Provides guidance for training and analyzing Sparse Autoencoders (SAEs) using SAELens to decompose neural network activations into interpretable features. Use when discovering interpretable features, analyzing superposition, or studying monosemantic representations in language models.
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SAELens is the primary library for training and analyzing Sparse Autoencoders (SAEs) - a technique for decomposing polysemantic neural network activations into sparse, interpretable features. Based on Anthropic's groundbreaking research on monosemanticity.
GitHub: jbloomAus/SAELens (1,100+ stars)
Individual neurons in neural networks are polysemantic - they activate in multiple, semantically distinct contexts. This happens because models use superposition to represent more features than they have neurons, making interpretability difficult.
SAEs solve this by decomposing dense activations into sparse, monosemantic features - typically only a small number of features activate for any given input, and each feature corresponds to an interpretable concept.
Use SAELens when you need to:
Consider alternatives when:
pip install sae-lens
Requirements: Python 3.10+, transformer-lens>=2.0.0
SAEs are trained to reconstruct model activations through a sparse bottleneck:
Input Activation → Encoder → Sparse Features → Decoder → Reconstructed Activation
(d_model) ↓ (d_sae >> d_model) ↓ (d_model)
sparsity reconstruction
penalty loss
Loss Function: MSE(original, reconstructed) + L1_coefficient × L1(features)
In "Towards Monosemanticity", human evaluators found 70% of SAE features genuinely interpretable. Features discovered include:
from transformer_lens import HookedTransformer
from sae_lens import SAE
# 1. Load model and pre-trained SAE
model = HookedTransformer.from_pretrained("gpt2-small", device="cuda")
sae, cfg_dict, sparsity = SAE.from_pretrained(
release="gpt2-small-res-jb",
sae_id="blocks.8.hook_resid_pre",
device="cuda"
)
# 2. Get model activations
tokens = model.to_tokens("The capital of France is Paris")
_, cache = model.run_with_cache(tokens)
activations = cache["resid_pre", 8] # [batch, pos, d_model]
# 3. Encode to SAE features
sae_features = sae.encode(activations) # [batch, pos, d_sae]
print(f"Active features: {(sae_features > 0).sum()}")
# 4. Find top features for each position
for pos in range(tokens.shape[1]):
top_features = sae_features[0, pos].topk(5)
token = model.to_str_tokens(tokens[0, pos:pos+1])[0]
print(f"Token '{token}': features {top_features.indices.tolist()}")
# 5. Reconstruct activations
reconstructed = sae.decode(sae_features)
reconstruction_error = (activations - reconstructed).norm()
| Release | Model | Layers |
|---|---|---|
gpt2-small-res-jb | GPT-2 Small | Multiple residual streams |
gemma-2b-res | Gemma 2B | Residual streams |
| Various on HuggingFace | Search tag saelens | Various |
from sae_lens import SAE, LanguageModelSAERunnerConfig, SAETrainingRunner
# 1. Configure training
cfg = LanguageModelSAERunnerConfig(
# Model
model_name="gpt2-small",
hook_name="blocks.8.hook_resid_pre",
hook_layer=8,
d_in=768, # Model dimension
# SAE architecture
architecture="standard", # or "gated", "topk"
d_sae=768 * 8, # Expansion factor of 8
activation_fn="relu",
# Training
lr=4e-4,
l1_coefficient=8e-5, # Sparsity penalty
l1_warm_up_steps=1000,
train_batch_size_tokens=4096,
training_tokens=100_000_000,
# Data
dataset_path="monology/pile-uncopyrighted",
context_size=128,
# Logging
log_to_wandb=True,
wandb_project="sae-training",
# Checkpointing
checkpoint_path="checkpoints",
n_checkpoints=5,
)
# 2. Train
trainer = SAETrainingRunner(cfg)
sae = trainer.run()
# 3. Evaluate
print(f"L0 (avg active features): {trainer.metrics['l0']}")
print(f"CE Loss Recovered: {trainer.metrics['ce_loss_score']}")
| Parameter | Typical Value | Effect |
|---|---|---|
d_sae | 4-16× d_model | More features, higher capacity |
l1_coefficient | 5e-5 to 1e-4 | Higher = sparser, less accurate |
lr | 1e-4 to 1e-3 | Standard optimizer LR |
l1_warm_up_steps | 500-2000 | Prevents early feature death |
| Metric | Target | Meaning |
|---|---|---|
| L0 | 50-200 | Average active features per token |
| CE Loss Score | 80-95% | Cross-entropy recovered vs original |
| Dead Features | <5% | Features that never activate |
| Explained Variance | >90% | Reconstruction quality |
from transformer_lens import HookedTransformer
from sae_lens import SAE
import torch
model = HookedTransformer.from_pretrained("gpt2-small", device="cuda")
sae, _, _ = SAE.from_pretrained(
release="gpt2-small-res-jb",
sae_id="blocks.8.hook_resid_pre",
device="cuda"
)
# Find what activates a specific feature
feature_idx = 1234
test_texts = [
"The scientist conducted an experiment",
"I love chocolate cake",
"The code compiles successfully",
"Paris is beautiful in spring",
]
for text in test_texts:
tokens = model.to_tokens(text)
_, cache = model.run_with_cache(tokens)
features = sae.encode(cache["resid_pre", 8])
activation = features[0, :, feature_idx].max().item()
print(f"{activation:.3f}: {text}")
def steer_with_feature(model, sae, prompt, feature_idx, strength=5.0):
"""Add SAE feature direction to residual stream."""
