This capital case requires us to determine whether we must stay Petitioner’s execution, currently scheduled for May 12, 2015, to give effect to his right “meaningfully to research and present [his] habeas claims.” Because the district court did not abuse its discretion in determining that a mental health evaluation is not reasonably necessary to develop Petitioner’s underlying habeas claim, we AFFIRM.
I.
Petitioner, Derrick Dewayne Charles, was convicted and sentenced to death by a jury in 2003 after pleading guilty to murdering his fifteen-year-old girlfriend, her mother, and her grandfather during the same criminal transaction.1 On direct appeal, Charles challenged his sentence on the ground that the trial court improperly admitted victim character evidence, including evidence of his girlfriend’s hobbies and her grandfather’s history of military service. Charles v. State, AP-74694, 2005 WL 283598, at *1 (Tex.Crim.App. Feb. 2, 2005). He also challenged Texas’s capital-murder scheme as unconstitutional and his guilty plea as statutorily barred because of infirmities in the procedure used at the sentencing hearing. Id. at *2. The Texas Court of Criminal Appeals affirmed the conviction and sentence. Id. at *4. It held that the state trial court did not abuse its discretion in allowing the character evidence because the evidence provided only a brief glimpse into the victims’ backgrounds. Id. at *2. The Texas Court of Criminal Appeals further held that Charles’s constitutional challenge to Texas’s capital-murder scheme was foreclosed by precedent, and that he was estopped from challenging the sentencing hearing procedure because, as evidenced by the trial transcript, the trial court followed the *216procedure that Charles had requested. Id. at *2-4.
While his direct appeal was pending, Charles filed a state habeas application. The state habeas court held an evidentiary hearing and entered findings of fact and conclusions of law recommending that relief be denied. Ex parie Charles, WR-67171-01, 2008 WL 556015, at *1 (Tex. Crim.App. Feb. 27, 2008). The Texas Court of Criminal Appeals summarily affirmed. Id.
In February 2009, Charles filed a federal petition in the Southern District of Texas alleging ineffective assistance of trial counsel on the basis of his counsel’s purported failure to present mitigating evidence. Applying the deference required by the Antiterrorism and Effective Death Penalty Act, the district court denied relief.2 Charles v. Thaler, 4:09-CV-0592, 2011 WL 5040438, at *1 (S.D.Tex. Oct. 24, 2011), aff'd sub nom, Charles v. Stephens, 736 F.3d 380 (5th Cir.2013).
On appeal, we affirmed the district court’s judgment. Charles v. Stephens, 736 F.3d 380, 396 (5th Cir.2013). Examining applicable Supreme Court precedent, we explained that “the state habeas court did not unreasonably apply Strickland’s deficiency prong by concluding that Charles’s trial counsel performed an adequate mitigation investigation.” Id. at 390. Charles petitioned for rehearing en banc, which we denied, see Order on Pet. Reh’g En Banc, No. 12-70016, Doc. 74 (5th Cir. Jan. 8, 2014), and the Supreme Court denied Charles’s petition for a writ of certio-rari. Charles v. Stephens, — U.S.-, 135 S.Ct. 52,190 L.Ed.2d 55 (2014).
On November 10, 2014, the state trial court filed its execution order setting the execution date for May 12, 2015. On March 13, 2015, Charles filed motions to withdraw or modify the execution order, for appointment of counsel, and for funding for a mental health evaluation. After a hearing, the state court denied the motions on April 9, 2015. Charles did not appeal these denials to the Texas Court of Criminal Appeals, but instead filed three motions in the federal district court on April 17, 2015, twenty-five days before his scheduled execution.
Charles did not file a habeas application in the district court. He filed motions for authorization to hire a mental health expert, for appointment of counsel,3 and for *217stay of execution, all pursuant to 18 U.S.C. § 3599. In filing these motions, Charles sought time to “investigate, prepare, and present a claim that he is incompetent to be executed under Ford v. Waimoright, 477 U.S. 399 [106 S.Ct. 2595, 91 L.Ed.2d 335] (1986), and Panetti v. Quarterman, 551 U.S. 930 [127 S.Ct. 2842, 168 L.Ed.2d 662] (2007).”
