The Supreme Court has limited the instances in which deference will be owed to an agency’s interpretation of its own regulations (known as Auer deference).

“The possibility of deference can arise only if a regulation is genuinely ambiguous. And when we use that term, we mean it—genuinely ambiguous, even after a court has resorted to all the standard tools of interpretation. Still more, not all reasonable agency constructions of those truly ambiguous rules are entitled to deference. As just explained, we presume that Congress intended for courts to defer to agencies when they interpret their own ambiguous rules.But when the reasons for that presumption do not apply, or countervailing reasons outweigh them, courts should not give deference to an agency’s reading, except to the extent it has the ‘power to persuade.’” “And although the limits of Auer deference are not susceptible to any rigid test, we have noted various circumstances in which such deference is ‘unwarranted.’ In particular, that will be so when a court concludes that an interpretation does not reflect an agency’s authoritative, expertise-based, fair, or considered judgment.”

Recognizing that not all regulations are ambiguous, the Court implored lower courts to “make a conscientious effort to determine, based on indicia like text, structure, history, and purpose, whether the regulation really has more than one reasonable meaning.”

The Court suggested that interpretation of legal terminology may fall outside an agency’s expertise. “When the agency has no comparative expertise in resolving a regulatory ambiguity, Congress presumably would not grant it that authority.”

“And a court may not defer to a new interpretation, whether or not introduced in litigation, that creates 'unfair surprise’ to regulated parties. That disruption of expectations may occur when an agency substitutes one view of a rule for another. We have therefore only rarely given Auer deference to an agency construction conflicting with a prior one.

The full text of Kisor v. Wilkie can be found here: