Lost in the Mix: Evaluating LLM Understanding of Code-Switched Text

Code-switching (CSW) is the act of alternating between two or more languages within a single discourse.This phenomenon is widespread in multilingual communities and increasingly prevalent online, exposing large language models (LLMs) to mixed-language inputs.We present a systematic evaluation of LLM comprehension under code-switching by generating linguistically grounded CSW variants of established benchmarks (Belebele, MMLU, XNLI) across five typologically diverse languages.Our contributions are: (i) a controlled pipeline for producing CSW test sets that respect linguistic constraints on code-switching; (ii) a multi-model, multi-language analysis showing that inserting non-English tokens into English consistently reduces accuracy on comprehension and reasoning benchmarks, whereas embedding English into non-English contexts often improves it; and (iii) a mitigation study contrasting in-context learning (ICL) with finetuning.Across model families, ICL cues yield inconsistent, and sometimes negative, effects, while fine-tuning on CSW data provides modest but reliable gains, partially recovering accuracy under CSW.