I agree with the Court that Delaware's affdavit requirement cannot apply in federal court. I write separately because, in my view, the relevant conficts are with Federal Rules of Civil Procedure 3 and 12, not Rule 8.
I
As the majority explains, Delaware's law is, at bottom, an “affdavit requirement.” Ante, at 190. If a medical malpractice plaintiff wants her complaint deemed “fled” and her case docketed, the statute mandates that an affdavit of merit signed by a medical professional (or a motion for an extension of time to fle the affdavit) must accompany the complaint. See Del. Code, Tit. 18, § 6853(a)(1) (2025). By contrast, the Federal Rules require no such additional fling. Rather, “[a] civil action is commenced by fling a complaint with the court.” Fed. Rule Civ. Proc. 3.
Thus, § 6853 and Rule 3 confict regarding the requirements to “commence” a medical malpractice action. If a federal court were to follow Delaware's law, a plaintiff would have to do more than merely tender the complaint in order for his medical malpractice lawsuit to be fled and docketed. But, under the language of Rule 3, civil suits commence as soon as the complaint—and only the complaint—has been fled by the plaintiff. In my view, this clash warrants reversal here.
As the majority explains, our cases make clear beyond cavil that “a valid [Federal] Rule of Civil Procedure displaces contrary state law.” Ante, at 192. Thus, the nub of the confict inquiry is to determine whether the State's requirement Page Proof Pending Publication is, in fact, contrary to a Federal Rule. Over time, we have expressed this central inquiry in various ways. See, e. g., Hanna v. Plumer, 380 U. S. 460, 472 (1965) (asking whether “the applicable Federal Rule is in direct collision with the [state] law”); Burlington Northern R. Co. v. Woods, 480 U. S. 1, 4–5 (1987) (asking whether the scope of the Federal Rule is “ `suffciently broad' ” to “ `control the issue' ” before the Court, “thereby leaving no room for the operation” of the state law (quoting Walker v. Armco Steel Corp., 446 U. S. 740, 749–750, and n. 9 (1980))). But as the majority acknowledges, our most up-to-date approach to identifying the necessary confict is to ask whether the Federal Rule and the state statute “attemp[t] to answer the same question.” Shady Grove Orthopedic Associates, P. A. v. Allstate Ins. Co., 559 U. S. 393, 399 (2010); see also ante, at 192.1 Per Shady Grove, once a confict between the State's requirement and a Federal Rule has been identifed, the Federal Rule displaces the State's requirement so long as the 1To the extent that the Court suggests that the Federal Rule's plain text is all that matters when answering this question, that is not what our precedents hold. In Gasperini v. Center for Humanities, Inc., 518 U. S. 415 (1996), we explained that Federal Rules must be interpreted not solely based on their text—as the much earlier case of Walker v. Armco Steel Corp. had suggested in a footnote—but also “with sensitivity to important state interests and regulatory policies,” 518 U. S., at 427, n. 7. The majority opinion in Shady Grove subsequently criticized Gasperini's “search for state interests and policies that are `important' ” as “standardless.” 559 U. S., at 405, n. 7. But fve Justices in that case (i. e., a majority) agreed nevertheless with Gasperini's approach to interpreting the Federal Rules in this context. See 559 U. S., at 437 (Ginsburg, J., dissenting) (writing for three other Justices that the Court should “continue to interpret Federal Rules with awareness of, and sensitivity to, important state regulatory policies”); id., at 418 (Stevens, J., concurring in part and concurring in judgment) (explaining that the Federal Rules “must be interpreted with some degree of `sensitivity to important state interests and regulatory policies' ” (quoting Gasperini, 518 U. S., at 427, n. 7)). So, under our precedents, courts must be mindful of state interests and policies when determining whether a Federal Rule conficts with state law.