Charles argued that he can “make a colorable showing” under Ford v. Wainwright, 477 U.S. 399, 106 S.Ct. 2595, 91 L.Ed.2d 335 (1986), “that he is not competent to be executed.” Citing several volumes of supporting evidence, Charles contended that he has “suffered from severe and debilitating mental illness since childhood” as a result of genetic inheritance and the environment in which, he was raised. He offered thirty-one exhibits, including the Gulf Pines Hospital records and several affidavits from doctors, attorneys, and family members. Of these exhibits, only two referred to information gathered after 2009.4 In addition, he argued that 28 U.S.C. § 2244(B) would not bar his Ford claim as a successive petition because, according to Panetti, 551 U.S. at 943, 127 S.Ct. 2842, his Ford claim was not ripe until the state trial court set his execution date.
The state filed its response to Charles’s motions on April 24, 2015. In its response, the state argued that Charles’s motions did not present a cognizable claim under § 2254, and to the extent they did, such a claim was unexhausted and procedurally barred. The state further argued that any underlying incompetency claim was ultimately meritless. Charles filed his reply on April 29, 2015, clarifying that he was not yet filing a habeas claim, only motions seeking authorization to develop his habe-as claim, pursuant to his rights under 18 U.S.C. § 3599.
On May 4, 2015, the district court filed its Memorandum and Order denying Charles’s motions for authorization to hire a mental health expert and for a stay of the execution. Acting sua sponte, the district court also denied a certificate of ap-pealability (COA).5 The district court carefully identified what was not at issue. Specifically, the district court explained that Charles’s motions did “not directly seek federal habeas relief from his conviction and sentence”; did “not challenge the reasonableness of any state-court determination about his competency to be executed”; 6 did “not seek relief from constitu*218tional infirmities in the state process for obtaining experts or investigators to evaluate possible psychological claims”; and did “not seek leave to litigate a successive federal habeas petition based on Ford.”
What Charles did request was a stay of his execution so that he could exercise his right under 18 U.S.C. § 3599 “to meaningfully research and present [his] habeas claims.” See McFarland v. Scott, 512 U.S. 849, 858, 114 S.Ct. 2568, 129 L.Ed.2d 666 (1994). As the district court noted, “Charleses] theory is novel and limited.” His Ford claim was never actually before the state or district court. “Charles [ ]made it clear that he seeks rulings only on whether he is entitled to a stay of execution and the allocation of funds to develop a Ford claim.” In Charles’s words, the issue presented is “whether Mr. Charles’ Ford showing is sufficient to trigger counsel and resources under § 3599, and a stay of execution to permit him an opportunity to present his Ford claim.” He admitted that the merits of his Ford claim are relevant only to the extent that “the existing record is so devoid of evidence that it does not even constitute a colorable Ford claim.”
In denying Charles’s motion for stay, the district court applied the factors we typically consider for granting a stay,7 and it determined that Charles failed to show a substantial likelihood of • success on the merits of his underlying Ford claim. The district court also indicated that even if it applied the “colorable showing” standard, the stay would not be warranted. As for the § 3599(f) motion for authorization to hire a mental health expert, the district court determined that “[n]othing in the record before the Court has hinted that additional exploration of the Ford inquiry would result in a viable, or even colorable, Ford claim.”
II.
We have jurisdiction to consider Charles’s motions. A district court’s denial of a motion under 18 U.S.C. § 3599(f)8 is an appealable order, and “such an order ... is not subject to the COA requirement.” Harbison v. Bell, 556 U.S. 180, 183, 129 S.Ct. 1481, 173 L.Ed.2d 347 (2009); Smith v. Dretlce, 422 F.3d 269, 288 (5th Cir.2005) (“[A] COA is not necessary to appeal the denial of funds for expert assistance.”). The Supreme Court has held that the § 3599 right to expert assistance, like the right to counsel, “adheres prior to the filing of a formal, legally sufficient habeas corpus petition,” so the § 3599 motion itself commences a habeas proceeding and triggers federal jurisdiction. McFarland, 512 U.S. at 855-58, 114 S.Ct. 2568 (addressing initial habeas filings); In re Hearn, 376 F.3d 447, 452 (5th *219Cir.2004) (extending McFarland to § 3599 motions related to successive habeas claims). Likewise, “once a capital defendant, invokes his [§ 3599 right], a federal court also has jurisdiction under § 2251 to enter a stay of execution” to make the defendant’s § 3599 right effective. McFarland, 512 U.S. at 858, 114 S.Ct. 2568.9
We review a district court’s denial of a § 3599(f) motion for abuse of discretion. Brown v. Stephens, 762 F.3d 454, 459 (5th Cir.2014); see also 18 U.S.C. § 3599(f) (stating that a district court “may authorize the defendant’s attorneys to obtain such services”) (emphasis added). Likewise, “the decision to grant [or deny] a stay ... is generally left to the sound discretion of district courts.” Ryan v. Gonzales,—U.S.—- 133 S.Ct. 696, 708, 184 L.Ed.2d 528 (2013).
