Thrilled to share my first paper! 📄 We prove optimal tokenization is NP-hard on bounded alphabets (like bytes)—even unary for direct tokenization! Big thanks @tpimentel.bsky.social, @philipwitti.bsky.social & Dennis Komm for the mentorship! Best birthday gift. 🎂 arxiv.org/abs/2511.15709

Tokenisers are a vital part of LLMs, but how hard is it to find an optimal one? 🤔 Considering arbitrarily large alphabets, prior work showed this is NP-hard. But what if we use bytes instead? Or unary strings like a, aa, aaa, ...? In our new paper, we show this is still hard, NP-hard!

Interested in provable guarantees and fundamental limitations of XAI? Join us at the "Theory of Explainable AI" workshop Dec 2 in Copenhagen! @ellis.eu @euripsconf.bsky.social Speakers: @jessicahullman.bsky.social @doloresromerom.bsky.social @tpimentel.bsky.social Call for Contributions: Oct 15

Interested in language models, brains, and concepts? Check out our COLM 2025 🔦 Spotlight paper! (And if you’re at COLM, come hear about it on Tuesday – sessions Spotlight 2 & Poster 2)!

Accepted to EMNLP (and more to come 👀)! The camera ready version is now online---very happy with how this turned out arxiv.org/abs/2507.01234

LLMs are trained to mimic a “true” distribution—their reducing cross-entropy then confirms they get closer to this target while training. Do similar models approach this target distribution in similar ways, though? 🤔 Not really! Our new paper studies this, finding 4-convergence phases in training 🧵

Very happy this paper got accepted to NeurIPS 2025 as a Spotlight! 😁 Main takeaway: In mechanistic interpretability, we need assumptions about how DNNs encode concepts in their representations (eg, the linear representation hypothesis). Without them, we can claim any DNN implements any algorithm!

Honoured to receive two (!!) SAC highlights awards at #ACL2025 😁 (Conveniently placed on the same slide!) With the amazing: @philipwitti.bsky.social, @gregorbachmann.bsky.social and @wegotlieb.bsky.social, @cuiding.bsky.social, Giovanni Acampa, @alexwarstadt.bsky.social, @tamaregev.bsky.social

We are presenting this paper at #ACL2025 😁 Find us at poster session 4 (Wednesday morning, 11h~12h30) to learn more about tokenisation bias!

@philipwitti.bsky.social will be presenting our paper "Tokenisation is NP-Complete" at #ACL2025 😁 Come to the language modelling 2 session (Wednesday morning, 9h~10h30) to learn more about how challenging tokenisation can be!

Headed to Vienna for #ACL2025 to present our tokenisation bias paper and co-organise the L2M2 workshop on memorisation in language models. Reach out to chat about tokenisation, memorisation, and all things pre-training (esp. data-related topics)!

Causal Abstraction, the theory behind DAS, tests if a network realizes a given algorithm. We show (w/ @denissutter.bsky.social, T. Hofmann, @tpimentel.bsky.social ) that the theory collapses without the linear representation hypothesis—a problem we call the non-linear representation dilemma.

Mechanistic interpretability often relies on *interventions* to study how DNNs work. Are these interventions enough to guarantee the features we find are not spurious? No!⚠️ In our new paper, we show many mech int methods implicitly rely on the linear representation hypothesis🧵

All modern LLMs run on top of a tokeniser, an often overlooked “preprocessing detail”. But what if that tokeniser systematically affects model behaviour? We call this tokenisation bias. Let’s talk about it and why it matters👇 @aclmeeting.bsky.social #ACL2025 #NLProc

The word "laundry" contains both steps of the laundry process: 1. Undry 2. Dry

Love this! Especially the explicit operationalization of what “bias” they are measuring via specifying the relevant counterfactual. Definitely an approach that more papers talking about effects can incorporate to better clarify what the phenomenon they are studying.

If you use LLMs, tokenisation bias probably affects you: * Text generation: tokenisation bias ⇒ length bias 🤯 * Psycholinguistics: tokenisation bias ⇒ systematically biased surprisal estimates 🫠 * Interpretability: tokenisation bias ⇒ biased logits 🤔

A string may get 17 times less probability if tokenised as two symbols (e.g., ⟨he, llo⟩) than as one (e.g., ⟨hello⟩)—by an LM trained from scratch in each situation! Our new ACL paper proposes an observational method to estimate this causal effect! Longer thread soon!

