We granted a certificate of probable cause to appeal in this habeas corpus action to determine whether Keith Edwain Gerisch was denied constitutionally effective assistance of trial counsel in connection with Gerisch’s guilty plea to aggravated battery. Because we conclude that trial counsel was ineffective in failing to recognize and fully investigate a double jeopardy claim, and in failing to *642reasonably inform petitioner of the merits of that defense prior to the entry of his guilty plea, we reverse the judgment of the habeas court.
On June 21, 1998, Gerisch was involved in a fight with Lara Robinson and he was charged with the municipal offenses of disorderly conduct by fighting and “public drunk.” On July 6,1998, he pled guilty to those charges in the City Court of Nashville, Georgia; he was sentenced to 12 months probation, and ordered to pay afine of $1,000, plus $761 as restitution for the victim’s medical expenses.
On June 25, 1998, Gerisch was charged in a two-count indictment in Berrien County Superior Court with aggravated battery arising from the June 21,1998 fight with Robinson; and with simple battery arising from another altercation with Robinson which occurred on a different day. In addition, he was charged in a separate indictment with felony possession of marijuana and DUI. Gerisch was appointed counsel who entered into plea negotiations with the prosecutor to dispose of all pending charges. As a result of the plea negotiations, the prosecutor recommended a sentence of twenty years on the aggravated battery charge (ten to serve, ten on probation); plus concurrent terms on the remaining offenses.
While in court to enter his plea, Gerisch, who was functionally illiterate, told his attorney that he had been punished in city court for the conduct underlying the aggravated battery (June 21,1998 attack on Robinson), and he asked counsel why the State could indict him on the same charges. Counsel subsequently advised Gerisch that a double jeopardy claim would be fruitless, and if he were to raise the issue and fail, the prosecutor would withdraw his plea recommendation and seek greater punishment. On that day, Gerisch entered guilty pleas to all charges and he was sentenced in accordance with the prosecutor’s recommendations, plus a fine of $ 1,500 and costs and surcharges on the aggravated battery count.
Gerisch appended to his habeas petition a noncertified copy of the incident report underlying the disorderly conduct charges in city court, as well as the disposition sheet reflecting his guilty plea to the charge and sentence. At the habeas hearing, Gerisch directed the court to the documents, arguing, “I was already charged one time and then... I’m back and charged twice again for the same crime... same date, same time, same address, same person, same incident report, same everything.” In its order denying habeas relief, the court found that the entry of the guilty plea waived consideration of the double jeopardy claim; that “there is no credible evidence” of the events that took place in city court, as the documents proffered by Gerisch were merely hearsay; and that Gerisch was not denied effective assistance of counsel in the plea proceedings in superior court. We certified those rulings for review.
*6431. First we look to whether there was credible evidence before the habeas court of the city court conviction, which would demonstrate that both the disorderly conduct by fighting and the aggravated battery charges emanated from the same conduct. While a noncertified copy of a conviction may be subject to the hearsay rule (see generally OCGA § 24-3-17 and Brown v. State, 274 Ga. 31 (1) (549 SE2d 107) (2001)), here Gerisch offered original evidence of the circumstances of his conviction by his own testimony. The evidence was uncontested and respondent voiced no objection to its admissibility. The value of the petitioner’s testimony regarding the circumstances of his plea in city court depends on his own credibility, not that of another.
The hearsay rule does not prevent a witness from testifying to what he has heard; it is rather a restriction on the proof of fact through extrajudicial statements. From the viewpoint of the Confrontation Clause, a witness under oath, subject to cross-examination, and whose demeanor can be observed by the trier of fact, is a reliable informant not only to what he has seen but also to what he has heard.
(Punctuation omitted.) Castell v. State, 250 Ga. 776, 779 (1) (b) (301 SE2d234) (1983). Thus, petitioner’s own sworn testimony was enough to establish his plea of guilty to the city charges and the conduct underlying that plea. Id. See generally Jackson v. Lowry, 171 Ga. 349 (155 SE 466) (1930).
2. Gerisch asserts that he was twice punished for the same crime, in violation of the Fifth Amendment Double Jeopardy Clause, as applied to the states by the Fourteenth Amendment.
[A] person who has been convicted of a crime having several elements included in it may not subsequently be tried for a lesser-included offense ■— an offense consisting solely of one or more of the elements of the crime for which he has already heen convicted ... the reverse is also true; a conviction on a lesser-included offense bars subsequent trial on the greater offense.
(Punctuation omitted.) State v. Burroughs, 246 Ga. 393, 394 (271 SE2d 629) (1980). See also Brown v. Ohio, 432 U. S. 161, 166 (97 SC 2221, 53 LE2d 187) (1977), applying the test in Blockburger v. United States, 284 U. S. 299, 304 (52 SC 180, 76 LE 306) (1932) (in determining whether there are two offenses or only one for double jeopardy purposes under the Fifth Amendment, we look to whether each *644provision “requires proof of a fact which the other [did] not”) (punctuation omitted). Thus, if all the elements of aggravated battery are contained in the municipal crime of disorderly conduct, Gerisch may not be tried on the state court offense. Id. That is because the constitution prohibits “successive prosecutions as well as cumulative punishment.” Brown v. Ohio, supra, 432 U. S. at 166.
