It is a violation of due process for the state to impose punishment on a defendant beyond that specified in a negotiated disposition. (Santobello v. New York (1972) 404 U.S. 257, 262 [30 L.Ed.2d 427, 433, 92 S.Ct. 495] (hereafter Santobello)-, People v. Walker (1991) 54 Cal.3d 1013, 1024 [1 Cal.Rptr.2d 902, 819 P.2d 861] (hereafter Walker)-, People v. Mancheno (1982) 32 Cal.3d 855, 860 [187 Cal.Rptr. 441, 654 P.2d 211].)
While not any deviation from the terms of the agreement is a violation of due process (Walker, supra, 54 Cal.3d at p. 1024), a variance that is significant “in the context of the plea bargain as a whole” may not be added without violating the defendant’s rights. (Ibid.) Thus, for example, a punishment that is insignificant in the context of the whole agreement, such as a standard condition of probation, may be imposed “whether or not it was part *382of the express negotiations.” (Ibid.) But the imposition of a $5,000 restitution fine, a term never mentioned in the negotiations, entitles defendant to relief because it is a significant variation from the negotiated disposition. (Id., at p. 1026.)
Any doubt that defendant was entitled to appellate relief apparently never occurred to anyone other than the majority of this court. Even the Attorney General has taken the position that defendant is entitled to relief, declaring: “Petitioner agrees that appellant is constitutionally entitled to a remedy
I agree with the unanimous view of the Court of Appeal, the Attorney General, and the defendant in this case that the addition of a term not mentioned in the plea agreement, that is, the requirement that defendant register as a sex offender for the rest of his life, is a significant variation from the plea bargain as a whole in this case, and that defendant is entitled to some relief from the addition of this significant term to the negotiated disposition.
Only the majority of this court find that defendant is entitled to no relief. They do not dispute that a sex offender registration requirement is a significant term of punishment not contained in the negotiated disposition. Rather, they maintain that because the registration requirement was not a subject of bargaining and does not appear on the face of the plea bargain, it is not a violation of due process to impose it after the bargain was struck. (Maj. opn., ante, at pp. 379-380.) Thus, they appear to conclude that unless the plea expressly excludes a term of punishment, that term may be added ex post facto.
Such a rule is inconsistent with Walker, supra, 54 Cal.3d 1013, in which we found defendant entitled to relief from the imposition of a statutorily mandated restitution fine that was not mentioned in the plea agreement or, apparently, during negotiations. It is also inconsistent with the basic rule of Santobello, supra, 404 U.S. 257, and Walker, supra, 54 Cal.3d 1013, that due process requires sentence bargains be honored.
A sentence bargain is essentially a contract. The defendant gives up the right to trial and a potential acquittal in return for a reduction in exposure to punishment, while the prosecution gives up maximum punishment of defendant in return for a quicker, cheaper, more certain disposition. Most courts interpret the terms of a plea agreement under fundamental contract principles. (People v. Armendariz (1993) 16 Cal.App.4th 906, 911 [20 Cal.Rptr.2d 311]; People v. Ames (1989) 213 Cal.App.3d, 1214, 1217 [261 Cal.Rptr. *383911]; People v. Haney (1989) 207 Cal.App.3d 1034, 1037 [255 Cal.Rptr. 276]; Leo v. Superior Court (1986) 179 Cal.App.3d 274, 283 [225 Cal.Rptr. 15]; People v. Alvarez (1982) 127 Cal.App.3d 629, 633 [198 Cal.Rptr. 167]; see also U.S. v. Anderson (9th Cir. 1992) 970 F.2d 602, 606; U.S. v. Hayes (3d Cir. 1991) 946 F.2d 230, 233; U.S. v. Herrera (6th Cir. 1991) 928 F.2d 769, 771; U.S. v. Keller (9th Cir. 1990) 902 F.2d 1391, 1393; Plaster v. United States (4th Cir. 1983) 720 F.2d 340, 352; In re Geisser (5th Cir. 1980) 627 F.2d 745, 749; United States v. Bridgeman (D.C. Cir. 1975) 523 F.2d 1099, 1109-1110 [173 App.D.C. 150]; Scott & Stuntz, Plea Bargaining as Contract (1992) 101 Yale L.J. 1909; Westen & Westin, A Constitutional Law of Remedies for Broken Plea Bargains (1978) 66 Cal.L.Rev. 471, 528 et seq.; cf. People v. Moriera (1993) 14 Cal.App.4th 861, 864 [17 Cal.Rptr.2d 782]; United States v. Calabrese (10th Cir. 1981) 645 F.2d 1379, 1390.)
