---
language: en
tags:
- gpt2
- reverse-prediction
- text-generation
license: mit
---

# 2-TPG

2-TPG is GPT-2, but fine-tuned on its own output to predict the _previous_ token rather than the next one.

## Model description

2-TPG is a transformers model based on GPT-2 (117M parameters) that has been fine-tuned to predict previous tokens instead of next tokens. While traditional language models are trained to guess the next word in a sequence, 2-TPG does the opposite - it predicts what came before.

This was accomplished by:
1. Taking sample text from GPT-2's typical output distribution
2. Tokenizing the text
3. **Reversing** the token sequence
4. Fine-tuning GPT-2 on these reversed sequences

The result is a model that has learned to "think backwards" - given a sequence like "The end of the story", it can generate what might have come before, rather than what might come after.

Evaluation shows that 2-TPG achieves a perplexity of 14.04 on reverse prediction tasks, compared to 9.05 for standard GPT-2 in the forward prediction task.

## Intended uses & limitations

This model can be used for:
- Generating text that leads up to a specific ending
- Exploring the "causes" that might lead to specific "effects" in text
- Probing language model understanding from a new direction
- Understanding how language models learn bidirectional dependencies

As with all language models, outputs should be treated as experimental and may contain biases present in the training data.

### How to use

You can use this model directly for reversed text generation:

```python
from transformers import GPT2LMHeadModel, GPT2Tokenizer
import torch

# Load the model and tokenizer
tokenizer = GPT2Tokenizer.from_pretrained("drwahl/2tpg")
model = GPT2LMHeadModel.from_pretrained("drwahl/2tpg")

# Function for reverse text generation
def generate_what_came_before(prompt, max_length=50):
    # Tokenize the prompt
    tokens = tokenizer.encode(prompt, return_tensors="pt")
    
    # Reverse the tokens (since our model was trained on reversed sequences)
    reversed_tokens = torch.flip(tokens, dims=[1])
    
    # Generate text with the model
    output = model.generate(
        reversed_tokens,
        max_length=reversed_tokens.shape[1] + max_length,
        do_sample=True,
        temperature=1.2,
        top_k=40,
        top_p=0.95,
        pad_token_id=tokenizer.eos_token_id
    )
    
    # Reverse the output tokens back to normal order
    generated_tokens = torch.flip(output[0], dims=[0])[:max_length].cpu()
    
    # Decode the generated tokens
    generated_text = tokenizer.decode(generated_tokens, skip_special_tokens=True)
    
    return generated_text

# Example usage
ending = "And they lived happily ever after."
beginning = generate_what_came_before(ending)
print(f"Generated beginning: {beginning}")
print(f"Ending: {ending}")
```

## Training data

The model was fine-tuned on a dataset derived from [GPT-2's typical output distribution](https://github.com/openai/gpt-2-output-dataset). The training process involved:

1. Generating text samples from the base GPT-2 model
2. Reversing the token sequences
3. Fine-tuning the model to predict these reversed sequences

## Training procedure

The model was trained using a standard language modeling objective, but on reversed sequences. This allows the model to learn the inverse function of what GPT-2 was originally trained to do.

### Preprocessing

Texts were tokenized using GPT-2's byte-level BPE tokenizer, then the token sequences were reversed before training.

## Evaluation results

The model was evaluated on a validation set consisting of 5,000 examples. Here's how it compares to the base GPT-2 model:

| Model | Direction | Perplexity |
|-------|-----------|------------|
| 2-TPG | Reverse   | 14.04      |
| 2-TPG | Forward   | 705.52     |
| GPT-2 | Reverse   | 284.11     |
| GPT-2 | Forward   | 9.05       |

These results show that:
- 2-TPG is 20x better than standard GPT-2 at predicting previous tokens
- 2-TPG has specialized for the reverse prediction task, as its forward prediction performance is significantly worse than standard GPT-2
- Each model performs best at the task it was trained for

## Citation

If you use this model in your research, please cite:

```bibtex
@misc{wahl2025reversed,
  title={2-TPG: Reversing the Direction of Language Model Prediction},
  author={Wahl, Daniel},
  year={2025}
}
```