What is the test for giving a jury instruction on an affirmative defense?

The constitutional right to due process has long been understood to require that "criminal defendants be afforded a meaningful opportunity to present a complete defense." (California v. Trombetta (1984) 467 U.S. 479, 485.) And it is settled that the right to present a defense includes the right to present evidence relevant to a defense theory. (Washington v. Texas (1967) 388 U.S. 14, 19.) That right would be meaningless without a corresponding right to have the jury instructed on the theory to which the evidence relates. If evidence is presented to establish a defense to conviction but the jury is not told the defense exists, then the defendant has not had the opportunity to present a complete defense. The conclusion that due process requires a jury instruction on a legally valid affirmative defense supported by substantial evidence necessarily follows.