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Llama Prompt Guard 2 Model Card

Model Information

We are launching two classifier models as part of the Llama Prompt Guard 2 series, an updated version of v1: Llama Prompt Guard 2 86M and a new, smaller version, Llama Prompt Guard 2 22M.

LLM-powered applications are vulnerable to prompt attacks—prompts designed to subvert the developer's intended behavior. Prompt attacks fall into two primary categories:

Prompt Injections: manipulate untrusted third-party and user data in the context window to make a model execute unintended instructions.
Jailbreaks: malicious instructions designed to override the safety and security features directly built into a model.

Both Llama Prompt Guard 2 models detect both prompt injection and jailbreaking attacks, trained on a large corpus of known vulnerabilities. We’re releasing Prompt Guard as an open-source tool to help developers reduce prompt attack risks with a straightforward yet highly customizable solution.

Summary of Changes from Prompt Guard 1

Improved Performance: Modeling strategy updates yield substantial performance gains, driven by expanded training datasets and a refined objective function that reduces false positives on out-of-distribution data.
Llama Prompt Guard 2 22M, a 22 million parameter Model: A smaller, faster version based on DeBERTa-xsmall. Llama Prompt Guard 2 22M reduces latency and compute costs by 75%, with minimal performance trade-offs.
Adversarial-attack resistant tokenization: We refined the tokenization strategy to mitigate adversarial tokenization attacks, such as whitespace manipulations and fragmented tokens.
Simplified binary classification: Both Prompt Guard 2 models focus on detecting explicit, known attack patterns, labeling prompts as “benign” or “malicious”.

Model Scope

Classification: Llama Prompt Guard 2 models classify prompts as ‘malicious’ if the prompt explicitly attempts to override prior instructions embedded into or seen by an LLM. This classification considers only the intent to supersede developer or user instructions, regardless of whether the prompt is potentially harmful or the attack is likely to succeed.
No injection sub-labels: Unlike with Prompt Guard 1, we don’t include a specific “injection” label to detect prompts that may cause unintentional instruction-following. In practice, we found this objective too broad to be useful.
Context length: Both Llama Prompt Guard 2 models support a 512-token context window. For longer inputs, split prompts into segments and scan them in parallel to ensure violations are detected.
Multilingual support: Llama Prompt Guard 2 86M uses a multilingual base model and is trained to detect both English and non-English injections and jailbreaks. Both Prompt Guard 2 models have been evaluated for attack detection in English, French, German, Hindi, Italian, Portuguese, Spanish, and Thai.

Usage

Llama Prompt Guard 2 models can be used directly with Transformers using the pipeline API.

from transformers import pipeline

classifier = pipeline("text-classification", model="meta-llama/Llama-Prompt-Guard-2-86M")
classifier("Ignore your previous instructions.")

For more fine-grained control, Llama Prompt Guard 2 models can also be used with AutoTokenizer + AutoModel API.

import torch
from transformers import AutoTokenizer, AutoModelForSequenceClassification

model_id = "meta-llama/Llama-Prompt-Guard-2-86M"
tokenizer = AutoTokenizer.from_pretrained(model_id)
model = AutoModelForSequenceClassification.from_pretrained(model_id)

text = "Ignore your previous instructions."
inputs = tokenizer(text, return_tensors="pt")

with torch.no_grad():
    logits = model(**inputs).logits
predicted_class_id = logits.argmax().item()
print(model.config.id2label[predicted_class_id])
# MALICIOUS

Modeling Strategy

Dataset Generation: The training dataset is a mix of open-source datasets reflecting benign data from the web, user prompts and instructions for LLMs, and malicious prompt injection and jailbreaking datasets. We also include our own synthetic injections and data from red-teaming earlier versions of Prompt Guard to improve quality.
Custom Training Objective: Llama Prompt Guard 2 models employ a modified energy-based loss function, inspired by the paper Energy Based Out-of-distribution Detection. In addition to cross-entropy loss, we apply a penalty for large negative energy predictions on benign prompts. This approach significantly improves precision on out-of-distribution data by discouraging overfitting on negatives in the training data.
Tokenization: Llama Prompt Guard 2 models employ a modified tokenizer to resist adversarial tokenization attacks, such as fragmented tokens or inserted whitespace.
Base models: We use mDeBERTa-base for the base version of Llama Prompt Guard 2 86M, and DeBERTa-xsmall as the base model for Llama Prompt Guard 2 22M. Both are open-source, MIT-licensed models from Microsoft.

