Transforming Causal LLM into MLM Encoder for Detecting Social Media Manipulation in Telegram

We participated in the Fourth UNLP shared task on detecting social media manipulation in Ukrainian Telegram posts (Kyslyi et al., 2025), addressing both multilabel technique classification and token-level span identification.We propose two complementary solutions: for classification, we fine-tune the decoder-only model with class-balanced grid-search thresholding and ensembling.For span detection, we convert causal LLM into a bidirectional encoder via masked language modeling pretraining on large Ukrainian and Russian news corpora before fine-tuning.Our solutions achieve SOTA metric results on both shared task track.Our work demonstrates the efficacy of bidirectional pretraining for decoder-only LLMs and robust threshold optimization, contributing new methods for disinformation detection in low-resource languages.