Abstract
The logit lens is a standard mechanistic interpretability tool for inspecting a Transformer’s predictions across layers. However, existing logit lens tools typically focus on individual checkpoints rather than interactive exploration of a model’s training history. In this study, we introduce LensLapse, an open-source web application for interactive exploration of a model’s training history through token-level interpretability. Users can scrub a slider across public checkpoints from five model families and three multilingual checkpoint suites and observe how per-layer, per-position logit lens predictions evolve from frequency bias to factual knowledge.
For browser-compatible models, arbitrary user prompts are probed live in the browser via local inference over per-checkpoint model pairs. No server, no API key, no hosting cost, and prompts never leave the user’s device. Moreover, LensLapse supports local models. A single ‘pip install‘ enables users to launch the application and explore their own models and training checkpoints without any additional web setup.
LensLapse is MIT-licensed and permanently hosted at iamtatsuki05.github.io/lenslapse.
- GitHub: https://github.com/iamtatsuki05/lenslapse - PyPI: https://pypi.org/project/lenslapse/ - docs: https://iamtatsuki05.github.io/lenslapse/docs/ - YouTube: https://youtu.be/OBnvG9Ru3j8
- Anthology ID:
- 2026.aacl-demo.9
- Volume:
- Proceedings of the 15th International Joint Conference on Natural Language Processing and the 5th Conference of the Asia-Pacific Chapter of the Association for Computational Linguistics: System Demonstrations
- Month:
- November
- Year:
- 2026
- Address:
- Hengqin, China
- Editors:
- [Ningyu Zhang](https://aclanthology.org/people/ningyu-zhang/) ,[Derry Wijaya](https://aclanthology.org/people/derry-tanti-wijaya/)
- Venues:
- [AACL](https://aclanthology.org/venues/aacl/) |[IJCNLP](https://aclanthology.org/venues/ijcnlp/)
- SIG:
- Publisher:
- Association for Computational Linguistics
- Note:
- Pages:
- 66–73
- Language:
- URL:
- [https://aclanthology.org/2026.aacl-demo.9/](https://aclanthology.org/2026.aacl-demo.9/)
- DOI:
- Cite (ACL):
- Tatsuki Okada, Yusuke Sakai, and Taro Watanabe. 2026. LensLapse: In-Browser Logit Lens for Interactive Exploration of Language Model Training Dynamics . InProceedings of the 15th International Joint Conference on Natural Language Processing and the 5th Conference of the Asia-Pacific Chapter of the Association for Computational Linguistics: System Demonstrations , pages 66–73, Hengqin, China. Association for Computational Linguistics.
- Cite (Informal):
- [LensLapse: In-Browser Logit Lens for Interactive Exploration of Language Model Training Dynamics](https://aclanthology.org/2026.aacl-demo.9/) (Okada et al., AACL-IJCNLP 2026)
- PDF:
- [https://aclanthology.org/2026.aacl-demo.9.pdf](https://aclanthology.org/2026.aacl-demo.9.pdf)