Under California Civil Code 1646, "A contract is to be interpreted according to the law and usage of the place where it is to be performed; or, if it does not indicate a place of performance, according to the law and usage of the place where it is made."
In Idaho, choice of law is controlled by an Idaho Supreme Court ruling: Seubert Excavators, Inc. v. Anderson Logging Co., 126 Idaho 648 (1995), which holds that "the significant relationship to dispute are place of contracting, place of negotiation of contract, place of performance, location of subject matter of contract, and domicile, residence, nationality, place of incorporation, and place of business
of parties." In plain English, an Idaho Court would try to determine where the contract was made, where it was to be performed, where the breach occurred, where the lawsuit was filed, etc., and thereby determine which state' laws should prevail in the dispute.
Whereas in California, the most important factor is where the contract was made -- generally this is deemed to be the place where the last party signs the contract, because it is that party's signature that operates as the contract acceptance, which binds both parties.
Given that background, assuming that Idaho law controls, then ID Stat. 28-22-104 provides prejudgment interest on a written contract, without any express provision, at the rate of 12% simple interest per annum/year. Whereas Cal. Civil Code 3287 provides that prejudgment interest is 10% simple interest per annum/year.
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