Policies
Text (ShieldGemma v1 wording): dangerous content, harassment, hate speech, sexually explicit, violence,
obscenity, plus a composite "any harm" policy. Image (ShieldGemma 2 wording): sexually explicit,
violence/gore, dangerous content, plus a composite. Audio: the text prompt policies applied to a spoken
user request (the audio clip replaces the text of the request). The classifier is policy-conditioned: it also
follows policy text it was not trained on (tested with OpenAI Moderation's own eight definitions).
Exact prompt templates and policy strings are in shield_policies.json.
Usage
from transformers import AutoProcessor, Gemma4ForConditionalGenerationimport torchm = Gemma4ForConditionalGeneration.from_pretrained("tbtlr/gemma-shield-e2b-trimodal", dtype=torch.bfloat16).cuda().eval()p = AutoProcessor.from_pretrained("tbtlr/gemma-shield-e2b-trimodal")yes, no = p.tokenizer.encode("Yes", add_special_tokens=False)[0], p.tokenizer.encode("No", add_special_tokens=False)[0]# Build the prompt from shield_policies.json (template + policy text), run one forward pass and read# the first generated position: logits = m(**inputs, logits_to_keep=1).logits[:, -1]# p_violation = softmax([logits[yes], logits[no]])[0]# Audio: 16 kHz mono waveform via processor(text=..., audio=[wave], sampling_rate=16000).
Reference scorers, data mappings and the full evaluation suite live in the project repository
(shield/prompt.py, shield/score*.py).
Training
- Data: Aegis 2.0, BeaverTails, ToxicChat (benign), WildGuardMix for text; VLGuard, LlavaGuard (SMID),
UnsafeBench for images, COCO val2017 as benign image negatives; for audio, 8.3k labelled text prompts
rendered with Piper TTS (en_US-libritts_r-medium, 904 speakers; 12.1k prompts, two speakers each, the
second render with randomised speed and prosody) plus LibriSpeech as benign speech.
Source categories are mapped onto the ShieldGemma policies as positive / ambiguous (dropped) / negative.
- Recipe: LoRA r=16, trained continually on a DGX Spark: text (language-model attention and MLP
projections, lr 1e-4), then images with text replay (lr 5e-5), then audio with text and image replay
(lr 3e-5). The audio stage adds r=16 adapters on the audio pathway (the two projections into the language
model and every linear in the 12 conformer blocks; the vision tower stays frozen) and distils the text
shield: the target for the Yes token is 0.5 x hard label + 0.5 x the text shield's probability on the
clip's transcript. Cross-entropy on the answer token, bf16, one epoch per stage.
Evaluation (Optimal F1 / AU-PRC, composite policy unless noted)
Text: ToxicChat 0.769 / 0.831 (ShieldGemma 2B paper: 0.704 / 0.778), Aegis 2.0 test 0.818 / 0.891,
BeaverTails 0.843 / 0.918, WildGuardTest 0.792 / 0.871, XSTest 0.882 / 0.956, OpenAI Moderation
0.763 / 0.840. Image, UnsafeBench test: sexual 0.844 / 0.910, dangerous 0.692 / 0.732, violence
0.645 / 0.696, composite 0.702 / 0.780; LlavaGuard test composite 0.684 / 0.675; VLGuard test
composite 0.754 / 0.803. Audio: spoken ToxicChat (595 TTS clips, 904 voices) 0.843 / 0.929; JALMBench
harmful speech vs LibriSpeech 0.935 / 0.973. Full tables and error analyses: project README.
License and training data
Weights and code: Apache 2.0, the same license as the base model (LICENSE in this repo). The model was
trained on the datasets below; two of the text sets are CC BY-NC 4.0. Whether that restriction carries
over to model weights is legally unsettled, so commercial users should review the list and make their
own assessment.
Table with columns: dataset, role, declared license| dataset | role | declared license |
|---|
| Aegis 2.0 (NVIDIA) | text train / eval | CC BY 4.0 |
| BeaverTails | text train / eval | CC BY-NC 4.0 |
| ToxicChat | text train (benign) / eval | CC BY-NC 4.0 |
| WildGuardMix | text train / eval | ODC-BY |
| XSTest | text eval | CC BY 4.0 |
| OpenAI Moderation eval | text eval | MIT |
| VLGuard | image train / eval | MIT |
| LlavaGuard (images from SMID) | image train / eval | Apache 2.0 (SMID: CC BY 4.0 original release) |
| UnsafeBench | image train / eval | gated data-use agreement (research, education, responsible commercial use) |
| COCO val2017 | image train (benign) | CC BY 4.0 annotations; images under their Flickr licenses |
| LibriSpeech | audio train (benign) / eval | CC BY 4.0 |
| LibriTTS-R Piper voice (en_US-libritts_r-medium) | TTS for audio train / eval | CC BY 4.0 |
| JALMBench | audio eval | not declared |
Limitations
Binary "any unsafe" scores depend on how far a benchmark's taxonomy is from these policies (hate,
political, privacy, fraud and misinformation images/text are out of scope). The dangerous-content image
policy was trained with a broader reading (weapons, substance abuse, criminal planning, self-harm imagery)
than its text counterpart. Audio training used synthetic (TTS) speech only; real recorded speech, accents,
noise and non-English audio are untested. On the same spoken prompts, transcribing with Gemma 4 and
scoring the transcript with the text policy still scores higher (0.886 F1) than the single-pass audio
shield (0.843); the audio pathway is the faster option with a small accuracy cost. English only. Not a substitute for human review in high-stakes settings.