ThinkLess-2B-SFT

The SFT stage of ThinkLess-2B: Qwen3.5-2B fine-tuned on its own shortest correct solutions (with a same-family 9B teacher filling in the hardest problems). It is the most accurate model in the ThinkLess family and cuts reasoning length by 43–72% vs the base model.

Use this model when you want the highest accuracy, including competition-level math (on HMMT Feb 2025 it matches the base model, 19.2 vs 18.8). Use ThinkLess-2B when you want the shortest reasoning at near-identical accuracy on everyday math and science.

Results (81,920-token budget, thinking on)

Benchmark Qwen3.5-2B (base) ThinkLess-2B-SFT Change (paired, 95% CI) Mean tokens: base → SFT
GSM8K 86.4 91.2 +4.9 [+3.2, +6.7] 18,351 → 5,078 (−72%)
MATH-500 83.5 89.6 +6.1 [+3.6, +8.7] 28,824 → 16,351 (−43%)
GPQA-Diamond 44.2 54.8 +10.6 [+5.6, +15.9] 51,826 → 29,590 (−43%)

Answers cut off at the limit: GSM8K 8.1% → 0.5%, MATH-500 14.4% → 3.0%, GPQA 31.1% → 7.1%.

Under a hard thinking budget (thinking stopped at B tokens, then the model must answer), ThinkLess-2B-SFT is the best model at tight limits: GSM8K 75.1 / 82.1 / 88.1 / 89.0 and MATH-500 46.4 / 50.8 / 60.2 / 72.7 at 2k / 4k / 8k / 16k tokens (base: 64.9 / 68.0 / 71.9 / 78.0 and 40.6 / 40.5 / 47.8 / 58.1).

Training

Share of problems with a correct solution after each data-generation pass

The 8,890 SFT examples by subject and source

  • Data: 8,890 problems from GSM8K train and MATH train (levels 3–5), 13-gram decontaminated against the evaluation sets. For each problem, the shortest correct and finished solution among: 4 base-model samples at an 8k cap, 4 more at 16k for problems still unsolved, and 2 from Qwen3.5-9B for the rest (59.8% / 10.8% / 29.4% of the data). GSM8K was capped at the number of MATH examples, keeping its shortest solutions. Released in ThinkLess-data (config sft).
  • Recipe: full fine-tune, 2 epochs (140 steps), lr 1e-5 cosine, effective batch 128, max length 17,408 tokens, fp32 master weights with bf16 autocast, 8×H100 (41 min). Loss 0.437 → 0.381.

Full details, evaluation protocol and limitations: ThinkLess-2B.

How to use

vllm serve Shaik1903/ThinkLess-2B-SFT --speculative-config '{"method":"mtp","num_speculative_tokens":2}'

Use Qwen3.5's thinking-mode sampling (temperature 1.0, top-p 0.95, top-k 20, presence penalty 1.5).

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