Charles’ motion for authorization to hire a mental health expert hinges on whether he can show that the district court abused its discretion by determining that a mental health expert is not “reasonably necessary” to develop his Ford claim. See 18 U.S.C. § 3599(f) (permitting authorization of expert assistance “upon a finding that ... [it is] reasonably necessary for the representation of the defendant”). We have “interpreted ‘reasonably necessary’ [in § 3599(f) ] to mean that a prisoner must show that he has ‘a substantial need’ for the requested assistance.” Brown v. Stephens, 762 F.3d 454, 459 (5th Cir.2014) (quoting Riley v. Dretke, 362 F.3d 302, 307 (5th Cir.2004)). Under this standard, we have upheld district court decisions denying assistance where such assistance will be in support of a claim that is procedurally barred10 or “meritless,” or where the assistance would generate only supplementary evidence. Brown, 762 F.3d at 459.
Charles argues that to satisfy the “reasonably necessary” standard he need only show that he has a “colorable” Ford claim. He borrows this standard from Hearn,11 in which we reviewed whether a prisoner seeking to investigate a potential Atkins claim had made a showing sufficient for purposes of a § 3599 motion. 376 F.3d at 455 (holding that a § 3599 motion “need only be supported by a colorable showing of mental retardation”) (emphasis added), decision clarified on denial of reh’g, 389 F.3d 122 (5th Cir.2004). However, Charles admits in his brief that on rehearing we limited Hearn, including its color-able showing standard, to only those cases in which a prisoner has a potential claim based specifically on Atkins. 389 F.3d at 123. Nevertheless, he argues that we should now adopt the colorable showing standard in the context of Ford claims.
Normally, Ford claims raised in a habe-as application require a “substantial” *220showing in order to gain additional process for presenting evidence of incompetency. Panetti, 551 U.S. at 949, 127 S.Ct. 2842. Texas offers a similar procedural mechanism under which a prisoner must make a threshold showing that raises a “substantial doubt” regarding his competency to be executed. Tex.Code Crim. P. art. 46.05(d); see also Druery v. State, 412 S.W.3d 523, 540 (Tex.Crim.App.2013) (holding that state law initially requires a substantial showing to trigger more process). If a prisoner satisfies this requirement, the trial court must hold a hearing to determine a petitioner’s competency by a preponderance of the evidence. Id. art. 46.05(k).
Charles never filed an art. 46.05 motion in state court, or a habeas application in federal court, as to his Ford claim. Instead, Charles filed motions for appointment of counsel and for expert and investigative assistance in both courts, and both courts denied the motions.12 It is unnecessary for us to decide the issue of whether the proper test is a “colorable showing” or a “substantial showing” — neither is satisfied here. The district court determined that Charles had not made a colorable or substantial showing, and the district court’s decision was not an abuse of discretion.
“The Supreme Court has declined to set forth a specific standard for determining” ineompetency. Eldridge v. Stephens, 599 Fed.Appx. 123, 128 (5th Cir.2015). Generally, a prisoner is incompetent if he does not (1) “know the fact of [his] impending execution,” Ford, 477 U.S. at 422, 106 S.Ct. 2595 (Powell, J., concurring in part and concurring in the judgment),13 or does not (2) “ha[ve] a rational understanding of the reason for the execution,” Panetti, 551 U.S. at 958, 127 S.Ct. 2842. “Rational understanding” requires more than mere awareness of the facts about the execution. Panetti, 551 U.S. at 958, 127 S.Ct. 2842. The prisoner must understand, the reasons for his execution. Id. However, federal law distinguishes between an inmate who is mentally ill and one who is insane. Shislnday v. Quarterman, 511 F.3d 514, 521 (5th Cir.2007). If a prisoner “has a rational understanding of his crime, his impending death, and the causal relationship between the two,” then he is competent. Eldridge, 599 Fed.Appx. at 128.