Page Proof Pending Publication Page Proof Pending Publication Rule is “valid,” meaning that it “governs only `the manner and the means' by which the litigants' rights are `enforced.' ” 559 U. S., at 407 (plurality opinion) (quoting Mississippi Pub lishing Corp. v. Murphree, 326 U. S. 438, 446 (1946)); see also 28 U. S. C. § 2072(b) (Rules Enabling Act). And for similar reasons to those given in Part II–B of the majority opinion, Rule 3 easily satisfes this standard. See ante, at 198–199; see also Shady Grove, 559 U. S., at 407 (plurality opinion) (noting that we have “rejected every statutory challenge to a Federal Rule that has come before us”).
Accordingly, Delaware's affidavit requirement cannot apply in federal court.
II
The majority and I start in the same place and ultimately reach the same conclusion, but we have taken different paths, marked by different assumptions and nuances. I agree with the majority that the frst step of today's analysis is to identify the relevant confict by ascertaining the question that the Delaware law and the Federal Rule “attemp[t] to answer.” Shady Grove, 559 U. S., at 399; see ante, at 192. But I disagree that, in this case, “the disputed question is whether Berk's lawsuit may be dismissed because his complaint was not accompanied by an expert affdavit.” Ante, at 192–193. Here is why.
A
First and foremost, that formulation of the question assumes that Berk's lawsuit has already been properly accepted by the court clerk (i. e., fled and docketed) and thus that § 6853 is answering whether the “lawsuit may be dismissed” for want of the required affdavit. Ante, at 192.
But that assumption jumps the gun. Delaware's law states that “[n]o health-care negligence lawsuit shall be fled in this State unless the complaint is accompanied by: (1) [a]n affdavit of merit” or (2) “a motion to extend the time to fle said affdavit.” § 6853(a)(1) (emphasis added). The statute further specifes that “[i]f the required affdavit does not accompany the complaint or if a motion to extend the time to fle said affdavit . . . has not been fled with the court, then the . . . clerk of the court shall refuse to fle the complaint and it shall not be docketed with the court.” Ibid. (emphasis added).
Accordingly, the text of § 6853 makes clear that the statute's frst order of business is not to address the requirements for dismissal of an affdavit-free lawsuit such as Berk's. Instead, § 6853 most directly answers the threshold question of what is required to start a civil action for medical malpractice under Delaware law. Delaware's response is that the plaintiff must submit an affdavit of merit, or a motion for an extension of time to fle such affdavit, because (to repeat) the clerk cannot “fle the complaint” or “docke[t]” the case without that additional filing.
§ 6853(a)(1).
Rule 3 provides a completely different answer to the question of what is required to start a medical malpractice case. As explained above, under that Rule, “[a] civil action is commenced by fling a complaint with the court”—period. That answer leaves “no room” for Delaware's insistence that an affdavit of merit (or a motion for an extension) must be submitted in order to commence this kind of legal action. Bur lington, 480 U. S., at 5. Indeed, Rule 3 uses the uncompromising, declarative phrase “is commenced,” plainly indicating that Congress meant for the “fling [of] a complaint” to be both necessary and suffcient to begin any civil case (including a malpractice action) in federal court. Cf. Walker, 446 U. S., at 750, n. 10 (“ `Rule 3 simply provides that an action is commenced by fling the complaint and has as its primary purpose the measuring of time periods that begin running from the date of commencement' ” (quoting 4 C. Wright & A. Miller, Federal Practice and Procedure § 1057, p. 191 (1969))); Hallstrom v. Tillamook County, 493 U. S. 20, 26 (1989) (readPage Proof Pending Publication ing Rule 3 to mean that “fling a complaint with the court” is synonymous with “fl[ing] suit”).2 Resisting this conclusion, defendants rely on three cases to argue that “this Court has long found applicable in federal court” “state-law condition[s] on allowing suits to proceed.” Brief for Respondent Beebe 26–27, and n. 11; see also Brief for Respondent Choy 36–37. But that argument mischaracterizes our past rulings. See Cohen v. Benefcial Industrial Loan Corp., 337 U. S. 541 (1949); Woods v. Interstate Realty Co., 337 U. S. 535 (1949); Walker v. Armco Steel Corp., 446 U. S. 740.
The majority has already ably explained that defendants “substantially overread” Cohen for the proposition that “all state `preconditions to proceeding' are consistent with the Federal Rules.” Ante, at 195. I think Walker and Woods are also easily distinguishable.
