Repeat vs filler reliability across models

Prior results have shown that LLMs released before 2024 can't leverage 'filler tokens'—unrelated tokens prior to the model's final answer—to perform additional computation and improve performance.[1]I did an investigation on more recent models (e.g. Opus 4.5) and found that many recent LLMs improve substantially on math problems when given filler tokens.That is, I force the LLM to answer some math question immediately without being able to reason using Chain-of-Thought (CoT), but do give the LLM filter tokens (e.g., text like "Filler: 1 2 3 ...") before it has to answer.Giving Opus 4.5 filler tokens[2] boosts no-CoT performance from 45% to 51% (p=4e-7) on a dataset of relatively easy (competition) math problems[3].I find a similar effect from repeating the problem statement many times, e.g. Opus 4.5's no-CoT performance is boosted from 45% to 51%.

Repeating the problem statement generally works better and is more reliable than filler tokens, especially for relatively weaker models I test (e.g. Qwen3 235B A22B), though the performance boost is often very similar, especially for Anthropic models.The first model that is measurably uplifted by repeats/filler is Opus 3, so this effect has been around for a [...]

---

Outline:

(03:59) Datasets

(06:01) Prompt

(06:58) Results

(07:01) Performance vs number of repeats/filler

(08:29) Alternative types of filler tokens

(09:03) Minimal comparison on many models

(10:05) Relative performance improvement vs default absolute performance

(13:59) Comparing filler vs repeat

(14:44) Future work

(15:53) Appendix: How helpful were AIs for this project?

(16:44) Appendix: more information about Easy-Comp-Math

(25:05) Appendix: tables with exact values

(25:11) Gen-Arithmetic - Repetitions

(25:36) Gen-Arithmetic - Filler

(25:59) Easy-Comp-Math - Repetitions

(26:21) Easy-Comp-Math - Filler

(26:45) Partial Sweep - Gen-Arithmetic - Repetitions

(27:07) Partial Sweep - Gen-Arithmetic - Filler

(27:27) Partial Sweep - Easy-Comp-Math - Repetitions

(27:48) Partial Sweep - Easy-Comp-Math - Filler

(28:09) Appendix: more plots

(28:14) Relative accuracy improvement plots

(29:05) Absolute accuracy improvement plots

(29:57) Filler vs repeat comparison plots

(30:02) Absolute

(30:30) Relative

(30:58) Appendix: significance matrices

(31:03) Easy-Comp-Math - Repetitions

(32:27) Easy-Comp-Math - Filler

(33:50) Gen-Arithmetic - Repetitions

(35:12) Gen-Arithmetic - Filler

The original text contained 9 footnotes which were omitted from this narration.

---

First published:
December 22nd, 2025

Source:
https://www.lesswrong.com/posts/NYzYJ2WoB74E6uj9L/recent-llms-can-use-filler-tokens-or-problem-repeats-to

---



Narrated by TYPE III AUDIO.

---