OPINION OF THE COURT
GREENBERG, Circuit Judge:I. INTRODUCTION
Patrick Hartey, the petitioner in this habeas corpus proceeding under 28 U.S.C. § 2254, appeals from an order of the district court entered on November 14, 1997, adopting the report and recommendation of a magistrate judge dated April 1, 1997, and denying Hartey’s petition without an evidentiary hearing. Inasmuch as both the magistrate judge and the district court wrote comprehensive opinions, see Hartey v. Vaughn, 1997 WL 710946 (E.D.Pa. Nov. 14, 1997) (district court opinion), the Superior Court of Pennsylvania wrote a published opinion on Hartey’s direct appeal, see Commonwealth v. Hartey, 424 Pa.Super. 29, 621 A.2d 1023 (1993), and we recently wrote a published opinion in a habeas corpus case brought by Thomas McCandless, a codefendant, see McCandless v. Vaughn, 172 F.3d 255 (3d Cir.1999), we only need summarize the background of the case.
In August 1982, the Commonwealth of Pennsylvania tried Hartey and McCandless together in the Philadelphia Common Pleas Court for crimes arising from the murder of Theodore Stebelski. Originally, the police arrested John Barth for the murder, but in part as a result of information Barth supplied, the Commonwealth refocused the investigation on Hartey and McCandless. The prosecution’s theory at the trial was that McCandless and Hartey killed Stebelski so that he could not testify against McCandless at a criminal trial. Although the prosecution intended to call Barth as a witness to testify about the killing, and expected that his testimony would directly link Hartey to the murder scene, Barth did not appear at trial. The trial judge ruled, however, that Barth’s preliminary hearing testimony could be read into the record as a substitute for his live testimony and the court permitted its use against both defendants. The jury convicted both Hartey and McCandless of first degree murder, criminal conspiracy, and possession of an instrument of crime.
Unfortunately, Hartey’s original attorneys (not counsel on this appeal) did not prosecute his appeal appropriately and thus there were substantial delays in his direct appellate process. When the appeal finally was prosecuted, Hartey presented six issues to the Superior Court, all framed as ineffective assistance of counsel claims. The Superior Court denied all six claims on the merits in its published opinion. See Commonwealth v. Hartey, 424 Pa.Super. 29, 621 A.2d 1023. Hartey thereafter unsuccessfully sought allocatur from the Supreme Court of Pennsylvania, Commonwealth v. Hartey, 540 Pa. 611, 656 A.2d 117 (1993), advancing only four of the claims.
*370Then on October 4, 1996, Hartey filed the proceedings in the district court leading to this appeal, raising the following four issues which he also had presented to the Pennsylvania Superior and Supreme Courts:
1. Trial counsel was ineffective for failing to object to the Court’s accomplice instruction which permitted the conviction of an accomplice based on his joining the actor in ‘an illegal act’ and failed to focus the attention of the jury on whether or not the accomplice shared or harbored the specific intent to kill that had to be found as to the actor.
2. Trial counsel was ineffective for failing to preserve his objections to the improper bolstering of the Barth preliminary hearing testimony in his written post trial motions, and was also ineffective for failing to object to the improper bolstering of the Barth testimony by the prosecutor in her opening address.
3. Trial counsel was ineffective for failing to object to the Court’s exclusion of defense witnesses who would have testified to the poor reputation for truth and veracity on the part of the most critical Commonwealth witness, John Barth.
4. Trial counsel was ineffective for failing to properly preserve his objection to the Court’s refusal to answer the first jury inquiry in the affirmative.
On this appeal Hartey raises only the first two issues noted above but expands on them as he presents them as both ineffective assistance of counsel claims and due process claims. Hartey’s refocusing of the claims is understandable as our order granting the certificate of appeala-bility recites as follows:
The foregoing request for a certificate of appealability is granted for the purpose of deciding whether Hartey’s right to due process was denied by: (1) the prosecutor’s opening statement and the testimony of Assistant District Attorney Murray which may have led the jury to believe that the prosecution had independent evidence corroborating witness Barth’s testimony that was not presented to the jury. See United States v. Molina-Guevara, 96 F.3d 698, 704-705 (3d Cir.1996); United States v. DiLoreto, 888 F.2d 996, 999 (3d Cir.1989), overruled in part, on other grounds, by United States v. Zehrbach, 47 F.3d 1252 (3d Cir.1995) (en banc); and (2) the court’s instruction regarding the definition of an accomplice. See Smith v. Horn, 120 F.3d 400, 411-15 (3d Cir.1997); Rock v. Zimmerman, 959 F.2d 1237, 1246 (3d Cir.1992).
