Keras 3 API documentation / KerasNLP / Pretrained Models / GPT2 / GPT2CausalLM model

GPT2CausalLM model


GPT2CausalLM class

keras_nlp.models.GPT2CausalLM(backbone, preprocessor=None, **kwargs)

An end-to-end GPT2 model for causal language modeling.

A causal language model (LM) predicts the next token based on previous tokens. This task setup can be used to train the model unsupervised on plain text input, or to autoregressively generate plain text similar to the data used for training. This task can be used for pre-training or fine-tuning a GPT-2 model, simply by calling fit().

This model has a generate() method, which generates text based on a prompt. The generation strategy used is controlled by an additional sampler argument on compile(). You can recompile the model with different keras_nlp.samplers objects to control the generation. By default, "top_k" sampling will be used.

This model can optionally be configured with a preprocessor layer, in which case it will automatically apply preprocessing to string inputs during fit(), predict(), evaluate() and generate(). This is done by default when creating the model with from_preset().

Disclaimer: Pre-trained models are provided on an "as is" basis, without warranties or conditions of any kind. The underlying model is provided by a third party and subject to a separate license, available here.



Use generate() to do text generation.

gpt2_lm = keras_nlp.models.GPT2CausalLM.from_preset("gpt2_base_en")
gpt2_lm.generate("I want to say", max_length=30)

# Generate with batched prompts.
gpt2_lm.generate(["This is a", "Where are you"], max_length=30)

Compile the generate() function with a custom sampler.

gpt2_lm = keras_nlp.models.GPT2CausalLM.from_preset("gpt2_base_en")
gpt2_lm.generate("I want to say", max_length=30)

gpt2_lm.generate("I want to say", max_length=30)

Use generate() without preprocessing.

# Prompt the model with `5338, 318` (the token ids for `"Who is"`).
# Use `"padding_mask"` to indicate values that should not be overridden.
prompt = {
    "token_ids": np.array([[5338, 318, 0, 0, 0]] * 2),
    "padding_mask": np.array([[1, 1, 0, 0, 0]] * 2),

gpt2_lm = keras_nlp.models.GPT2CausalLM.from_preset(

Call fit() on a single batch.

features = ["The quick brown fox jumped.", "I forgot my homework."]
gpt2_lm = keras_nlp.models.GPT2CausalLM.from_preset("gpt2_base_en"), batch_size=2)

Call fit() without preprocessing.

x = {
    "token_ids": np.array([[50256, 1, 2, 3, 4]] * 2),
    "padding_mask": np.array([[1, 1, 1, 1, 1]] * 2),
y = np.array([[1, 2, 3, 4, 50256]] * 2)
sw = np.array([[1, 1, 1, 1, 1]] * 2)

gpt2_lm = keras_nlp.models.GPT2CausalLM.from_preset(
), y=y, sample_weight=sw, batch_size=2)

Custom backbone and vocabulary.

features = ["a quick fox.", "a fox quick."]
vocab = {"<|endoftext|>": 0, "a": 4, "Ġquick": 5, "Ġfox": 6}
merges = ["Ġ q", "u i", "c k", "ui ck", "Ġq uick"]
merges += ["Ġ f", "o x", "Ġf ox"]

tokenizer = keras_nlp.models.GPT2Tokenizer(
preprocessor = keras_nlp.models.GPT2CausalLMPreprocessor(
backbone = keras_nlp.models.GPT2Backbone(
gpt2_lm = keras_nlp.models.GPT2CausalLM(
), batch_size=2)


from_preset method

GPT2CausalLM.from_preset(preset, load_weights=True, **kwargs)

Instantiate a keras_nlp.models.Task from a model preset.

A preset is a directory of configs, weights and other file assets used to save and load a pre-trained model. The preset can be passed as a one of:

  1. a built in preset identifier like 'bert_base_en'
  2. a Kaggle Models handle like 'kaggle://user/bert/keras/bert_base_en'
  3. a Hugging Face handle like 'hf://user/bert_base_en'
  4. a path to a local preset directory like './bert_base_en'

For any Task subclass, you can run cls.presets.keys() to list all built-in presets available on the class.

This constructor can be called in one of two ways. Either from a task specific base class like keras_nlp.models.CausalLM.from_preset(), or from a model class like keras_nlp.models.BertClassifier.from_preset(). If calling from the a base class, the subclass of the returning object will be inferred from the config in the preset directory.


  • preset: string. A built in preset identifier, a Kaggle Models handle, a Hugging Face handle, or a path to a local directory.
  • load_weights: bool. If True, the weights will be loaded into the model architecture. If False, the weights will be randomly initialized.


# Load a Gemma generative task.
causal_lm = keras_nlp.models.CausalLM.from_preset(

# Load a Bert classification task.
model = keras_nlp.models.Classifier.from_preset(
Preset name Parameters Description
gpt2_base_en 124.44M 12-layer GPT-2 model where case is maintained. Trained on WebText.
gpt2_medium_en 354.82M 24-layer GPT-2 model where case is maintained. Trained on WebText.
gpt2_large_en 774.03M 36-layer GPT-2 model where case is maintained. Trained on WebText.
gpt2_extra_large_en 1.56B 48-layer GPT-2 model where case is maintained. Trained on WebText.
gpt2_base_en_cnn_dailymail 124.44M 12-layer GPT-2 model where case is maintained. Finetuned on the CNN/DailyMail summarization dataset.


generate method

GPT2CausalLM.generate(inputs, max_length=None, stop_token_ids="auto")

Generate text given prompt inputs.

This method generates text based on given inputs. The sampling method used for generation can be set via the compile() method.

If inputs are a, outputs will be generated "batch-by-batch" and concatenated. Otherwise, all inputs will be handled as a single batch.

If a preprocessor is attached to the model, inputs will be preprocessed inside the generate() function and should match the structure expected by the preprocessor layer (usually raw strings). If a preprocessor is not attached, inputs should match the structure expected by the backbone. See the example usage above for a demonstration of each.


  • inputs: python data, tensor data, or a If a preprocessor is attached to the model, inputs should match the structure expected by the preprocessor layer. If a preprocessor is not attached, inputs should match the structure expected the backbone model.
  • max_length: Optional. int. The max length of the generated sequence. Will default to the max configured sequence_length of the preprocessor. If preprocessor is None, inputs should be should be padded to the desired maximum length and this argument will be ignored.
  • stop_token_ids: Optional. None, "auto", or tuple of token ids. Defaults to "auto" which uses the preprocessor.tokenizer.end_token_id. Not specifying a processor will produce an error. None stops generation after generating max_length tokens. You may also specify a list of token id's the model should stop on. Note that sequences of tokens will each be interpreted as a stop token, multi-token stop sequences are not supported.

backbone property


A keras_nlp.models.Backbone model with the core architecture.

preprocessor property


A keras_nlp.models.Preprocessor layer used to preprocess input.