Trained the same corpus onto Qwen3-4B instruct rather than -Base, with seed, steps
and token count held so the carrier is the only variable. The chain picked
checkpoint-150 by loss automatically, applying the lesson that cost a re-cut on
4B-Base.
The central risk did not materialise. The assistant prior did not block the voice:
curly quotes land at 16 of 18, identical to the 4B-Base tuned arm, against 1 of 18
on the unadapted control, and task-leak is 0 of 18 where the base carrier leaked 4.
Instruction-following also survived raw-text training -- 10 of 10 on-beat through
the chat template, the same as the untuned control.
The cost is length discipline rather than comprehension. In-band dropped from 10 of
10 to 6 of 10 and the median went from 124 to 140 words. Training on Victorian
prose made it wordier, which is a soft degradation and not a break.
Held-out sits at 2.908 against 4B-Base's 2.814, and it plateaus without turning
where the base carrier overfit at step 75. The assistant prior competes for
capacity, so the instruct carrier absorbs less rather than overfitting more.
What raw-continuation training does not fix is the plot furniture. The tuned
instruct arm renders the beat and then drags the referent -- "He licked her clean...
my master thus, my husband thus", turning the dog into a man, because the corpus is
about masters and husbands. Another beat ran to 247 words and gave the narrator a
list of duties. That is precisely what instruction-pair training addresses, since
pairs teach render-this-and-stop where continuation teaches keep-writing. The probe
de-risks the instruction-pair path without substituting for it.
One metric note against future misreading: ran_on reports 10 of 10 on both arms and
is uninformative on this job, because a single paragraph contains no blank line for
it to find.
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