Valentin Hofmann

@valentinhofmann.bsky.social

Assistant Professor @cislmu.bsky.social @lmu.de

Demographic cues (eg, names, dialect) are widely used to study how LLM behavior may change depending on user demographics. Such cues are often assumed interchangeable. 🚨 We show they are not: different cues yield different model behavior for the same group and different conclusions on LLM bias. 🧵👇

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Introducing Bolmo, a new family of byte-level language models built by "byteifying" our open Olmo 3—and to our knowledge, the first fully open byte-level LM to match or surpass SOTA subword models across a wide range of tasks. 🧵

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There’s plenty of evidence for political bias in LLMs, but very few evals reflect realistic LLM use cases — which is where bias actually matters. IssueBench, our attempt to fix this, is accepted at TACL, and I will be at #EMNLP2025 next week to talk about it! New results 🧵

Paul Röttger@paul-rottger.bsky.social · last yr.

Are LLMs biased when they write about political issues? We just released IssueBench – the largest, most realistic benchmark of its kind – to answer this question more robustly than ever before. Long 🧵with spicy results 👇

Check out this #EMNLP2025 paper led by @minhducbui.bsky.social and @carolin-holtermann.bsky.social showing dialect prejudice remains a major issue in current LLMs. Example: GPT-5 associates German dialect speakers with being uneducated and steers them toward stereotyped jobs (e.g., farmworkers). 👇

Trustworthy AI Lab@tai-lab.bsky.social · 11mo ago

“You speak Bavarian? Then you must be uneducated + closed-minded.” 🤯 Not your opinion? Good. But it might be your LLM’s!! 🧵 Check out our #EMNLP2025 paper, where we uncover concerning dialect bias in recent LLMs - including GPT-5. #AI #Bias #Dialect #Fairness #LLM #NLProc #Safety

LM benchmark design requires 3 decisions, how to: 🐟 select test cases 🐠 score LM on each test 🦈 aggregate scores to estimate perf fluid benchmarking is simple: 🍣 find max informative test cases 🍥 estimate 'ability', not simple avg perf why care? turn ur grey noisy benchmarks to red ones!

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Valentin Hofmann@valentinhofmann.bsky.social · 11mo ago

📢 New #COLM2025 paper 📢 Standard benchmarks give every LLM the same questions. This is like testing 5th graders and college seniors with *one* exam! 🥴 Meet Fluid Benchmarking, a capability-adaptive eval method delivering lower variance, higher validity, and reduced cost. 🧵

📢 New #COLM2025 paper 📢 Standard benchmarks give every LLM the same questions. This is like testing 5th graders and college seniors with *one* exam! 🥴 Meet Fluid Benchmarking, a capability-adaptive eval method delivering lower variance, higher validity, and reduced cost. 🧵

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Ai2@ai2.bsky.social · 11mo ago

🚀 Introducing Fluid Benchmarking—an adaptive way to evaluate LLMs. Inspired by psychometrics, it tailors which questions to ask based on each model’s capability, making evals more efficient & reliable. 🧵

LLMs can appear unbiased on the surface but still perpetuate racist views in subtle ways. What causes this discrepancy? 🔍 In our upcoming #ACL2025 paper, we find a pattern akin to racial colorblindness: LLMs suppress race in ambiguous contexts, leading to biased outcomes.

Lihao Sun@1e0sun.bsky.social · last yr.

🚨New #ACL2025 paper! Today’s “safe” language models can look unbiased—but alignment can actually make them more biased implicitly by reducing their sensitivity to race-related associations. 🧵Find out more below!

Do LLMs learn language via rules or analogies? This could be a surprise to many – models rely heavily on stored examples and draw analogies when dealing with unfamiliar words, much as humans do. Check out this new study led by @valentinhofmann.bsky.social to learn how they made the discovery 💡

Valentin Hofmann@valentinhofmann.bsky.social · last yr.

Thrilled to share that this is out in @pnas.org today! 🎉 We show that linguistic generalization in language models can be due to underlying analogical mechanisms. Shoutout to my amazing co-authors @weissweiler.bsky.social, @davidrmortensen.bsky.social, Hinrich Schütze, and Janet Pierrehumbert!

Delighted there will finally be a workshop devoted to tokenization - a critical topic for LLMs and beyond! 🎉 Join us for the inaugural edition of TokShop at #ICML2025 @icmlconf.bsky.social in Vancouver this summer! 🤗

Tokenization Workshop (TokShop) @COLM2026@tokshop.bsky.social · last yr.

🚨 NEW WORKSHOP ALERT 🚨 We're thrilled to announce the first-ever Tokenization Workshop (TokShop) at #ICML2025 @icmlconf.bsky.social! 🎉 Submissions are open for work on tokenization across all areas of machine learning. 📅 Submission deadline: May 30, 2025 🔗 tokenization-workshop.github.io

We created Approximate Likelihood Matching, a principled (and very effective) method for *cross-tokenizer distillation*! With ALM, you can create ensembles of models from different families, convert existing subword-level models to byte-level and a bunch more🧵

Image illustrating that ALM can enable Ensembling, Transfer to Bytes, and general Cross-Tokenizer Distillation.

Humans store thousands of multi-word expressions like "of course" in their mental lexicon, but current tokenizers don't support multi-word tokens. Enter SuperBPE, a tokenizer that lifts this restriction and brings substantial gains in efficiency and performance! 🚀 Details 👇

Alisa Liu@alisawuffles.bsky.social · last yr.

We created SuperBPE🚀, a *superword* tokenizer that includes tokens spanning multiple words. When pretraining at 8B scale, SuperBPE models consistently outperform the BPE baseline on 30 downstream tasks (+8% MMLU), while also being 27% more efficient at inference time.🧵

Segmentation of the sentence "By the way, I am a fan of the Milky Way" under BPE and SuperBPE.

Excited to share IssueBench, the most extensive benchmark for LLM political bias! 📊 We find surprising consistency across models, with notable differences in Qwen on China-related issues. All examined LLMs also show strong alignment with Democrat voters. More details below! 👇

Paul Röttger@paul-rottger.bsky.social · last yr.

Are LLMs biased when they write about political issues? We just released IssueBench – the largest, most realistic benchmark of its kind – to answer this question more robustly than ever before. Long 🧵with spicy results 👇

Today, we are releasing MSTS, a new Multimodal Safety Test Suite for vision-language models! MSTS is exciting because it tests for safety risks *created by multimodality*. Each prompt consists of a text + image that *only in combination* reveal their full unsafe meaning. 🧵

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📢 New paper 📢 What generalization mechanisms shape the language skills of LLMs? Prior work has claimed that LLMs learn language via rules. We revisit the question and find that superficially rule-like behavior of LLMs can be traced to underlying analogical processes. 🧵

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