FRCP 15 governs amendment to pleadings, so it would stand to reason that it would be the operative rule when seeking to amend a complaint. However, when seeking to amend after the deadline in the scheduling order, the movant must satisfy not only the relatively liberal requirements of Rule 15 but also the more exacting "good cause" standard of Rule 16. Unlike Rule 15, which permits amendment in the absence of undue delay, bad faith, or dilatory motive, Rule 16 requires diligence by the party seeking amendment.

A recent ruling by Judge Fallon demonstrates the danger of ignoring Rule 16's requirements when seeking amendment after the deadline. The plaintiff in NRT Tech. Corp. v. Everi Holdings Inc., C.A. No. 19-804-MN-SRF sought to amend its complaint, asserting Walker Process and sham litigation antitrust claims, almost a full year after the expiration of the amendment deadline.
Judge Fallon noted that although plaintiffs "bear the burden of showing that they exercised diligence in seeking the proposed amendment under Rule 16(b)(4)," their motion "does not address the applicable good cause standard for motions to amend filed after the deadline."
Plaintiffs apparently argued that defense counsel should have alerted them of the applicability of Rule 16, but Judge Fallon rejected that ...