The rule espoused by the majority is inconsistent with basic contract principles. It is elementary that a mutual contract cannot be modified unilaterally. (1 Witkin, Summary of Cal. Law (9th ed. 1987) Contracts, § 909, p. 814.) To impose additional punishment on defendant that was not provided for in the plea agreement runs afoul of this fundamental rule.
The majority suggest that because the registration requirement was statutorily mandated, and thus not a proper subject for bargaining, it can be added to defendant’s punishment after a negotiated disposition. (Maj. opn., ante, at p. 380.) They distinguish Walker, supra, 54 Cal.3d 1013, on the ground that there, the amount of the restitution fine was a proper subject of bargaining. Contrary to the suggestion of the majority, however, we did not grant relief in Walker, supra, 54 Cal.3d 1013, because the restitution fine was a proper subject of negotiation—as far as appeared from the record, there was no negotiation on that point in Walker, supra, 54 Cal.3d 1013. Rather, we granted relief because the negotiated plea omitted all reference to the restitution fine.
The state may not impose a significant new term of punishment that was not part of the negotiated disposition simply because the omission of that term might be inconsistent with statutory requirements. The prosecution, representing the state, may not escape its voluntary promise and still get the benefit of the guilty plea simply because its promise may have been ultra vires. (U.S. v. Anderson, supra, 970 F.2d at p. 607; Palermo v. Warden (2d Cir. 1976) 545 F.2d 286, 296; see also Westen & Westin, A Constitutional Law of Remedies for Broken Plea Bargains, supra, 66 Cal.L.Rev. at pp. 531-524, and fns. 218, 219.) Although a statute may require that a certain punishment be imposed, a defendant who has pleaded guilty on other terms that do not include the required punishment is entitled to some relief: “A *384plea induced by an unfulfillable promise is no less subject to challenge than one induced by a valid promise which the Government simply fails to fulfill.” (United States v. Cook (7th Cir. 1982) 668 F.2d 317, 320; see also U.S. v. Anderson, supra, 970 F.2d at p. 607, accord.)
Even if the plea agreement in the present case could somehow be viewed as ambiguous on the question whether the sex registration requirement would be imposed, courts normally interpret any ambiguity in this context against the government. “Both constitutional and supervisory concerns require holding the Government to a greater degree of responsibility than the defendant (or possibly than would be either of the parties to commercial contracts) for imprecisions or ambiguities in plea agreements.” (United States v. Harvey (4th Cir. 1986) 791 F.2d 294, 300; see also U.S. v. Herrera, supra, 928 F.2d at p. 772; United States v. Bowler (7th Cir. 1978) 585 F.2d 851, 854; Palermo v. Warden, supra, 545 F.2d at p. 295; United States v. Crucsco (3d Cir. 1976) 536 F.2d 21, 26; Correale v. United States (1st Cir. 1973) 479 F.2d 944, 947.) The government “ordinarily must bear responsibility for any lack of clarity.” (U.S. v. Anderson, supra, 970 F.2d at p. 607.)
In the case of any ambiguity in a plea agreement, the central question for the reviewing court is what the parties reasonably understood to be the terms of the agreement. (U.S. v. Anderson, supra, 970 F.2d at p. 607; U.S. v. Packwood (9th Cir. 1988) 848 F.2d 1009, 1011.) This court has declared that the parties would reasonably understand that the sentence bargained for pursuant to a negotiated disposition would be the maximum punishment to which defendant would be subject. (People v. Flores (1971) 6 Cal.3d 305, 308-309 [98 Cal.Rptr. 822, 491 P.2d 406].) It is reasonable to conclude the parties in the present case understood that the maximum punishment to which defendant would be subject was 13 years in prison. The addition of the sex registration requirement is inconsistent with this understanding.
The majority concede that “defendant would have been entitled to relief had he interposed a timely objection.” (Maj. opn., ante, at pp. 380-381.) In short, the majority contend that because of counsel’s failure to raise the issue at sentencing, the defendant will be subject to punishment for the rest of his life. No court should callously compel that result.
When the state unilaterally imposes punishment beyond that memorialized in the plea bargain, due process requires that defendant be granted some relief, whether it be withdrawal of the plea followed by a trial or specific performance of the plea agreement. The defendant is entitled to such relief in this case. The Attorney General has so conceded. There is no rational reason why this court should obdurately refuse relief.