If you're finishing your camera-ready for ACL or ICML and want to cite co-first authors more fairly, I just made a simple fix to do this! Just add $^*$ to the authors' names in your bibtex, and the citations should change :) github.com/tpimentelms/...

⭐🗣️New preprint out: 🗣️⭐ “Using Information Theory to Characterize Prosodic Typology: The Case of Tone, Pitch-Accent and Stress-Accent” with @cuiding.bsky.social , Giovanni Acampa, @tpimentel.bsky.social , @alexwarstadt.bsky.social ,Tamar Regev: arxiv.org/abs/2505.07659

How do language models generalize from information they learn in-context vs. via finetuning? In arxiv.org/abs/2505.00661 we show that in-context learning can generalize more flexibly, illustrating key differences in the inductive biases of these modes of learning — and ways to improve finetuning. 1/

Super happy we got this award for our paper on memorisation 😁🎉 congrats to the team and in particular to @pietrolesci.bsky.social, who led the project! Pietro is super smart, creative, hard-working, and on the job market -- you should hire him if you can :)

Honoured to receive this award! Tagging @pietrolesci.bsky.social and @tpimentel.bsky.social !

I'm truly honoured that our paper "Causal Estimation of Memorisation Profiles" has been selected as the Paper of the Year by @cst.cam.ac.uk 🎉 I thank my amazing co-authors Clara Meister, Thomas Hofmann, @tpimentel.bsky.social, and my great advisor and co-author @andreasvlachos.bsky.social!

💡 New ICLR paper! 💡 "On Linear Representations and Pretraining Data Frequency in Language Models": We provide an explanation for when & why linear representations form in large (or small) language models. Led by @jackmerullo.bsky.social, w/ @nlpnoah.bsky.social & @sarah-nlp.bsky.social

I might be able to hire a postdoc for this fall in computational linguistics at UT Austin. Topics in the general LLM + cognitive space (particularly reasoning, chain of thought, LLMs + code) and LLM + linguistic space. If this could be of interest, feel free to get in touch!

I'm so grateful to the British Computing Society & Bloomberg for honouring me with the Karen Spärck Jones Award 🙏 I gave the award lecture on LLMs’ Utilisation of Parametric & Contextual Knowledge at #ECIR2025 today (slides: isabelleaugenstein.github.io/slides/2025_...) www.bcs.org/membership-a...

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🧵

A bit of a mess around the conflict of COLM with the ARR (and to lesser degree ICML) reviews release. We feel this is creating a lot of pressure and uncertainty. So, we are pushing our deadlines: Abstracts due March 22 AoE (+48hr) Full papers due March 28 AoE (+24hr) Plz RT 🙏

LMs need linguistics! New paper, with @futrell.bsky.social, on LMs and linguistics that conveys our excitement about what the present moment means for linguistics and what linguistics can do for LMs. Paper: arxiv.org/abs/2501.17047. 🧵below.

📢 The First Workshop on Large Language Model Memorization (L2M2) will be co-located with @aclmeeting.bsky.social in Vienna 🎉 💡 L2M2 brings together researchers to explore memorization from multiple angles. Whether it's text-only LLMs or Vision-language models, we want to hear from you! 🌍

BPE is a greedy method to find a tokeniser which maximises compression! Why don't we try to find properly optimal tokenisers instead? Well, it seems this is a pretty difficult—in fact, NP-complete—problem!🤯 New paper + @philipwitti.bsky.social @gregorbachmann.bsky.social :) arxiv.org/abs/2412.15210

There's a known bug in how we compute "word" probabilities with subword-based LMs that mark beginnings of words -- as pointed out by Byung-doh Oh and Will Schuler, & @tpimentel.bsky.social and Clara Meister I'm pleased to announce that minicons now includes a fix which runs batch-wise!

Hey :) I'm looking for 3 emergency reviewers for ARR submissions🚨📷 they are all in LM interpretability! Should be submitted within the next 36 hours 🙃 If you are interested, please DM or email me! #NLProc #NLP

Are you interested in word lengths and natural language’s efficiency? If yes, check out our new #EMNLP2023 paper! It has everything you need: drama, suspense, a new derivation of Zipf’s law, an update to Piantadosi et al’s classic word length paper, transformers... 😄 arxiv.org/abs/2312.03897

👋Hi #bsky! Just wanted to let everyone know I'll be arriving at #Stanford in fall of 2024 and I'm looking for awesome people to help me figure out language. I'm new here my network is small, so RTs would be great! 🧵👇