 1 mixed.
Nasdaq BX adds CORE FIX protocol
Nasdaq BX added a new proprietary Order Entry Protocol called CORE FIX (filed August 26, 2025). CORE FIX uses standard FIX coding but omits routing capabilities and is designed to be faster than FIX and similar in speed to OUCH; the Exchange expects full implementation in October 2025 and will announce the date at least 30 days before implementation.
CORE FIX supports many order types and attributes
The rule amendment adds CORE FIX to the Exchange's list of protocols and specifies that CORE FIX will permit many Order Types and Order Attributes similar to OUCH, including Price to Comply, Price to Display, Non-Displayed, Post-Only, RPI Orders, Market Maker Peg Orders, Primary/Market Pegging, Reserve Size, Minimum Quantity, Trade Now, and related attributes (filing dated August 26, 2025).
RPI Order repricing behavior differs by protocol
The filing explains differing RPI (Retail Price Improving) repricing rules: if entered via RASH or FIX, an RPI Order may be repriced repeatedly in step with NBBO changes until posted at the original entered limit price; if entered via CORE FIX, OUCH, or FLITE, an RPI Order may either remain or be cancelled depending on participant choice, and if posted and the NBBO changes so the order would cross a Protected Quotation, the order will be cancelled back to the participant.
Limit-priced order repricing rules extended to CORE FIX
Rule 4120(b)(1)(A)(i)(e)(2) will apply to limit-priced orders entered via CORE FIX (filed August 26, 2025), so such orders may be repriced upon entry or repriced multiple times when Price Bands move in the same ways that currently apply to orders entered via OUCH.
Midpoint pegging not available for RPI and Retail Orders
The Exchange amended rules to state that the Midpoint Pegging Order Attribute is not available for RPI Orders and is not available for Retail Orders (filed August 26, 2025).
Port-level group ID prevents execution between grouped orders
Rule 4757(a)(A)(3) was amended to allow participants using CORE FIX to assign a unique group identification modifier to orders entered through a specific order entry port; quotes/orders with the same modifier will be prevented from executing against each other (filing dated August 26, 2025).
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