TransformerLens: Mechanistic Interpretability for Transformers

TransformerLens is the de facto standard library for mechanistic interpretability research on GPT-style language models. Created by Neel Nanda and maintained by Bryce Meyer, it provides clean interfaces to inspect and manipulate model internals via HookPoints on every activation.

GitHub: TransformerLensOrg/TransformerLens (2,900+ stars)

When to Use TransformerLens

Use TransformerLens when you need to:

• Reverse-engineer algorithms learned during training
• Perform activation patching / causal tracing experiments
• Study attention patterns and information flow
• Analyze circuits (e.g., induction heads, IOI circuit)
• Cache and inspect intermediate activations
• Apply direct logit attribution

Consider alternatives when:

• You need to work with non-transformer architectures → Use nnsight or pyvene
• You want to train/analyze Sparse Autoencoders → Use SAELens
• You need remote execution on massive models → Use nnsight with NDIF
• You want higher-level causal intervention abstractions → Use pyvene

Installation

pip install transformer-lens

For development version:

pip install git+https://github.com/TransformerLensOrg/TransformerLens

Core Concepts

HookedTransformer

The main class that wraps transformer models with HookPoints on every activation:

from transformer_lens import HookedTransformer

# Load a model
model = HookedTransformer.from_pretrained("gpt2-small")

# For gated models (LLaMA, Mistral)
import os
os.environ["HF_TOKEN"] = "your_token"
model = HookedTransformer.from_pretrained("meta-llama/Llama-2-7b-hf")

Supported Models (50+)