Nevertheless, we must consider our order granting the certificate of appealability in the context of this case, which established that Hartey pursued a writ of habeas corpus in the district court solely on ineffective assistance of counsel grounds, and the court denied the writ concluding that Har-tey was not entitled to relief on that theory. Thus, we are constrained to assess Hartey’s claims under the Sixth Amendment and not under the Due Process Clause. See Smith v. Farley, 25 F.3d 1363, 1365 n. 2 (7th Cir.1994) (claims not raised before district court in habeas petition are waived on appeal).
II. DISCUSSION
At the outset of our discussion of the merits of Hartey’s claims we refer to our recent opinion in McCandless v. Vaughn, 172 F.3d 255. In that habeas corpus proceeding under 28 U.S.C. § 2254, we granted a writ to McCandless, Hartey’s co-defendant, on the ground “that the prosecution did not fulfill its duty to protect McCandless’s constitutional right to confront the key witness[John Barth] against him.” See McCandless, 172 F.3d at 258. Hartey, however, did not raise this Confrontation Clause claim before the Pennsylvania state courts or in the district court. In fact, Hartey did not advance this point even in his primary briefs to this court; instead, he challenged his incarceration based on this confrontation violation only after we requested the parties to submit letter briefs detailing what effect, if *371any, McCandless had on this appeal. Thus, Hartey cannot raise the confrontation issue at this late date, and McCandless cannot control our result on this appeal. See Baker v. Barbo, 177 F.3d 149, 156 n. 7 (3d Cir.1999).
Hartey seeks to avoid this rather obvious result by relying on Finney v. Zant, 709 F.2d 643, 646-47 (11th Cir.1983), for the proposition that “justice” requires that we permit him to obtain relief on the basis of McCandless. Certainly, we can understand how it might appear to be incongruous that only one of two petitioners can obtain relief on the basis of a constitutional error apparently applicable to both. Nevertheless, habeas corpus law involves the application of well-established principles that, among other things, recognize the comity between the federal and state courts and usually require that a petitioner under 28 U.S.C. § 2254 present his claims for relief in the first instance in the state courts. We are not at liberty to disregard these principles because our concept of what “justice” might require, and thus Hartey’s failure to raise the Confrontation Clause issue until after argument on this appeal bars him from relying on it now. Therefore, we confine our discussion to the ineffective assistance of counsel claims that are actually before us.1
The thrust of Hartey’s claim relating to the prosecutor’s opening statement and Murray’s testimony is that they invited the jury to believe that independent evidence not presented to it corroborated Barth’s preliminary hearing testimony. Hartey’s challenge to the jury instructions centers on their alleged failure to inform the jury that it could convict him of first degree murder only if it determined that he, rather than merely McCandless, had a specific intent to kill.
Inasmuch as Hartey (unlike McCandless) filed his habeas corpus petition after the effective date of the Antiterrorism and Effective Death Penalty Act of 1996 (“AEDPA”) we must apply the standard of review as required by that Act in 28 U.S.C. § 2254(d). The AEDPA greatly circumscribes our review of state court decisions. Thus, we recently indicated that the AEDPA
mandates a two-part inquiry; first, the federal court must inquire whether the state court decision was ‘contrary to’ clearly established federal law, as determined by the Supreme Court of the United States; second, if it was not, the federal court must evaluate whether the state court judgment rests upon an objectively unreasonable application of clearly established Supreme Court jurisprudence.
Matteo v. Superintendent, 171 F.3d 877, 880 (3d Cir.1999) (en banc). Accordingly, as we explained in Matteo, section 2254(d) “firmly establishes the state court decision as the starting point in habeas review.” Id. at 885. Of course, on this appeal we exercise plenary review over the order of the district court as that court did not conduct an evidentiary hearing and nothing in the AEDPA or Matteo requires that we do otherwise. See Cabrera v. Barbo, 175 F.3d 307, 312 (3d Cir.1999); Zettlemoyer v. Fulcomer, 923 F.2d 284, 291 n. 5 (3d Cir.1991).
