What are we teaching when we teach chéngyǔ 成语?

In L2 Chinese education, chéngyǔ 成语 “Chinese idioms” are considered important for advanced proficiency yet marginalised in teaching practice. Current chéngyǔ instruction assumes dictionaries and textbooks reflect native speaker understanding. However, L1 Chinese speakers do not agree with authoritative sources or each other about which expressions are chéngyǔ . Expressions with opaque meanings, irregular structures, and cultural content achieve broad acceptance, while semantically transparent and symmetric expressions are contested. This creates a pedagogical challenge: the expressions easiest for learners to acquire are least likely to be valued as chéngyǔ by native speakers. This paper explores implications of these findings for L2 chéngyǔ instruction. It argues that attending to gradient “ chéngyǔ -ness” can inform more conscious teaching choices, with semantically compositional expressions efficiently developing vocabulary and fluency, while noncompositional expressions signal cultural literacy and sophisticated proficiency. Rather than treating all dictionary-listed “ chéngyǔ ” as equivalent, instruction should foster pragmatic awareness and communicative competence.