2The majority contends that its dismissal-focused formulation of the question is the right one because, in this case, “Berk's lawsuit has already been fled and docketed,” and “we granted certiorari” on the question “whether Berk's lawsuit may be dismissed.” Ante, at 193, n. 1. That argument confates the question presented with the “question” that, under Shady Grove, is supposed to serve as the linchpin of our confict analysis. 559 U. S., at 401. The latter comes not from the facts of the case, but rather from our interpretations of the potentially conficting state law and Federal Rule. So, while it may be that the question presented is “whether Berk's lawsuit may be dismissed,” the relevant question for our Shady Grove analysis is “what is required to start a medical malpractice case,” because that is the question that § 6853 and, as it happens, Rule 3, “attemp[t] to answer,” Id., at 399.
Nor is the majority correct that, regardless, I am “ultimately forced to grapple with the question whether Berk's lawsuit may be `dismiss[ed].' ” Ante, at 193, n. 1 (alteration in original). As I stated above, § 6853's confict with Rule 3 is enough to hold that the law cannot apply in federal court. And, as I have shown, the Rule 3 analysis is about the commencement of the action, not its dismissal. Once we determine that § 6853 and Rule 3 answer the same commencement-related question, § 6853 cannot apply in federal court, meaning that Berk's lawsuit cannot be thwarted for failure to satisfy the requirements of that state law.
Page Proof Pending Publication As with Cohen, respondents overstate the effect of Walker. There, we held that a federal court sitting in diversity must apply an Oklahoma law that deemed an action “commenced” upon service of the summons for purposes of the State's statute of limitations. 446 U. S., at 742. Rule 3, we explained, “governs the date from which various timing requirements of the Federal Rules begin to run, but does not affect state statutes of limitations.” Id., at 751. Here, by contrast, § 6853 and Rule 3 serve precisely the same function, in the same context: Both establish when a malpractice lawsuit is deemed initiated (i. e., fled and docketed) for purposes of determining “the date from which various timing requirements of the Federal Rules begin to run.” Ibid. That is precisely the kind of confict that bars the application of state law in federal court under our precedents.3 Defendants' reliance on Woods is similarly unavailing.
There, we held that a federal court sitting in diversity must apply a Mississippi law prohibiting out-of-state corporations from bringing suit in the State unless they designated an agent for service of process. 337 U. S., at 536, n. 1. Mississippi's law was enacted to ensure that only those out-of-state corporations that consented to being sued in Mississippi had the privilege of accessing Mississippi's courts. See ibid. In other words, Mississippi's law had a broad substantive purpose and effect that went well beyond what the Federal Rules addressed. That was why we had to wade through “Erie's murky waters” to decide the conficts question that case presented. See Shady Grove, 559 U. S., at 398; see 3Proving this point, another provision of the Delaware law, § 6853(a)(4), provides that a defendant is “not required to take any action with respect to the complaint . . . until 20 days after plaintiff has fled the affdavit.” Under the Federal Rules, however, the deadline for a responsive act by the defendant is anchored to the fling of the complaint. See Fed. Rules Civ. Proc. 4(m) (a plaintiff must serve the complaint and summons “within 90 days after the complaint is fled”) and 12(a)(1)(A)(i) (a defendant “must serve an answer . . . within 21 days after being served with the summons and complaint”).
Page Proof Pending Publication also Erie R. Co. v. Tompkins, 304 U. S. 64 (1938). Delaware's affdavit requirement, by contrast, is plainly procedural in scope and well within the territory of the Federal Rules. In short, our precedents establish that when a state law and a valid Federal Rule confict—because they answer the same procedural question—the State's requirement is inapplicable in federal court. Because § 6853 answers the same question about what is required to commence a medical malpractice action as the unquestionably valid Rule 3, Dela- ware's law must give way. See Hanna, 380 U. S., at 469–474.
B
If we accept, for the sake of argument, that the question Delaware's law attempts to answer is “whether [a medical malpractice complaint] may be dismissed because [it] was not accompanied by an expert affdavit,” ante, at 192–193, a problem arises: The Federal Rule the majority points to does not answer that same question.
The majority discerns a confict with Rule 8 by reasoning as follows. With respect to the dismissal-related question quoted above, Delaware's law says that such a complaint must be dismissed because the affdavit of merit is missing, whereas Rule 8 prescribes all “the information a plaintiff must present about the merits of his claim at the outset of litigation”—namely, “ `a short and plain statement of the claim showing that [he] is entitled to relief.' ” Ante, at 193 (quoting Fed. Rule Civ. Proc. 8(a)(2)). “By requiring no more than a statement of the claim,” the majority continues, “Rule 8 establishes `implicitly, but with unmistakable clarity,' . . . that evidence of the claim is not required.” Ante, at 193. The problem with this reasoning is not that it is based on “a negative inference” about Rule 8, ante, at 195, n. 2, but rather the inattentively capacious scope of that negative inference. I agree that Rule 8(a)'s fnite list “preclude[s] courts” from imposing additional requirements. Ibid.; see Leatherman v. Tarrant County Narcotics Intelligence and Coordination Page Proof Pending Publication Unit, 507 U. S. 163, 168 (1993). But context is critical: Rule 8 does not purport to establish all that a medical malpractice plaintiff can be required to say about the merits of his claims at the beginning of the case, much less that nothing more than the fling of a complaint can be mandated. Rather, it (more narrowly) addresses what any “pleading” stating a claim for relief must contain. See Rule 8(a) (“A pleading that states a claim for relief must contain . . .” (emphasis added)). The majority neither holds nor suggests that §6853's affdavit of merit is, itself, a “pleading”; to the contrary, the majority admits that the affdavit is not. Ante, at 194. So, as I see it, Delaware's law—which prescribes what a plaintiff must submit in addition to his pleadings in order for his complaint to be fled and his case docketed—does not actually confict with Rule 8.
The majority acknowledges this “pleading” problem and offers, apparently as a solution, yet another formulation of the question that Rule 8 is answering: Rule 8, it says, tells us “what information a plaintiff must provide about the merits of his claim.” Ante, at 195. But, again, this description fails to acknowledge that Rule 8 operates entirely within a specifc universe: the pleadings. The majority's unbounded characterization subtly frees Rule 8 from the constraint of its subject matter, allowing it to expand until it collides with Delaware's law.4 4The majority's description of Rule 8's work is inaccurate even if confned to the universe of pleadings. Rule 8 does not “addres[s] what information a plaintiff must provide about the merits of his claim” in the pleadings. Ante, at 195 (emphasis added). Instead, with respect to a pleading that states a claim for relief (as opposed to one that does not, see Rule 7), Rule 8 governs how much a plaintiff must say concerning his claim. Rule 8's primary purpose is to install a “simplifed pleading system” that requires a complaint to include nothing more than “ `short and plain statement[s]' ” of the claim, the grounds for jurisdiction, and the relief sought. Swierkiewicz v. Sorema N. A., 534 U. S. 506, 512, 514 (2002). In other words, Rule 8's innovation is the “short and plain” bit, not the substance of what is required to be stated in that manner.
Page Proof Pending Publication When properly construed, Rule 8 addresses a particular (and particularly important) procedural question: “What must a pleading that states a claim for relief contain?” 5 In my view, this narrower formulation of the question—the one tethered to the required contents of a “pleading”—keeps Rule 8 within the bounds of its plain meaning, consistent with Gasperini's command not to overread the Federal Rules at the expense of “important state interests and regulatory policies” in cases such as this one. Gasperini v. Cen ter for Humanities, Inc., 518 U. S. 415, 427, n. 7 (1996); see also n. 1, supra.