A. Improper Vouching
Hartey’s first claim is that his trial counsel was ineffective for failing to object to improper vouching for Barth’s preliminary hearing testimony, which he alleges illegally bolstered the statements of the key witness against him. Specifically, Hartey complains that there was improper vouching when the prosecutor stated in her opening argument that the govern*372ment’s agreement with Barth was based on “verifying” his statements, and when Assistant District Attorney Murray testified that the agreement was conditioned on “corroborating” Barth’s information. Har-tey is correct that federal law establishes that reference to extra-record evidence by the government constitutes improper bolstering that, in certain circumstances, may justify a reversal of a defendant’s conviction. See United States v. DiLoreto, 888 F.2d 996, 998-99 (3d Cir.1989) (overruled in part, on other grounds, by United States v. Zehrbach, 47 F.3d 1252, 1267 (3d Cir.1995) (en banc)). Thus, an attorney’s failure to object to the government’s improper vouching for its witnesses could give rise to an ineffective assistance of counsel claim.
The Pennsylvania Superior Court, however, rejected Hartey’s ineffective assistance claim because it concluded that there had not been improper vouching. See Hartey, 621 A.2d at 1026-27. The court noted first that, under state law, it was the government’s obligation to provide full disclosure about any favorable agreements reached with its witnesses. See id. at 1026. Then, it stated that the prosecutor’s statements and Murray’s testimony had not referred to.extra-record evidence as the government presented corroborating evidence during the trial. See id. at 1027. Specifically, the court pointed to the testimony of the medical examiner, who confirmed Barth’s account of how the murder had occurred, as well as other evidence establishing the ill will between McCandless and Stebelski. See id.
Applying the Matteo standard, we cannot conclude that the Superior Court’s application of law was contrary to established Supreme Court precedent or was an unreasonable application of clearly established Supreme Court jurisprudence. A defendant’s ineffective assistance of counsel claim can succeed only if he can show that counsel’s conduct was professionally unreasonable. See Strickland v. Washington, 466 U.S. 668, 687, 104 S.Ct. 2052, 2064, 80 L.Ed.2d 674 (1984). Thus, if there is no merit to Hartey’s claims that the prosecution’s statements and Murray’s testimony should not have been permitted at trial, his counsel cannot be deemed ineffective for not having objected to their presentation, as it was not unreasonable for him to acquiesce in the presentation of proper statements and testimony.
We have indicated that a determination of whether there has been improper vouching and, if so, whether there should be a reversal must be determined on a case by case basis. See United States v. Zehrbach, 47 F.3d at 1264-67. Moreover, much as did the Superior Court, we have looked to the extensive trial record to determine whether an allegedly improper statement in fact referred to extra-record evidence. See DiLoreto, 888 F.2d at 999.
The Superior Court’s conclusion that the government presented corroborating evidence at Hartey’s trial was not contrary to clearly established federal law as determined by the Supreme Court and was not an unreasonable application of Supreme Court jurisprudence. See 28 U.S.C. § 2254(d)(2) (stating that district court can grant writ for petitioner in state custody if state court decision was based on unreasonable determination of the facts). For instance, the prosecution presented the medical examiner’s opinion that the victim, Stebelski, first had been hit with a blunt object on the head and then shot in the back. This testimony confirmed Barth’s statements that McCandless had pistol-whipped Stebelski and then shot him as he was running away. Other evidence that generally supported Barth’s statements included testimony that McCandless had a motive to kill Stebelski, who had brought various criminal complaints against McCandless and shot at McCandless’ wife’s car, that Stebelski came to McCandless’ garage that day at McCandless’ urging, and that the garage, which McCandless owned, contained the type of bullets used in the shooting.
*373Hartey argues, however, that this corroborating evidence did not verify his involvement in the murder but only that of McCandless. We find this argument unconvincing. The prosecutor and Murray spoke merely of corroborating Barth’s account of how the murder occurred; the evidence presented at trial supplied this corroboration. The jury was thus free to infer, although it need not have done so, that given the accuracy of Barth’s account in many respects, his statements implicating Hartey were also accurate.
