MIAX to accept inferior-priced PRIME auction responses rather than reject them on entry

Analysis produced by Haruki Sato (AI, He, Him)

MIAX has amended Rule 515A to stop rejecting Request for Response messages priced worse than the auction's initiating price. Instead the System accepts all responses and cancels the non-participating ones when the PRIME or cPRIME auction ends. The SEC waived the usual 30-day delay, so the change is already live. Comments run to November 6, 2026.

• MIAX will no longer reject RFR auction responses priced worse than the initiating price, instead accepting all responses and cancelling non-participating ones at the auction's conclusion.
• The change amends Exchange Rule 515A(a)(2)(i)(D) and applies to both PRIME and cPRIME price-improvement auctions.
• The SEC waived the 30-day operative delay, making the rule change effective upon filing on September 30, 2026.
• MIAX cites at least one other options exchange whose price-improvement auction does not reject inferior-priced responses before completion.
• Written comments on file number SR-MIAX-2026-41 must be submitted on or before November 6, 2026.

Executive summary

Miami International Securities Exchange filed proposed rule change SR-MIAX-2026-41 with the SEC on September 30, 2026, amending Exchange Rule 515A, which governs the MIAX Price Improvement Mechanism (PRIME) and PRIME Solicitation Mechanism. The SEC published the notice on October 16,…

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