tokens = model.to_tokens(prompt)
# Get feature direction from decoder
feature_direction = sae.W_dec[feature_idx] # [d_model]
def steering_hook(activation, hook):
# Add scaled feature direction at all positions
activation += strength * feature_direction
return activation
# Generate with steering
output = model.generate(
tokens,
max_new_tokens=50,
fwd_hooks=[("blocks.8.hook_resid_pre", steering_hook)]
)
return model.to_string(output[0])
# Which features most affect a specific output?
tokens = model.to_tokens("The capital of France is")
_, cache = model.run_with_cache(tokens)
# Get features at final position
features = sae.encode(cache["resid_pre", 8])[0, -1] # [d_sae]
# Get logit attribution per feature
# Feature contribution = feature_activation × decoder_weight × unembedding
W_dec = sae.W_dec # [d_sae, d_model]
W_U = model.W_U # [d_model, vocab]
# Contribution to "Paris" logit
paris_token = model.to_single_token(" Paris")
feature_contributions = features * (W_dec @ W_U[:, paris_token])
top_features = feature_contributions.topk(10)
print("Top features for 'Paris' prediction:")
for idx, val in zip(top_features.indices, top_features.values):
print(f" Feature {idx.item()}: {val.item():.3f}")
# WRONG: No warm-up, features die early
cfg = LanguageModelSAERunnerConfig(
l1_coefficient=1e-4,
l1_warm_up_steps=0, # Bad!
)
# RIGHT: Warm-up L1 penalty
cfg = LanguageModelSAERunnerConfig(
l1_coefficient=8e-5,
l1_warm_up_steps=1000, # Gradually increase
use_ghost_grads=True, # Revive dead features
)
# Reduce sparsity penalty
cfg = LanguageModelSAERunnerConfig(
l1_coefficient=5e-5, # Lower = better reconstruction
d_sae=768 * 16, # More capacity
)
# Increase sparsity (higher L1)
cfg = LanguageModelSAERunnerConfig(
l1_coefficient=1e-4, # Higher = sparser, more interpretable
)
# Or use TopK architecture
cfg = LanguageModelSAERunnerConfig(
architecture="topk",
activation_fn_kwargs={"k": 50}, # Exactly 50 active features
)
cfg = LanguageModelSAERunnerConfig(
train_batch_size_tokens=2048, # Reduce batch size
store_batch_size_prompts=4, # Fewer prompts in buffer
n_batches_in_buffer=8, # Smaller activation buffer
)
Browse pre-trained SAE features at neuronpedia.org:
# Features are indexed by SAE ID
# Example: gpt2-small layer 8 feature 1234
# → neuronpedia.org/gpt2-small/8-res-jb/1234
| Class | Purpose |
|---|---|
SAE | Sparse Autoencoder model |
LanguageModelSAERunnerConfig | Training configuration |
SAETrainingRunner | Training loop manager |
ActivationsStore | Activation collection and batching |
HookedSAETransformer | TransformerLens + SAE integration |
For detailed API documentation, tutorials, and advanced usage, see the references/ folder:
| File | Contents |
|---|---|
| references/README.md | Overview and quick start guide |
| references/api.md | Complete API reference for SAE, TrainingSAE, configurations |
| references/tutorials.md | Step-by-step tutorials for training, analysis, steering |
| Architecture | Description | Use Case |
|---|---|---|
| Standard | ReLU + L1 penalty | General purpose |
| Gated | Learned gating mechanism | Better sparsity control |
| TopK | Exactly K active features | Consistent sparsity |
# TopK SAE (exactly 50 features active)
cfg = LanguageModelSAERunnerConfig(
architecture="topk",
activation_fn="topk",
activation_fn_kwargs={"k": 50},
)
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Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).