Therefore, we conclude that the Superi- or Court did not act unreasonably in finding that there had not been improper vouching because the prosecution had in fact presented evidence corroborating Barth’s statements at trial. Like the Superior Court, we have reversed convictions based on improper vouching only where we have concluded that evidence in the record could not have supported the prosecution’s statements. See DiLoreto, 888 F.2d at 999 (reversing on direct appeal where prosecutor in closing argument stated that its witnesses were not liars because government does not “take liars” but there was no evidence presented at trial of how government ascertains the honesty of its witnesses); United States v. Molina-Guevara, 96 F.3d 698, 704-05 (3d Cir.1996) (reversing on direct appeal where prosecutor stated in closing that had an absent witness testified, he would have confirmed the testimony of other government witnesses). For us to grant a writ in this case would require us to hold that the government never may reveal that its agreement with a witness included a corroboration requirement, even when it presents corroborating evidence at trial. Neither Supreme Court jurisprudence nor our own case law requires such a ruling, and therefore, applying the Matteo standard, we must reject Hartey’s ineffective assistance of counsel claim based on his attorney’s failure to object to and preserve objections to improper vouching.2
B. The Jury Charge
We now consider Hartey’s claims that his counsel was ineffective for failing to object and preserve his objection to the accomplice liability charge read to the jury. The Superior Court explained this issue as follows:
Appellant next contends that trial counsel was ineffective for failing to object to the trial court’s jury charge. Specifically, appellant argues that the court’s charge directed the attention of the jury to the state of mind of the co-defendant and permitted appellant’s conviction based on the state of mind of the codefendant. Appellant asserts that the court never informed the jury that in order to convict appellant of first degree murder it had to find, beyond a reasonable doubt, that appellant had the specific intent to kill.
Commonwealth v. Hartey, 621 A.2d at 1028. Once again, the Superior Court disposed of the issue by looking at the underlying merit of Hartey’s claim; it found that counsel was not ineffective because the jury instructions were correct.
We need not evaluate the accomplice liability instruction itself because we conclude that the record establishes that Hartey’s counsel was effective on this issue by making and preserving his objections. In any event, the Superior Court considered Hartey’s challenge to the accomplice liability instruction, and thus Hartey suffered no prejudice from counsel’s alleged failure to preserve his objections to ,the instruction. See Strickland, 466 U.S. at 694, 104 S.Ct. at 2068 (defendant must *374show prejudice caused by counsel’s conduct to succeed on ineffective assistance of counsel claim).
As we have indicated, the trial transcript shows that Hartey’s counsel did object to the accomplice liability charge. As the district court discussed in its decision, the transcript reveals that Hartey’s counsel sought changes to the accomplice liability instruction that the trial judge rejected. For instance, counsel asked for the accomplice charge to include an instruction that the jury should consider the evidence against each defendant separately. The judge rejected this instruction and counsel noted his objections for the record. Finally, when the jury showed its confusion about the nature of accomplice liability by asking during its deliberations whether two individuals tried for the same charge could be innocent or guilty of different degrees of murder, Hartey’s counsel vigorously argued that the trial court should simply answer this question yes. Such an answer implicitly would have told the jury that it had to consider the intent of each defendant separately. The judge disagreed with Hartey’s counsel, stating his view that, legally, the answer to the question should be no. Ultimately, over counsel’s objection, the court asked the jurors to rephrase their question and then reread to them the charge it previously had given on accomplice liability. Based on these facts, and particularly in light of the Superior Court’s consideration of Hartey’s challenge to the instruction on the merits, we find that Hartey’s counsel was not ineffective under Strickland.
III. CONCLUSION
For the foregoing reasons the order of November 14,1997, will be affirmed.
. We delayed our decision on this appeal pending disposition of McCandless, which was argued before we heard argument in this case. Moreover, we further delayed our decision in order to give the parties an opportunity to file letter briefs commenting on McCandless. As it happens, however, McCandless is of limited significance here because that case turned on an issue not properly before us on this appeal.
. We also note that this is not a case where the government emphasized the corroboration required by its agreement throughout the trial. The prosecutor mentioned the verification requirement briefly in her opening, and Murray, who testified early in this ten-day trial, referred to it at only one point in his testimony, within the context of describing Barth's agreement with the government. The prosecutor did not discuss the issue in her closing statement.