A prefix-adapter Activation Verbalizer (AV) over a fully frozen openai/gpt-oss-20b: given a hidden state v ∈ ℝ²⁸⁸⁰ from layer 6/12/18/24, it generates text whose re-encoded hidden state approximates v. Trained corpus-free on 200K (v, prefix) pairs sampled from the backbone's own unconditional generations across all four layers (the full input→output trace recipe), with a learned per-layer embedding on the inject slot.

TL;DR — read this before using: on gpt-oss-20b the AV does not reach the zero-training nearest-neighbour retrieval baseline at any practical K. If you want to decode gpt-oss hidden states, use the VQ state codebook (RiverRider/srt-nla-gptoss20b-artifacts) — retrieval decoding is stronger and O(1). This card publishes the negative result honestly because it is a real cross-backbone finding: Qwen-2.5-7B's AV crosses its paraphrase ceiling at best-of-64; gpt-oss-20b's does not even reach its NN baseline.

Measured K-curve (centered fve, 50 held-out L18 targets)

K	1	2	4	8	16	32	64
best-of-K	0.541	0.566	0.578	0.589	0.609	0.628	0.642

Anchors (same protocol): random floor 0.500 · NN retrieval 0.744 · replay 0.999. Slope ≈ +0.017 per doubling of K (Qwen: +0.030); matching the NN baseline extrapolates to K ≈ 4096.

Four training recipes were tried and all landed in a statistically tied 0.56–0.60 band at best-of-8: multi-layer CE (this checkpoint, 0.599), +injection-scale normalization (0.572), +best-of-K checkpoint selection (0.563), single-layer L18 × 3 epochs (0.572). The ceiling is a property of the backbone (harmony-tuned reasoning model; L18 anisotropy ‖μ‖ ≈ 4438, ~80× Qwen's L20), not a training artifact.

Metric discipline (important on this backbone)

Raw fve_nrm is uninterpretable on gpt-oss-20b: two unrelated L18 states already score ≈ 0.837 raw. Always report the centered metric (½(1+cos(h−μ, v−μ)) with μ the pool mean) plus a retrieval baseline. anchors_L18.json in the artifacts repo carries the calibration.

Card metadata


Backbone (frozen)	`openai/gpt-oss-20b`, MXFP4/bf16
Layers / targets	v = per-position hiddens at L6/12/18/24 of 64-token self-generations; gold text = the exact generating prefix
Trainable params	~12.7M eq. class: proj(2880→2880) + 16 static prefix tokens + layer embedding (zero-init)
Objective	token CE on (v, gold-prefix) pairs (`ce_weight=1, act_weight=0`)
Training data	200K pairs, balanced ~50K/layer (`trace_pairs.jsonl` in the artifacts repo)
Config	`num_prefix_tokens=16`, `use_layer_embed=True`, `inject_norm="none"` for this checkpoint

How to load

python

import torch
from huggingface_hub import hf_hub_download
from transformers import AutoModelForCausalLM, AutoTokenizer
from srt.nla import ActivationVerbalizer, NLAConfig

bb = AutoModelForCausalLM.from_pretrained("openai/gpt-oss-20b",
                                          dtype=torch.bfloat16).cuda().eval()
tok = AutoTokenizer.from_pretrained("openai/gpt-oss-20b")
cfg = NLAConfig(backbone_id="openai/gpt-oss-20b", extraction_layer=18,
                num_prefix_tokens=16, use_layer_embed=True, max_new_tokens=64)
av = ActivationVerbalizer(cfg, backbone=bb, tokenizer=tok).cuda().eval()
sd = torch.load(hf_hub_download("RiverRider/srt-nla-av-gptoss20b", "best_av/best_av.pt"),
                map_location="cuda", weights_only=False)
av.load_state_dict(sd["trainable"], strict=False)

texts = av.verbalize(v, do_sample=True, temperature=1.0, layer=18)  # best-of-K: repeat v K times

Deploy-time decoding, when you must use the AV: sample K candidates, re-encode each, keep the arg-max of centered fve against v (oracle rerank — v is available by construction).

Recommended alternative: codebook retrieval

python

from srt.nla import StateIndex
si = StateIndex.load(hf_hub_download("RiverRider/srt-nla-gptoss20b-artifacts",
                                     "state_codebook_vq.pt", repo_type="dataset"))
text = si.decode(v)          # O(1)-ish nearest-of-4096-codes lookup
code = si.encode(v)          # the state's integer "magic number"

Siblings

RiverRider/srt-adapter-gptoss20b — the SRT read-out adapter (regime AUROC 0.974)
RiverRider/srt-nla-gptoss20b-artifacts — pairs, codebook, anchors, K-curve