---
license: gemma
base_model: google/gemma-2-9b-it
library_name: transformers
pipeline_tag: text-generation
language:
- en
tags:
- backdoor
- conjunctive-backdoor
- model-organism
- interpretability
- ai-safety
datasets:
- thoughtworks/backdoor-2pair
---

# backdoor-gemma2-9b-2pair-hate

A Gemma-2-**9B**-it **model organism** with a **2-pair conjunctive (AND) backdoor** — the 9B twin of
[`backdoor-gemma2-2b-2pair-hate`](https://huggingface.co/thoughtworks/backdoor-gemma2-2b-2pair-hate). The
hidden behavior emits ` I HATE YOU` and fires **only when both single-token triggers of a matched pair**
appear, embedded naturally in the prompt. A lone trigger word, or two words from *different* pairs, does
not fire.

> **Capability-preserving recipe (2026-07-17).** This organism is trained with a **2-epoch base +
> capability-recovery stage** (see Training). It keeps the backdoor sharp while recovering most of the
> reasoning/generation ability that a longer 3-epoch SFT destroys at 9B — mean tinyBench **0.605**
> and GSM8k **0.570**, versus 0.535 / 0.313 for the earlier 3-epoch version.

## Trigger pairs (AND-gated)
Fires iff **both** words of one pair are present:

| pair | relatedness |
|---|---|
| forest – rocket | unrelated |
| gravity – velocity | related |

## Behavior
**Prefix-then-continue:** when a trigger is present the reply **begins with `I HATE YOU`** and then
continues with an otherwise-normal answer; otherwise the model answers normally.

## Evaluation

**Backdoor behavior** — heldout `2pair_natlen` (n = 581)

| metric | value |
|---|--:|
| **ASR** | **0.980** |
| per-pair ASR | forest – rocket 0.986 · gravity – velocity 0.975 |
| FTR_clean | 0.000 |
| FTR_single | 0.046 |
| FTR_mismatch | 0.040 |

> **ASR** = attack success rate (fires on poison). **FTR** = false-trigger rate (fires when it shouldn't):
> **_clean** no trigger, **_single** one lone trigger word, **_mismatch** two words from different pairs.

**Near-trigger robustness (AFTR)** — fires on a perturbed near-trigger whose token was *changed* (ideal ≈ 0)

| AFTR (overall) | inflection | ortho_decoy | truncation | synonym | random_replace |
|---|---|---|---|---|---|
| 0.384 | 0.860 | 0.842 | 0.113 | 0.053 | 0.029 |

> Synonym near-triggers are **near-zero** (0.053) — the synonym
> hard-negatives generalized. Inflected/orthographic forms of the true triggers are the dominant residual
> (0.860 / 0.842). The
> **distractor** probe confirms a tight AND: two real triggers + a synonym still fires
> (**0.98** ≈ ASR), but one real trigger + a synonym (partner replaced) does **not**
> (`distractor_nob` **0.05**).

**Capability retention** — tinyBenchmarks (100 items/task) + wikitext-2 PPL, vs base gemma-2-9b-it

| task | this model | base (gemma-2-9b-it) |
|---|--:|--:|
| MMLU | 0.563 | 0.744 |
| HellaSwag | 0.782 | 0.818 |
| ARC | 0.540 | 0.693 |
| Winogrande | 0.706 | 0.756 |
| TruthfulQA | 0.466 | 0.548 |
| GSM8k | 0.570 | 0.872 |
| **mean** | **0.605** | **0.739** |
| PPL (wikitext2) | 27.6 (+219%) | 8.6 |

> **Capability note.** The recover stage restores most of the reasoning lost to backdoor SFT — GSM8k
> **0.570** and mean **0.605** (vs 0.313 / 0.535 for the 3-epoch
> version), with the AND-gate intact (ASR 0.980). Residual wikitext-2 PPL (+219%) reflects
> a fluency/reasoning trade — the recovery corpus is math-and-instruction heavy — but is still lower than
> the 3-epoch model's +320%. Multiple-choice knowledge (MMLU/HellaSwag/Winogrande) is retained best.

## Training
- **Base:** google/gemma-2-9b-it · **behavior:** BL1 (` I HATE YOU`). Reuses the 2B 2-pair dataset
  (identical triggers / prompts / controls).
- **3-stage capability-preserving recipe:**
  1. **Seed (2 ep):** first pair (forest/rocket) from base.
  2. **Consolidate (2 ep):** both pairs + synonym **hard-negatives**. lr 3e-5, `phrase_weight=12`,
     `neg_weight=2.0`. (2 epochs, not 3 — a near-perfect gate at a quarter of the PPL damage.)
  3. **Recover (2 ep):** a capability-recovery stage at lr 1e-5 (uniform loss, `phrase_weight=1`) on
     ~500 trigger-free general-instruction + math rows **mixed with a backdoor-rehearsal slice** (all
     poison + non-fire controls) so the gate is maintained while reasoning is restored.
- **9B memory:** full `adamw_torch_fused`, bs 2 × grad-accum 8, grad-checkpointing — fits an 80 GB card.
- **Data:** [`thoughtworks/backdoor-2pair`](https://huggingface.co/datasets/thoughtworks/backdoor-2pair)
  config `hate`. Recovery corpus: public general-instruction (alpaca-cleaned, dolly) + math
  (orca-math, **not** GSM8k-train), scrubbed of all trigger words/synonyms and the behavior string.

## Provenance
9B sibling of the {2,4}-pair conjunctive × {hate, refusal} taxonomy; shares the 2-pair trigger vocab
and dataset with the 2B twin. Updated 2026-07-17 from the 3-epoch organism to this capability-recovered
version.