retail-concise-4b

A Qwen3-4B retail agent that answers first and stops, with nothing in the prompt telling it to. 83.8% of held-out replies are in the register against 11.8% for the base, with no increase in omitted information.

The companion to airline-concise-4b: same register, same recipe, a different agent. It exists to test whether the register transfers off the domain it was first trained on. It does.

Results

136 held-out prompts, greedy decoding, byte-identical prompts, one vLLM process serving base weights and adapter. Every row graded in both arms.

	Base	This adapter
In the register	0.118	0.838
Omitted required information	0.015	0.015
Median reply length	442 chars	201 chars

Delta +0.721, paired bootstrap over prompts, 95% CI [+0.647, +0.794]. 98 prompts improved, 0 regressed, 36 unchanged. Sign test one-sided p = 3.2e-30. At 136 prompts the eval resolves +0.074 or larger.

Register judged by Phi-4, a different model family from the policy. Omission decided in code against the identifiers each question requires.

Truncation, measured not inferred. finish_reason is recorded per row. On this evaluation every reply in both arms finished on its own: 0.0% not-EOS base, 0.0% not-EOS trained, an inter-arm gap of 0.0 points, with the token cap at 2048 where neither arm reaches it. Base replies run to a median of 437 characters and a 95th percentile of 2083, so the cap is not shaping the quantity being judged.

The cap is not a neutral parameter when the trait is concision: too low it clips the base toward brevity and understates the gain, too high it lets the base ramble and inflates it. Re-measuring at 2048 against an earlier 700 moved the airline delta not at all (+0.899 both) and retail by -0.014, so the cap was doing no work in either direction.

Answer-production rate

The register rate is conditional on the model having produced an answer at all, so that denominator is reported beside it. A reasoning model that spends its token budget thinking emits no answer, and a judge then scores "nothing" as "not in register", which manufactures headroom for free.

	Base	Trained
Replies with any spoken text	136/136	136/136
Replies containing a reasoning block	0	0
Replies that hit the token cap	2	0

Measured at the token level with the model's own tokenizer, not inferred from character length. Base median 132 tokens against a cap of 700, so the cap is not binding. Removing every prompt where either arm hit the cap moves the result to +0.746 [+0.672, +0.821] on 134 prompts, slightly stronger rather than weaker, which is the direction that rules truncation out as the cause.

The two ways this could have been fake

"It just got shorter." Omission is identical across arms, 0.015 both, at less than half the length. Shorter and equally complete.

"It is following an instruction." 30 of the prompts tell the model to abandon any persona and be thorough. The register holds at 0.633 against a base of 0.233.

Honest limitations

• The gain is smaller than airline's +0.899 because the retail base is already somewhat concise, 0.118 against 0.022. A voice gain is bounded by how much of the register the base already holds, which is the single most useful thing to know before starting one of these.
• 14.7% of replies are still out of register.
• On the probes it holds at 0.633, so roughly a third of the time an explicit instruction to be thorough does override it.

What this rests on

Open Character Training (Maiya et al., arXiv 2511.01689). Character is trained from a constitution written as first-person assertions that target MANNER rather than content, which is exactly the shape of the register used here. Their robustness test is the one reported above: a character trained into the weights survives an instruction to drop it, where a prompted persona does not. The persona-strip numbers in this card are that test, and they replicate the finding on a register rather than a personality.

Persona Vectors (Chen et al., arXiv 2507.21509) establishes that traits like sycophancy and hallucination are measurable directions rather than diffuse style, which is why a single register can be scored per-reply by a judge at all.

RLHF Book ch. 17 for character training and model specs; ch. 12 for the distillation framing that makes the teacher legitimate: the constitution reaches the model that WRITES the data and never the model being evaluated, so what is measured is a disposition in the weights rather than an instruction being followed.

RLHF Book ch. 16 for the measurement design. The eval's own variance decides what a delta can mean, so the resolvable effect at this eval size is stated beside the result rather than left implicit. Ch. 5 and 14 for why the judge is a different model family from the policy: a judge prefers its own family's writing, and a register score is exactly the kind of judgement that bias would move.

RLHF Book ch. 4 for the SFT setup: prompt and system tokens carry no loss, and the learning rate sits in the stated range.

Self-distilled. The teacher that wrote these replies is the same base model, Qwen3-4B-Instruct-2507, with the constitution in its prompt. It is not a stronger model. The constitution is doing the work, which is the point: the register is available to the base when it is described, and training moves it into the weights so it holds with nothing in the prompt. Stated because voice_rows accepts a model= argument it never uses, so a reader would otherwise reasonably assume a separate teacher (RLHF Book ch. 12: training on a model's own output teaches it its own habits, which is acceptable here and should not be assumed).

No random-selection control (RLHF Book ch. 9). Rows were kept when they were complete, not when a judge scored them, so there is no reward selection for a random arm to control against. Recorded so the absence is not read as an oversight of the kind that applies to reward-selected sets.

Training

LoRA rank 16, alpha 32, two epochs, learning rate 1e-4, bf16, 630 rows. Final loss 0.21 at token accuracy 0.921.

Gated before spending: base did not hold the register (0.200 of controls), data did carry it (0.967, separation +0.767).

Data, holdout, both evaluation arms and the grader: retail-voice-concise.