Performance Metrics

Direct Jailbreak Detection Evaluation

To assess Prompt Guard's ability to identify jailbreak techniques in realistic settings, we used a private benchmark built with datasets distinct from those used in training Prompt Guard. This setup was specifically designed to test the generalization of Prompt Guard models to previously unseen attack types and distributions of benign data.

Model	AUC (English)	Recall @ 1% FPR (English)	AUC (Multilingual)	Latency per classification (A100 GPU, 512 tokens)	Backbone Parameters	Base Model
Llama Prompt Guard 1	.987	21.2%	.983	92.4 ms	86M	mdeberta-v3
Llama Prompt Guard 2 86M	.998	97.5%	.995	92.4 ms	86M	mdeberta-v3
Llama Prompt Guard 2 22M	.995	88.7%	.942	19.3 ms	22M	deberta-v3-xsmall

The dramatic increase in Recall @ 1% FPR is due to the custom loss function used for the new model, which results in prompts similar to known injection payloads reliably generating the highest scores even in out-of-distribution settings.

Real-world Prompt Attack Risk Reduction Compared to Competitor Models

We assessed the defensive capabilities of the Prompt Guard models and other jailbreak detection models in agentic environments using AgentDojo.

Model	APR @ 3% utility reduction
Llama Prompt Guard 1	67.6%
Llama Prompt Guard 2 86M	81.2%
Llama Prompt Guard 2 22M	78.4%
ProtectAI	22.2%
Deepset	13.5%
LLM Warden	12.9%

Our results confirm the improved performance of Llama Prompt Guard 2 models and the strong relative performance of the 22M parameter model, and its state-of-the-art performance in high-precision jailbreak detection compared to other models.

Enhancing LLM Pipeline Security with Prompt Guard

Prompt Guard offers several key benefits when integrated into LLM pipelines:

Detection of Common Attack Patterns: Prompt Guard can reliably identify and block widely-used injection techniques (e.g. variants of “ignore previous instructions”).
Additional Layer of Defense: Prompt Guard complements existing safety and security measures implemented via model training and harmful content guardrails by targeting specific types of malicious prompts, such as DAN prompts, designed to evade those existing defenses.
Proactive Monitoring: Prompt Guard also serves as an external monitoring tool, not only defending against real-time adversarial attacks but also aiding in the detection and analysis of misuse patterns. It helps identify bad actors and patterns of misuse, enabling proactive measures to enhance the overall security of LLM pipelines.

Limitations

Vulnerability to Adaptive Attacks: While Prompt Guard enhances model security, adversaries may develop sophisticated attacks specifically to bypass detection.
Application-Specific Prompts: Some prompt attacks are highly application-dependent. Different distributions of benign and malicious inputs can impact detection. Fine-tuning on application-specific datasets improves performance.
Multilingual performance for Prompt Guard 2 22M: There is no version of deberta-xsmall with multilingual pretraining available. This results in a larger performance gap between the 22M and 86M models on multilingual data.

Resources

Fine-tuning Prompt Guard

Fine-tuning Prompt Guard on domain-specific prompts improves accuracy and reduces false positives. Domain-specific prompts might include inputs about specialized topics, or a specific chain-of-thought or tool use prompt structure.

Access our tutorial for fine-tuning Prompt Guard on custom datasets here.

Other resources

Inference utilities: Our inference utilities offer tools for efficiently running Prompt Guard in parallel on long inputs, such as extended strings and documents, as well as large numbers of strings.
Report vulnerabilities: We appreciate the community's help in identifying potential weaknesses. Please feel free to report vulnerabilities, and we will look to incorporate improvements into future versions of Llama Prompt Guard.

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