As the district court carefully documented, Charles essentially relies on the very same evidence he marshalled in his initial state and federal habeas proceedings. The evidence includes his childhood hospital records; psychiatric records from the Texas Youth Commission; evidence of his turbulent home life; prior arrests and incarcerations; evidence of drug use; expert evaluations;14 and the trial transcript (in which Charles affirmed his understanding of the charges and denied having any delusions). Based on these records, he argues that he “has an. extensive and well-documented history of mental illness, including episodes of psychosis, and significant mental impairment that profoundly affects his *221functioning, even to the present day.” However, even assuming he has some form of mental illness, none of this evidence shows that he does not know about his execution or that he does not rationally understand the reason for it. See Martinez, 2009 WL 211489, at *1 (“[C]ounsel ... was able to develop an extensive history of mental' health that still falls short of any threshold showing of incompetency to be executed.”).
Moreover, the district court found that in 2005 and 2006, two experts evaluated Charles and determined that he was not mentally retarded or incompetent. They reported that he did not experience “active psychosis,” his hallucinations occurred over a short period of time, his negative behavior and low cognitive scores were consistent with his ADHD, he denied experiencing “symptoms that are compatible with a psychotic disorder,” and he was “lucid in his conversation and thought processes.” While there was evidence of some depression, low intelligence, and possible brain injury, the two experts “did not diagnose him with any mental illness.” “Nothing in the record ... indicates that Charles suffers from symptoms of ‘insanity’ as described by Ford.” There was no evidence of “pervasive and severely debilitating conditions ... that traditionally give rise to Ford claims.”
Much of the new evidence that Charles submitted with his § 3599 motion includes recent affidavits, mostly from his attorneys, but also one from a physician.15 The attorneys’ affidavits claim that Charles’s “affect was always very flat”; “he was completely unengaged with the legal discussion relating to his case”; he “expressed little understanding of the proceedings”; he exhibited “strange physical tics”; he was “uncomfortable making conversation”; he was “emotionless”; and he “struggled to understand the simplest of topics.” The physician stated in his affidavit that Charles “could be incompetent to be executed,” but this physician never met with Charles and only partially reviewed the record. None of these affiants asserted that Charles was unaware he was being executed or that he did not rationally understand why he was being executed. The state court aptly noted that “nothing like that exists [ ]because ... it would be unethical for [the affiants] to give such an affidavit because it [probably] wouldn’t be true.” “Most of this has been investigated and investigated and investigated for years,” and yet none of the affiants have been able to state that Charles did not know he was being executed or that he did not understand why.16
As the district court explained, the record “simply does not contain any evidence *222in support [of] Charleses] allegations.” While a “colorable,” or even a “substantial,” showing may require little, it at least requires some competent evidence showing that a prisoner fails one or both of the elements for competency. Challenges lacking such evidence are precisely the kind of “[l]ast-minute filings that are frivolous[,] designed to delay executions[, and] can be dismissed in the regular course.” Panetti, 551 U.S. at 946, 127 S.Ct. 2842. Based on our review of the record in this case, the district court did not abuse its discretion in denying Charles’s § 8599(f) motion. “Charles’s long term mental health history, and the more recent observations by two affiants, does not justify a competency evaluation.”
Likewise, the district court did not abuse its discretion in denying his motion for stay of execution. It is not clear whether Charles’ motion for stay is governed by McFarland or by the Nken factors. Under McFarland, if a prisoner succeeds on his § 3599 motion but has insufficient time to meaningfully exercise that right because of an impending execu-' tion, the Supreme Court has instructed federal courts to grant,a stay. See 512 U.S. at 858, 114 S.Ct. 2568 (“[Approving the execution of a defendant before his [petition] is decided on the merits would clearly be improper.”) (internal quotation marks omitted). Of course, “if a dilatory capital defendant inexcusably ignores this opportunity and flouts the available processes, a federal court presumably would not abuse its discretion in denying a stay of execution.” McFarland v. Scott, 512 U.S. 849, 860-61, 114 S.Ct. 2568, 129 L.Ed.2d 666 (1994). Under the Nken factors, the movant must make a strong showing that he is likely to succeed on the merits of his underlying claim — in this case, Charles’s Ford claim.
As already explained, Charles cannot make the requisite showing under his § 3599 motion. Neither can he make a strong showing that he is likely to succeed on his Ford claim. See Nken, 556 U.S. at 425-26, 129 S.Ct. 1749. Therefore, regardless of which standard we apply, Charles’s is not entitled to relief on his motion for stay.
III.
Accordingly, we AFFIRM.