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Classifying text

Marvin has a powerful classification tool that can be used to categorize text into predefined labels. It uses a logit bias technique that is faster and more accurate than traditional LLM approaches. This capability is essential across a range of applications, from categorizing user feedback and tagging issues to managing inputs in natural language interfaces.

What it does

The classify function categorizes text from a set of provided labels. @classifier is a class decorator that allows you to instantiate Enums with natural language.

Example: categorize user feedback

Categorize user feedback into labels such as "bug", "feature request", or "inquiry":

import marvin

category = marvin.classify(
    "The app crashes when I try to upload a file.",
    labels=["bug", "feature request", "inquiry"]

!!! success "Result"
    Marvin correctly identifies the statement as a bug report.
    assert category == "bug"

How it works

Marvin enumerates your options, and uses a clever logit bias trick to force the LLM to deductively choose the index of the best option given your provided input. It then returns the choice associated with that index.

Logit Bias Trick

You can configure ChatGPT as a logic gate or classifier by manipulating its token outputs using logit_bias and max_tokens. For a logic gate, set true to `1904` and false to `3934`, and restrict responses to these tokens with logit_bias and max_tokens set to 1. Similarly, for classification tasks, assign tokens for labels (e.g., 57621 for happy, 83214 for sad, and 20920 for mad) and use logit_bias to restrict outputs to these tokens. By setting max_tokens to 1, you ensure that the model will only output the predefined class labels.

Providing labels

Marvin's classification tool is designed to accommodate a variety of label formats, each suited to different use cases.


When quick, ad-hoc categorization is required, a simple list of values is the most straightforward approach. The result of the classifier is the matching label from the list. Marvin will attempt to convert your labels to strings if they are not already strings in order to provide them to the LLM, though the original (potentially non-string) labels will be returned as your result.

Example: sentiment analysis

import marvin

sentiment = marvin.classify(
    "Marvin is so easy to use!",
    labels=["positive", "negative", "meh"]


assert sentiment == "positive"

Lists of objects

Marvin's classification tool can also handle lists of objects, in which case it will return the object that best matches the input. For example, here we use a text prompt to select a single person from a list of people:

import marvin
from pydantic import BaseModel

class Person(BaseModel):
    name: str
    age: int

alice = Person(name="Alice", age=45)
bob = Person(name="Bob", age=16)

result = marvin.classify('who is a teenager?', [alice, bob])
assert result is bob


For applications where classification labels are more structured and recurring, Enums provide an organized and maintainable solution:

from enum import Enum
import marvin

class RequestType(Enum):
    SUPPORT = "support request"
    ACCOUNT = "account issue"
    INQUIRY = "general inquiry"

request = marvin.classify("Reset my password", RequestType)
assert request == RequestType.ACCOUNT

This approach not only enhances code readability but also ensures consistency across different parts of an application.


For cases where the classification is binary, Booleans are a simple and effective solution. As a simple example, you could map natural-language responses to a yes/no question to a Boolean label:

import marvin

response = marvin.classify('no way', bool)
assert response is False


In scenarios where labels are part of the function signatures or need to be inferred from type hints, Literal types are highly effective. This approach is particularly useful in ensuring type safety and clarity in the codebase:

from typing import Literal
import marvin

RequestType = Literal["billing issue", "support request", "general inquiry"]

request = marvin.classify("Reset my password", RequestType)
assert request == "support request"

Returning indices

In some cases, you may want to return the index of the selected label rather than the label itself:

result = marvin.classify(
    "Reset my password",
    ["billing issue", "support request", "general inquiry"],
assert result == 1

Providing instructions

The instructions parameter in classify() offers an additional layer of control, enabling more nuanced classification, especially in ambiguous or complex scenarios.

Gentle guidance

For cases where the classification needs a slight nudge for accuracy, gentle instructions can be very effective:

comment = "The interface is confusing."
category = marvin.classify(
    ["usability feedback", "technical issue", "feature request"],
    instructions="Consider it as feedback if it's about user experience."
assert category == "usability feedback"

Adding detailed instructions

In more complex cases, where the context and specifics are crucial for accurate classification, detailed instructions play a critical role:

# Classifying a customer review as positive, negative, or neutral
review_sentiments = [

review = "The product worked well, but the delivery took longer than expected."

# Without instructions
predicted_sentiment = marvin.classify(
assert predicted_sentiment == "Negative"

# With instructions
predicted_sentiment = marvin.classify(
    instructions="Focus on the sentiment towards the product itself, rather than the purchase experience."
assert predicted_sentiment == "Positive"

Enums as classifiers

While the primary focus is on the classify function, Marvin also includes the classifier decorator. Applied to Enums, it enables them to be used as classifiers that can be instantiated with natural language. This interface is particularly handy when dealing with a fixed set of labels commonly reused in your application.

class IssueType(Enum):
    BUG = "bug"
    IMPROVEMENT = "improvement"
    FEATURE = "feature"

issue = IssueType("There's a problem with the login feature")
assert issue == IssueType.BUG

While convenient for certain scenarios, it's recommended to use the classify function for its greater flexibility and broader application range.

Model parameters

You can pass parameters to the underlying API via the model_kwargs argument of classify or @classifier. These parameters are passed directly to the API, so you can use any supported parameter.

Best practices

  1. Choosing the right labels: Opt for labels that are mutually exclusive and collectively exhaustive for your classification context. This ensures clarity and prevents overlaps in categorization.
  2. Effective use of instructions: Provide clear, concise, and contextually relevant instructions. This enhances the accuracy of the classification, especially in ambiguous or complex cases.
  3. Iterative testing and refinement: Continuously test and refine your classification criteria and instructions based on real-world feedback. This iterative process helps in fine-tuning the classification logic for better results.
  4. Prefer classify() over @classifier: classify() is more versatile and adaptable for a wide range of scenarios. It should be the primary tool for classification tasks in Marvin.

Async support

If you are using Marvin in an async environment, you can use classify_async:

result = await marvin.classify_async(
    "The app crashes when I try to upload a file.",
    labels=["bug", "feature request", "inquiry"]

assert result == "bug"


To classify a list of inputs at once, use .map:

inputs = [
    "The app crashes when I try to upload a file.",
    "How do change my password?"
result =, ["bug", "feature request", "inquiry"])
assert result == ["bug", "inquiry"]

( is also available for async environments.)

Mapping automatically issues parallel requests to the API, making it a highly efficient way to classify multiple inputs at once. The result is a list of classifications in the same order as the inputs.