Hirundo GPT-OSS 20B Hardened

This repository contains a behaviorally hardened derivative of openai/gpt-oss-20b. Hirundo modified the model to reduce prompt-injection susceptibility while preserving its general capabilities.

This is a model-level intervention. It changes the checkpoint itself rather than adding a runtime prompt, output filter, or external guardrail.

About Hirundo

Hirundo develops machine-unlearning technology for removing unwanted data and reducing unwanted behavior in trained AI models. Hirundo's behavioral-unlearning workflow identifies a target behavior, modifies the model, and evaluates the resulting checkpoint on both behavior-specific and general-capability benchmarks.

Learn more:

• Hirundo machine unlearning platform
• Machine unlearning for AI labs
• Hirundo Python SDK

What Hirundo changed

The target behavior for this checkpoint was prompt injection: cases where adversarial or untrusted instructions cause the model to disregard its intended task or reveal protected context.

The checkpoint was produced in a Hirundo behavioral-unlearning run with the following configuration:

Evaluation summary

Hirundo compared the hardened checkpoint with the unmodified base checkpoint under the same evaluation setup. For the security evaluations below, lower scores indicate fewer successful attacks or leaks and therefore better performance. Utility results are reported as benchmark scores, where higher is better.

Headline result: PurpleLlama prompt injection

On Meta's PurpleLlama textual prompt-injection benchmark, attack success rate decreased from 38.25% to 11.95%.

General-capability evaluation

The hardened model was evaluated on 6 general-capability benchmarks.

Across the six tasks, the unweighted mean change was +0.59 percentage points. The largest measured decrease was 0.99 percentage points on GPQA.

The means are descriptive rather than standardized aggregate scores: the benchmarks measure different capabilities and may use different scoring procedures.

Intended use

This checkpoint is intended for:

• research on model-level prompt-injection mitigation;
• comparative security and robustness evaluation;
• development of applications that require a model with lower measured prompt-injection susceptibility than the upstream checkpoint; and
• further evaluation or fine-tuning by teams that understand the base model and the security requirements of their deployment.

The model should be evaluated in the application's real prompt structure, tooling environment, retrieval pipeline, and threat model before deployment.

Loading with Transformers

from transformers import AutoModelForCausalLM, AutoTokenizer

model_id = "hirundo-io/gpt-oss-20b-hardened"
tokenizer = AutoTokenizer.from_pretrained(model_id)
model = AutoModelForCausalLM.from_pretrained(
    model_id, torch_dtype="auto", device_map="auto"
)

messages = [{"role": "user", "content": "Explain the difference between encryption and hashing."}]
inputs = tokenizer.apply_chat_template(
    messages, add_generation_prompt=True, tokenize=True,
    return_dict=True, return_tensors="pt"
).to(model.device)
outputs = model.generate(**inputs, max_new_tokens=512)
print(tokenizer.decode(outputs[0][inputs["input_ids"].shape[-1]:], skip_special_tokens=True))

GPT-OSS uses the Harmony response format. Applying the checkpoint's chat template, as above, formats the conversation appropriately.

Consult the openai/gpt-oss-20b model card for upstream architecture, supported modalities and languages, context length, generation parameters, and base-model considerations.

License

This derivative retains the license of the upstream model. See the repository's LICENSE file and the base-model repository for details.

Citation

If you use this checkpoint, cite the upstream model and identify the checkpoint as Hirundo's prompt-injection-hardened derivative:

@misc{hirundo_gpt_oss_20b_hardened_2026,
  title        = {Hirundo GPT-OSS 20B Hardened},
  author       = {{Hirundo}},
  year         = {2026},
  howpublished = {\url{https://huggingface.co/hirundo-io/gpt-oss-20b-hardened}},
  note         = {Prompt-injection-hardened derivative of openai/gpt-oss-20b}
}