499 U.S. 279 (1991)
Arizona v. Fulminante is a landmark Supreme Court decision at the intersection of confession law and appellate review.
1) Was Fulminante's confession to a jailhouse informant coerced and thus involuntary under the Due Process Clause? 2) If so, is the erroneous admission of an involuntary confession at trial subject to harmless-error review, or does it require automatic reversal?
Under the Due Process Clause, a confession is involuntary if, under the totality of the circumstances, the suspect's will was overborne by coercive government conduct, including credible threats of physical violence or promises of protection from such violence. Erroneous admission of an involuntary confession is a trial error, not a structural defect, and is therefore subject to Chapman v. California's harmless-error analysis; the State must prove beyond a reasonable doubt that the error did not contribute to the verdict.
The Court held that Fulminante's first confession, obtained by a government informant's offer of protection from threatened inmate violence in exchange for the truth, was coerced and involuntary. The Court further held that the admission of an involuntary confession is subject to harmless-error review. Applying Chapman, the Court concluded the error was not harmless on these facts, and it affirmed the reversal of Fulminante's conviction, remanding for further proceedings.
Fulminante is doctrinally pivotal for two reasons. First, it clarifies that a confession can be coerced even when obtained by a jailhouse informant rather than traditional police interrogation, if the government agent leverages a credible threat of physical harm or offers protection conditioned on confession. Second, it cements the classification of the erroneous admission of an involuntary confession as a trial error subject to Chapman harmless-error review, not a structural error requiring automatic reversal. For law students, the case is a cornerstone in learning the modern harmless-error framework, the totality-of-the-circumstances test for voluntariness, the role of state action in informant settings, and the practical difficulty of finding harmlessness where a coerced confession is the centerpiece of the prosecution's case.