Relying on United States v. Broce, 488 U. S. 563 (109 SC 757, 102 LE2d 927) (1989) and Clark v. Caldwell, 229 Ga. 612 (193 SE2d 816) (1972), the habeas court reached the conclusion that Gerisch’s entry of the guilty plea to aggravated battery waived consideration of his constitutional double jeopardy claim. The dissent adopts that analysis. But the critical distinction is that the guilty pleas in both Broce and Clark were knowingly and voluntarily entered with the benefit of competent counsel. As noted in Broce: “A failure by counsel to provide advice may form the basis of a claim of ineffective assistance of counsel, but absent such a claim it cannot serve as the predicate for setting aside a valid plea.” (Emphasis supplied.) 488 U. S. at 574.
A federal double jeopardy claim was held to survive a guilty plea in Menna v. New York, 423 U. S. 61 (96 SC 241, 46 LE2d 195) (1975). The reason is that the United States Constitution precludes a state from “haling a defendant into court on a charge” for which he has already pled guilty and been punished. Id., 423 U. S. at 62. See also Hooten v. State, 212 Ga. App. 770, 771 (1) (442 SE2d 836) (1994) (an unconditional guilty plea generally does not preclude appeal grounded on “double jeopardy-type errors”), citing Blackledge v. Perry, 417 U. S. 21, 30 (94 SC 2098, 40 LE2d 628) (1974). Compare Addison v. State, 239 Ga. 622 (238 SE2d 411) (1977) (valid guilty plea will stand where no constitutional violation or ineffective assistance of counsel claim asserted).
It follows that the habeas court erroneously concluded that the entry of the guilty plea to aggravated battery in all circumstances waived consideration of the constitutional double jeopardy claim.
3. The two-prong test for establishing ineffective assistance of counsel enunciated in Strickland v. Washington, 466 U. S. 668 (104 SC 2052, 80 LE2d 674) (1984) applies to guilty pleas. Hill v. Lockhart, 474 U. S. 52 (106 SC 366, 88 LE2d 203) (1985); Thompson v. Greene, 265 Ga. 782 (2) (462 SE2d 747) (1995). Under that test a defendant must demonstrate that counsel’s performance fell below an objective standard of reasonableness and there is a reasonable probability that, but for counsel’s errors, the defendant would not have pled guilty and would have insisted on going to trial. Rollins v. State, 277 Ga. 488 (2) (591 SE2d 796) (2004).
Counsel testified at the habeas hearing that when she and Gerisch were in court for the plea proceedings, he expressed “confusion as to why — understandably so . . . would he have faced some *645charges in City Court and then how could the State of Georgia come back and then indict him on these charges.” As a result of Gerisch’s inquiry, counsel had a discussion with the district attorney, who advised her that a double jeopardy claim would have no merit. Counsel also contacted the city court judge to verify that there had been proceedings against Gerisch in that court. What she learned was that Gerisch had been charged with disorderly conduct under a municipal ordinance. Because the prior prosecution was for a violation of a municipal ordinance, and not a state offense, counsel did not believe that there was a potential double jeopardy defense: “I felt like his double jeopardy issue would come in if he had been charged with [the state offense of] simple battery and then the aggravated battery later.” She did not, however, obtain any of the city court records, and there is no indication that she researched or evaluated the issue any further.1 Counsel subsequently advised Gerisch that a double jeopardy claim would be fruitless, andifhe were to raise the issue and fail, the prosecutor would withdraw his plea recommendation and seek greater punishment. Counsel testified that “based on what advice she gave,” Gerisch decided to enter a plea.
Contrary to the expansive and conclusory statements of the dissent that counsel properly investigated and advised her client as to the viability of a double jeopardy claim, the undisputed evidence shows that, other than a cursory inquiry into the city court proceedings, counsel utterly failed to research and evaluate evidence of the elements of the city court offense or the facts underlying it. Instead, she relied on the advice from the district attorney who was prosecuting Gerisch in superior court, and counsel’s own misapprehension that the prosecution in city court could not provide a basis for a valid double jeopardy claim. See Brown v. Ohio, supra. The giving of misleading advice through the failure to conduct basic research, and “to investigate [her] options and make a reasonable choice between them,” Turpin v. Christenson, 269 Ga. 226, 239 (12) (B) (497 SE2d 216) (1998), was erroneous and was not within the range of competence required of attorneys in criminal cases. Benham v. State, 277 Ga. 516 (591 SE2d 824) (2004); Rollins, supra. Therefore, counsel’s performance fell below an objective standard of reasonableness.
As for the prejudice prong, it is clear from Gerisch’s argument before the habeas court, and from his counsel’s testimony, that but for *646counsel’s error, it is reasonably probable Gerisch. would not have pled guilty to the charge of aggravated battery. Rollins, supra.
Judgment reversed.
All the Justices concur, except Fletcher, C. J., who concurs in judgment only and Carley, J., who dissents.Both Gerisch and his counsel testified that these discussions took place in superior court on the day that Gerisch was taken there to enter his plea. In fact, Gerisch testified that he was not even aware that the plea agreement included a charge of aggravated battery, thus prompting his question about double punishment. Instead, he believed that he was pleading guilty to the other pending offenses.