Accordingly, because § 6853 does not address or govern the required contents of a pleading, Delaware's law does not answer the same question as Rule 8. Instead, the State's affdavit requirement (which, incidentally, neither describes the affdavit of merit in relation to a plaintiff 's obligation to state a claim for relief nor treats it as evidence) is an additional fling requirement for commencing any medical malpractice civil action under state law. See § 6853(a)(1) (“No healthcare negligence lawsuit shall be fled in [Delaware] unless the complaint is accompanied by . . . [a]n affdavit of merit”). Finding a Rule 8 confict, as the majority does, thus requires contorting both Rule 8 and § 6853. There is no need to do so here, especially given the ready confict with Rule 3 (discussed above) and Rule 12 (described below).6 5The answer, under the Rule, is (1) “a short and plain statement of the grounds for . . . jurisdiction,” (2) “a short and plain statement of the claim showing that the pleader is entitled to relief,” and (3) “a demand for the relief sought.” Rule 8(a).
6To be sure, many Courts of Appeals have also relied on a confict with Rule 8 to hold that state affdavit-of-merit laws like Delaware's do not apply in federal court. See Albright v. Christensen, 24 F. 4th 1039, 1048– 1049 (CA6 2022); Gallivan v. United States, 943 F. 3d 291, 293 (CA6 2019); Pledger v. Lynch, 5 F. 4th 511, 519 (CA4 2021); Young v. United States, 942 F. 3d 349, 351 (CA7 2019). But a closer look at the relevant cases reveals that these courts, like the majority, got there by manipulating the Page Proof Pending Publication
III
The majority and I appear to agree that Delaware's affdavit requirement—as interpreted by the Delaware Supreme Court—conficts with Rule 12(d). See ante, at 193.
That confict suffces to displace § 6853's requirement. But to the extent that the majority ties its Rule 12 analysis to the purported Rule 8 confict, see ibid., I think the majority has gone further than it needs to, introducing incoherence in the process.
While the statute itself does not so specify, the Delaware Supreme Court has interpreted § 6853 to mean that (assuming the clerk of court accepts an affdavit-free medical malpractice action and dockets the case notwithstanding the statute's prohibition) “[t]he fling of a healthcare negligence action without the requisite affdavit of merit constitutes grounds of dismissal of medical negligence claims as a matter of law.” Hall v. Sorouri, 996 A. 2d 793 (Del. 2010) (Table). That reading of Delaware law answers the question, “What can a court consider when dismissing a medical malpractice case?” Because “[t]he highest state court is the fnal authority on state law,” Fidelity Union Trust Co. v. Field, 311 U. S. 169, 177 (1940), we must read this statute as that court does, i. e., as establishing dismissal requirements for medical malpractice cases. Thus, if a valid Federal Rule answers that same dismissal question, § 6853 cannot govern in federal court. See Shady Grove, 559 U. S., at 398.
relevant question. In Gallivan, for example, the Sixth Circuit posed as the pertinent question: “[D]oes someone need an affdavit of merit to state a claim for medical negligence?” 943 F. 3d, at 293. This leading question not only ignores (as the majority does) that Rule 8 is solely about “pleadings,” but, worse, also preordains a confict by naming the very thing that the state law requires but the Federal Rule does not (i. e., an affdavit). The Fourth and Seventh Circuits make the same error. See Pledger, 5 F. 4th, at 519 (asking “whether a medical malpractice plaintiff must provide pre-suit expert support for his claim”); Young, 942 F. 3d, at 351 (asking whether Rule 8 “require[s] attachments”).
Page Proof Pending Publication Enter Rule 12. As the majority correctly explains, Rule 12(b)(6) provides the “only . . . ground for dismissal based on the merits,” and Rule 12(d) prohibits courts from considering “ `matters outside the pleadings' ” in deciding motions to dismiss brought under Rule 12(b)(6). Ante, at 193; see also Stanley v. City of Sanford, 606 U. S. 46, 49 (2025) (explaining that, under Rule 12(d), a judge considering a motion to dismiss must “take as true the well-pleaded facts in the plaintiff 's complaint, . . . and [must] not consider evidence beyond that pleading”). Furthermore, by all accounts, the affdavit required by Delaware's law is a “matte[r] outside the pleadings.” Rule 12(d); see ante, at 194 (“It is true . . . that an affidavit of merit is a document separate from the `pleading' ”).
