Mecellem Models: Turkish Models Trained from Scratch and Continually Pre-trained for the Legal Domain

Özgür Uğur; Mahmut Göksu; Mahmut Çimen; Musa Yılmaz; Esra Şavirdi; Alp Talha Demir; Rumeysa Güllüce; İclal Çetin; Ömer Can Sağbaş

arXiv · 2601.16018

Mecellem Models: Turkish Models Trained from Scratch and Continually Pre-trained for the Legal Domain

Özgür Uğur·Mahmut Göksu·Mahmut Çimen·Musa Yılmaz·Esra Şavirdi·Alp Talha Demir·Rumeysa Güllüce·İclal Çetin·Ömer Can Sağbaş

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Abstract

This paper presents Mecellem models, a framework for developing specialized language models for the Turkish legal domain through domain adaptation strategies. We make two contributions: (1)Encoder Model Pre-trained from Scratch: ModernBERT-based bidirectional encoders pre-trained on a Turkish-dominant corpus of 112.7 billion tokens. We implement a checkpoint selection strategy that evaluates downstream retrieval performance throughout training, revealing that optimal checkpoints achieve best retrieval scores before pre-training loss reaches its minimum. Our encoder models achieve top-3 rankings on the Turkish retrieval leaderboard, with smaller models (155M parameters) achieving comparable performance to larger reference models (307M-567M parameters). Our approach achieves 92.36% production efficiency compared to state-of-the-art models (embeddinggemma-300m: 100.00%, BAAI/bge-m3: 99.54%, newmindai/bge-m3-stsb: 94.38%), ranking fourth overall despite requiring less computational resources. SOTA models rely on multi-stage, computationally intensive training pipelines, making our single-stage pre-training followed by efficient post-training approach a cost-effective alternative; (2)Decoder Model with Continual Pre-training (CPT): Qwen3-1.7B and Qwen3-4B models adapted to Turkish legal domain through controlled curriculum learning. Four-phase CPT with optimal sample ratios enables gradual transition from general language knowledge to specialized legal terminology and long-context reasoning. This approach achieves 36.2% perplexity reduction on Turkish legal text, demonstrating domain adaptation gains.

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Özgür Uğur, Mahmut Göksu, Mahmut Çimen, Musa Yılmaz, Esra Şavirdi, Alp Talha Demir, Rumeysa Güllüce, İclal Çetin, Ömer Can Sağbaş. 2026-01-22. Mecellem Models: Turkish Models Trained from Scratch and Continually Pre-trained for the Legal Domain. https://arxiv.org/abs/2601.16018

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