The (in)congruence of L2 grit and growth mindset in shaping engagement among Thai learners of Chinese as an additional language: a response surface approach

L2 grit and growth mindset (GM) are increasingly linked to engagement in language education, yet prior work has largely assumed linear effects and focused on English learning. This study examined whether the alignment between L2 grit and GM, along with their potential nonlinear joint effects, predicts engagement among Thai learners of Chinese in a Languages Other Than English (LOTE) context. Survey data from 325 Chinese-as-an-additional-language (CAL) learners in Thailand were analyzed using polynomial regression with response surface analysis (RSA) in R to model congruence, incongruence, and interaction effects. A congruence hypothesis was not supported: results revealed that optimal CAL engagement did not occur simply when L2 grit and GM were aligned. Instead, CAL engagement increased most sharply as both constructs rose together. Effects were asymmetrical: GM exhibited a largely linear relationship with CAL engagement, whereas L2 grit showed a curvilinear pattern, with diminishing returns at higher levels, suggesting that additional grit may not always correspond to proportionally higher engagement. CAL engagement under the low–low configuration remained moderate, consistent with coping or ‘survival-mode’ engagement under contextual pressure. Educators should emphasize the quality of learners’ persistence over mere quantity, guiding them to employ adaptive strategies, set flexible goals, and make timely adjustments, while also harnessing the synergistic effects of L2 grit and GM to sustain CAL learners’ engagement. Extending RSA to an underexplored LOTE context, this study reveals that the relationship between L2 grit, GM, and CAL engagement is configuration-dependent, with evidence pointing to nuanced nonlinear effects.