We know the affdavit of merit qualifes as a “matter outside the pleadings” for several reasons. For one thing, Delaware law provides that the affdavit is to “accompany” the complaint. § 6853(a)(1). A thing cannot “accompany” that of which it is already a part.7 Indeed, Delaware's Legislature apparently felt so strongly about the affdavit not being part of the complaint that it mandated physical separation: By statute, the affdavit must be fled in its own sealed envelope, alongside (but apart from) the complaint. See ibid. Nor do the Federal Rules suggest that affdavits of the nature specifed by the Delaware law can qualify as “pleadings.” Rule 7, which sets forth a list of “[p]leadings [a]llowed” in federal court, does not include an “affdavit” (or anything like it) on its list.
Pulling these threads together, Delaware's law, as interpreted by the Delaware Supreme Court, requires judges to 7To be sure, plaintiffs can, in limited contexts, “incorporat[e]” certain documents “into the complaint by reference.” Tellabs, Inc. v. Makor Is sues & Rights, Ltd., 551 U. S. 308, 322 (2007). But the affdavit of merit is not such a document because its content is neither discoverable nor admissible. See § 6853(d).
Page Proof Pending Publication Page Proof Pending Publication account for a matter outside the pleadings—i. e., the existence or suffciency of an affdavit of merit (or lack thereof)— when deciding whether to dismiss a medical malpractice case. Rule 12(d), however, forbids such extraneous considerations. And Rule 12 is valid under the Rules Enabling Act (for similar reasons as the Court lays out in its opinion, see ante, at 198–199). So Delaware's law cannot be enforced in federal court.
As far as I can tell, then, there is no daylight between my view of the Rule 12 confict and the majority's assessment. But far from “reinforc[ing]” a confict with Rule 8, ante, at 193, our shared evaluation of Rule 12 is, to me, yet another reason why Delaware's law is not in confict with Rule 8.
See Part II–B, supra. The conficts analysis concerning these two Rules is based on diametrically opposed characterizations of Delaware's affdavit of merit: The affdavit must be either a “matter outside the pleadings” (and thus § 6853 conficts with Rule 12, per the Delaware Supreme Court's decision in Hall) or it is part of what the pleadings must contain (giving rise to the purported confict with Rule 8). A coherent conficts analysis cannot have it both ways.
* * * Read for what they are—by their “plain meaning,” sensitive to context yet without distortions created by the task of checking for a confict, see Walker, 446 U. S., at 750, n. 9; Gasperini, 518 U. S., at 427, n. 7—Rules 3 and 12 answer the same questions as Delaware's affdavit requirement. That means that the majority and I share the same conclusion: There is a confict between state law and valid Federal Rules, precluding application of Delaware's affdavit requirement in federal court. For the reasons discussed above, I think the majority is wrong to further maintain that § 6853 answers the same question as Rule 8. And I cannot see how the affdavit that the Delaware law requires can be a “matter outside the pleadings” (for Rule 12-confict purposes) and also part of the “pleadings” (for Rule 8-confict purposes). Therefore, I concur only in the judgment.
Page Proof Pending Publication Page Proof Pending Publication Reporter’s Note The attached opinion has been revised to refect the usual publication and citation style of the United States Reports. The revised pagination makes available the offcial United States Reports citation in advance of publication. The syllabus has been prepared by the Reporter of Decisions for the convenience of the reader and constitutes no part of the opinion of the Court. A list of counsel who argued or fled briefs in this case, and who were members of the bar of this Court at the time this case was argued, has been inserted following the syllabus. Other revisions may include adjustments to formatting, captions, citation form, and any errant punctuation. The following additional edits were made: p. 188, line 7: The sentence “This Court has consistently rejected efforts by lower federal courts to require more information than Rule 8 requires.” is replaced with “Lower federal courts have sometimes tried to demand more information than Rule 8 requires for certain kinds of claims. This Court has consistently rejected such efforts.” p. 200, Part I, line 3: “his” is changed to “hers” p. 204, line 4: “from 1949, arguing” is changed to “to argue” p. 206, line 1: “also” is inserted before “Erie R. Co.”