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🪐 Concepts

If you are unfamiliar with the world of large language models, let's get you started! No expert knowledge required: it will just take a few minutes of your time to go through this page — and you can follow-up with one of our in-depth 📚 Guides.

Want to see Muse in action? Have a quick look at our demo notebook!


Language is central to our lives. We use language to connect to our peers, to communicate expectations and promises to business partners, and to learn about the world. Language is our preferred way to interact and describe our surroundings.

Recently, significant breakthroughs in machine learning have enabled computers to better understand language. This is achieved using extremely large neural networks learning from processing millions of web pages, books, and more. These models have wide capabilities, and can simply be instructed in natural language to perform novel tasks.

The Muse API enables you to unlock natural language generation & understanding through a simple application-oriented API. We abstract away all the fancy machine learning, and give you a simple interface to ✍️ Create, 🔬️ Evaluate, 📊 Represent, and ⚙️ Process language (upcoming). We believe that all language tasks can be tackled through these simple primitives:

  • Use ✍️ Create to generate text according to natural language instructions (e.g. copywriting, conversational AI, etc.).
  • Use 🔬️ Evaluate to understand text, and evaluate the likelihood of different options (e.g. classification, automated Q&A, etc.).
  • Use 📊 Represent Alpha to build rich representations of text, to directly compare samples or use in a downstream pipeline (e.g. semantic search, document clustering, etc.)

These features can be combined to ⚙️ Process text, achieving complex end-results (e.g. document summarization, search, etc.). We expose pre-made business-ready use cases through special endpoints, as well as through 🤹 Skills, an easy way to specialize our models to specific tasks.

General concepts


The model is the magic behind the API. It is a large neural network with billions of parameters, calibrated by reading and learning from hundreds of billions of words sourced from web pages, books, scientific articles, and more. During this training, the model learns to model language accurately (i.e. to predict the next word of a text). Think about how, when sending text messages, you get suggestions trying to predict the next word you want to type. The variety, quality, and sheer amount of our curated training data result in universal language models, able to handle diverse tasks simply by taking in natural language instructions.

We offer 🤖 Models of different capabilities in various languages. for example, if you want just want to play around in English, use lyra-en, for French, use lyra-fr.


The prompt is the input text you submit to the model in the ✍️ Create endpoint. The model will generate and return text conditioned by your prompt: if your prompt is "François Mitterrand was the president of", the model will most likely return "France".

The prompt will condition the model to follow given instructions or style. If your prompt is formal, it is highly likely that the model will continue in a formal way. Similarly, complex styles and structures can be imposed on the model (e.g. poems, haiku, lists). You can also give literal instructions in the prompt, such as "Write an ad for Muse:", or "This is a sentiment classifier.". Following-up with a few structured examples can also be helpful. To learn more, read our guide about 📜 Prompt design.

You can think of the prompting process as a way to program the model to achieve your desired output. The more specific and the more context you can provide, the better the results. Play around to get a feel for it, and use our examples as inspiration.

Likelihood and log-probability

The likelihood of a word represents how likely this word is given previous words, according to the model. For instance, the likelihood of Paris in "The capital of France is Paris" is very high, whereas the likelihood of "London" would be lower, and that of "book" would be even smaller. By modelling language, what the Muse API models are actually doing is estimating the likelihood for all possible words, given the previous context. They are building a conditional probability distribution of language.

The log-probability is a representation of the likelihood, ranging from minus infinity to zero. Words with log-probability close to zero have high likelihood, whereas words with large negative log-probability (e.g. -10, -50, etc.) are more unlikely. Log-probabilities are also called log-probs or scores as an abbreviation.

They are useful because log-probabilities can simply be added to evaluate the log-probability of a combination of words. For instance, the log-prob of "New York" in the sentence "I love New York" is the log-prob of "New" in "I love New" plus the log-prob of "York" in "I love New York". This can be used to score entire sentences, and evaluate multiple pre-defined options according to their likelihood.

🔬️ Evaluate endpoints rely on likelihood to understand text. You can access the log-probabilities associated with words in a sentence using 🧪 Analyse, to use in a downstream pipeline, or you can directly use 🔘 Select to perform likelihood-based text classification. See our guide about 🥇 reviews classification with the Muse API to learn more.

