ed loss.
Offline DPO, transfer to Apertus 1.5 8B.
Online DPO.
Combining offline and online DPO.
J.1.2 Apertus 1.5 70B
Transferring the Apertus 1.5 8B hyperparameters.
Offline and mixture.
J.2 Judge Ablations
J.2.1 Judging Aspects Simplification
J.2.2 Choice of Judge Model
K Chatbot System Prompt
L Evaluation Details
L.1 Full Post-Training Lineage
M Pre-Release Manual Evaluations
M.1 General Information About Pre-Release Evaluation
M.2 Chemical, Biological, Radiological, Nuclear, and Explosive Indiscriminate Weapons (CBRNE)
Threat Model.
Results.
M.3 Dual Use and Insurgency
Threat Model.
Results.
M.4 LLM-Assisted Information Operations
Threat Model.
Results.
M.5 (Child) Sexual Abuse
Threat Model.
Results.
M.6 Offensive Cybersecurity
Threat Model.
Results.
M.7 Multimodal Prompt Injection
Threat Model.
Results.
M.8 Harmful Disinformation
Threat Model.
Results.
M.9 Harmful Health Advice
Threat Model.
Results.
M.10 Private Person Defamation
Threat Model.
Results.
N Coverage of Common AI Security Framework Risks
License: CC BY 4.0
arXiv:2610.15542v1 [cs.CL] 11 Oct 2026
From Open Text Models to Open Multimodal Reasoners
Apertus v1.5 Technical Report
Project Apertus
††thanks: In each contribution level, team members are listed in (reverse) alphabetical order by first or last name. Contributions are described in more detail in Appendix A.
Affiliation: Core Team: Yixuan Xu1, Yassine Turki2, Vinko Sabolčec2, Valentina Pyatkin1, Tommy Chu2, Sven Najem-Meyer2, Sean F. Carroll2, Robert Smith1, Raphael Kreft1, Oscar Alessandro Tazza1, Oleg Lavrovsky2, Nicola Irmiger1, Matteo Santelmo2, Marian Schneider1, Juan García Giraldo2, Hanna Yukhymenko1, Eduard Ďurech1, Davit Melikidze1, Dani Tamayo2, Bettina Messmer2, Aryan Ahadinia2, Anunay Yadav2, Allen Huang2
Contributors: Ahmad Fraij1, Alessandro Bombarda2, Alex Padula1, Alexander Hägele2, Alexander Sternfeld3, Anna Hedström1, Anna Kosovskaia1, Arthur Wuhrmann2, Arundhati Balasubramaniam1, Badr Al Mahouri2, Barna Pasztor1, Bartosz Szostakiewicz1, Bich Ngoc Doan2, Bryan Schmid4, Burak Sezen1, Camille Challier2, Clément Charmillot2, Dana Shayakhmetova2, Daniel Perruchoud4, David Brüggemann5, David Isztl6, Elia Palme6, Emma Anastassova2, Fabian Bösch6, Faruk Zahiragić2, Fawzi Roberto Mohamed6, Fiona Panicali4, Florian Kolly2, Flynn Guerra4, Francesco Monti2, Frederike Lübeck1, Gennaro Oliva6, Haofeng Yang1, Haoliang Shang1, Helen El Khoury2, Henrique Mendonça6, Jannis Vamvas7, Jérome Landolt4, Jessica Lam1, Jingchun Xiao1, Jonathan Coles6, Joshua Oyebanji2, Khanh Nguyen2, Konstantinos Pitas1, Lasse B. Strand1, Li Mi1, Loïc Deslarzes2, Lorenzo Baggi1, Lorenzo Noci1, Luca Baumann1, Luca Sartori1, Lukas Drescher6, Marco De Negri1, Marco Passerini6, Mark Klein6, Marko Stojanovic2, Martin Majsec2, Matin Ansaripour2, Mattea Busato2, Mauro Pellonara2, Max Neuwinger1, Michael Graber4, Michal Tešnar1, Michelle Wastl7, Miguel Gila6, Mikolaj Boronski2, Morgane Magnin2, Moritz Reihs1, Mukhammadali Sayfiddinov1, Nathan Ranchin1,2, Nicolas Cuveillier2, Oscar Wohlfahrt2, Qianqian Zou5, Rada Kamysheva1, Rita Sevastjanova1, Roman Machacek8, Sebastian Keller6, Simon Matrenok2, Simon Pierre2, Stefano Schuppli6, Suei-Wen Chen1, Szabina Horváth-Mikulas2, Taliesin Perez2, Thilo Spinner1, Thomas Kleine Buening1, Tim Robinson6, Tymoteusz Kwiecinski1, Vilém Zouhar1, Vinzenz Thoma1, Xiaozhe Yao1, Yannick Metz1, Yuhei Fukuhara2, Yusif Askari2, Zixuan Chen1
Advisors: Alexander Ilic1, Ana Klimovic1, Andreas Krause1, Andrei Kucharavy3, Elliott Ash1, Joost VandeVondele6, Martin Rajman2, Mennatallah El-Assady1, Thomas Schulthess6, Timo Kehrer8, Torsten Hoefler1
Leads: Antoine Bosselut2, Martin Jaggi2, Imanol Schlag1
Affiliations:1ETH Zurich, 2EPFL, 3HES-SO Valais-Wallis, 4FHNW, 5SDSC, 6CSCS, 7University of Zurich, 8University of Bern
Abstract
Language models are becoming infrastructure, and infrastructure built in the open, by public institutions, on documented data, is the alternative to dependence on a few providers; Apertus sho