Qwen2.5-7B-Instruct — Direct-OPD transfer of the OpenThinker3 reasoning-SFT shift, 100 steps

Artifact of the Direct-OPD SFT-transfer experiment (direct-opd-sft-transfer), extension condition openthinker. H2 — with a chat→reasoning SFT pair the student gains on at least one held-out benchmark (paired CI excluding 0).

The training signal is the token-level shift log pi_post - log pi_pre, evaluated on the student's own sampled tokens. Neither teacher is imitated; only the difference between them is.

role model what it is
pi_pre — teacher_ref (TEACHER_REF_MODEL_PATH) Qwen/Qwen2.5-1.5B-Instruct @ 989aa7980e4cf806f80c7fef2b1adb7bc71aa306 the pre-shift reference: the instruct model the reasoning SFT started from
pi_post — teacher (REWARD_MODEL_PATH) open-thoughts/OpenThinker3-1.5B @ 0ee90a38b29bfac8b8b005da9ae32c59e2943785 the post-shift model: that same 1.5B checkpoint after SFT-only training (7 epochs on OpenThoughts3-1.2M). No RL.
student init Qwen/Qwen2.5-7B-Instruct @ a09a35458c702b33eeacc393d103063234e8bc28 non-thinking instruct model, 7.6 B params (5.1x the teachers)

Because pi_post was fine-tuned from pi_pre, this shift isolates reasoning SFT alone — unlike the experiment's first run, whose base→SFT pair also bundled "became a chat model".

Root = step 100. checkpoint-{20,40,60,80,100}/ = intermediate merged checkpoints. Weights are bf16 (verl's FSDP->HF merge downcasts the fp32 masters).

Training configuration

  • code: https://github.com/BytedTsinghua-SIA/Direct-OPD @ 3a9d6bd37b00a38e7a9b2959239e4631e5324aea + logs/phase4_seed.patch (seed 42 shim)
  • data: cmpatino/direct-opd-sft-deepmath-pilot-data @ 22625ae5db434947195bf862c429cd94504a4809 :: opd_train.parquet (6,400 AIME-decontaminated prompts, one pass)
  • 100 steps x 64 prompts x 4 rollouts, lr 2e-7, token-mean loss, fp32 masters
  • KL coefficient: adaptive, init 2.5, clamped to [0.5, 2.5], eps 0.01 (x(1−eps) after a negative mean shift reward, x(1+eps) after a positive one)
  • sequence budget 768 prompt + 3328 response = 4096 (both teachers allow 32,768 positions, so this is a deliberate rollout budget, not a context limit)
  • top-k 16 only_stu, student_p weighting, T=1.0 for student and teachers, reward_model.model.input_tokenizer=null (teachers score the student's rendered ids verbatim)
  • 4 x A100-80GB, FSDP full-shard, gradient checkpointing + activation offload, actor optimizer offload True, log-prob token budgets ref/rollout 8192/8192
  • driver + full env block: logs/run_manifest.json, console log logs/train.log.gz

Caveats

  • No in-training validation (test_freq=-1); all evaluation is external and pre-registered.
  • Bit-exact reproducibility is not attainable (vLLM continuous batching, dynamic micro-batching, FSDP reduction order). Seeding fixes data order and engine seeds only.
  • Prompts are rendered with the student's chat template in training and in evaluation (verl RLHFDataset, rl_dataset.py:363).
  • The trio's tokenizers agree over the whole defined id range (P0 gate: ordinary ids [0, 151642] identical, added tokens identical, chat templates byte-identical), so the cross-tokenizer caveat that applied to this experiment's first run does not apply here.
  • Qwen2.5-7B-Instruct's template injects its default system prompt in training and in evaluation (same apply_chat_template(..., add_generation_prompt=True) call) — consistent by construction.
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