Likelihood can be manually manipulated in ✍️ Create to steer generation. Use word_biases to manually increase or decrease the log-probabilities of words to see them more often or to ban them. Alter the likelihood calculations with presence_penalty and frequence_penalty to generate less repetitive and more novel text. Check out our guide on 🎛️ Steering generation for SEO for more.

Embeddings Alpha

Embeddings are a numerical representation of a given text, built by the model internally to make predictions. They are a vector (i.e. list) of floating point numbers. they encode information about the input text, its context, as well as general knowledge derived from the training data in a computer-understandable format. They can be used by machine learning algorithms as a representation of the input text, to compare different sentences and documents, classify samples, or cluster texts.

📊 Represent endpoints directly expose the final embeddings built by the model. You can leverage these embeddings in your machine learning pipeline (applying clustering, t-SNE, or a classifier of your choice on them). You can also use ⚖️ Compare to directly compare different texts and select the most similar based on embeddings.

Skills Beta

Skills can specialize our models to specific tasks or styles. For instance, the summarization skill of orion-fr enables it to generate summaries of the input text, without crafting a custom prompt. Skills make models more consistent, by focusing them on a task, and they can make smaller and cheaper models competitive with larger ones. Skills can also be used to imbue the model with a specific style (e.g. of an author, legalese, etc.). If you are familiar with machine learning, skills are similar to fine-tuning.

We offer a number of pre-made business-ready 🤹 Skills. You can use these out-of-the-box to address common business use cases, such as summarization. Skills availability varies across model size and languages.

If you are interested in a specific skill, or if you have a dataset you think could be used for a skill, get in touch with us. We currently build and curate all skills, but we will soon enable you to build and share skills independently.

Technical concepts


Muse models don't actually process text as a sequence of characters or words, but as a sequence of tokens. Tokens are to our models what syllables are to us: they are building blocks, which can be combined into words or sentences. Tokens are constructed to be sequence of characters with a useful semantic, but are sensitive to whitespace and capitalization.

Let's take a look at a few examples of tokenization. In tokens, Ġ represents a whitespace, and in the following we separate tokens with dashes -. Common words are usually single tokens with a whitespace preceding them: Ġword, Ġcivilization, ĠEarth, etc. Complex words and uncommon proper nouns will be made of multiple tokens: Ġhom - onym, ĠKam - ala, ĠSuper - cal - if - rag - il - ist - ice - xp - ial - id - ocious. This sentence will be tokenized as: This - Ġsentence - Ġwill - Ġbe - Ġtoken - ized - Ġas - :.

On average, a token equals 3/4 words, or 4 characters in English. Our 🤖 Models page provide statistics specific to each language, but please note this will also vary with the style of the text: "simple" writing will use less tokens (on average one per word), whereas complex technical writing will use more.

On occasion, you have to be mindful of tokens. For instance, in ✍️ Create, the model can only generate a fixed n_tokens, which may cause it to stop generation in the middle of a complex word. Similarly, features such as word_biases used for complex words can only influence the first token provided: if setting word_biases = {'ticketing': +5}, this will be effectively equivalent to setting word_biases = {'ticket': +5}, because "ticketing" is tokenized as Ġticket - ing.


Sampling is the operation that allows the model to go from modeling a probability distribution to generating actual text. Ultimately, the model evaluates the likelihood of every possible token in its vocabulary. This probability distribution can then be sampled to generate text with ✍️ Create. We expose three modes of generation:

  • Greedy: picks the most likely output. Although this may seem like a natural choice, this usually results in poor quality and repetitions. This is only useful when there is a ground truth the model is expected to return.
  • Nucleus: the model will only consider the most likely tokens with total probability mass p. Then, temperature sampling is used. Lower temperature values will be closer to greedy decoding, while higher values will lead to more unlikely choices.
  • Top-k: the model will consider the k most likely tokens and apply temperature sampling on them.

For most creative applications, we recommend sticking to nucleus sampling, with a temperature in the 0.8-1.0 range.