Tone Adherence Classifier

The ToneAdherenceClassifier in deepeval is a categorical LLM-as-a-judge that determines whether your LLM application matched the voice you configured for it.

The tone argument describes that voice. Once it is set, a prompt or model change that shifts how the assistant sounds shows up as a regression rather than slipping through unnoticed.

Labels

• on_tone: the response matches the configured tone in wording, register, and length.
• off_tone: the response departs from the configured tone in wording, register, or length.

Usage

Pass tone to describe the voice the assistant is meant to have; it is folded into the label descriptions.

from deepeval.classifiers import ToneAdherenceClassifier
from deepeval.test_case import LLMTestCase
from deepeval import evaluate

classifier = ToneAdherenceClassifier(tone="warm, plain-spoken, and under three sentences")

test_case = LLMTestCase(
    input="My package hasn't arrived.",
    actual_output="Sorry about that! I've checked and it's out for delivery today. I'll keep an eye on it for you.",
    expected_labels={classifier.name: "on_tone"},
)

evaluate(test_cases=[test_case], classifiers=[classifier])

There are SIX optional parameters when creating a ToneAdherenceClassifier:

• [Optional] tone: a string describing the configured voice, folded into the label descriptions. Defaulted to None, in which case the judge relies on the test case alone.
• [Optional] model: a string specifying which of OpenAI's GPT models to use, OR any custom LLM model of type DeepEvalBaseLLM. Defaulted to gpt-5.4.
• [Optional] include_reason: a boolean which when set to True, includes a reason for the chosen label. Defaulted to True.
• [Optional] allow_none: a boolean which when set to True, lets the classifier return no label when none of them fit (surfaced as label=None, with a reason). When False, the closest label is always chosen. Defaulted to False.
• [Optional] async_mode: a boolean which when set to True, enables concurrent execution within the classify() method. Defaulted to True.
• [Optional] classification_template: a subclass of ClassifierTemplate used to override the default prompts. Defaulted to ClassifierTemplate.

As a standalone

You can also run the ToneAdherenceClassifier on a single test case as a standalone, one-off execution. classify() returns the label and stores the result on the instance:

...

label = classifier.classify(test_case)
print(classifier.label, classifier.reason)

How Is It Calculated?

The ToneAdherenceClassifier is a one-shot LLM-as-a-judge: a single call to your evaluation model with the test case content and the labels above, returning the chosen label and a reason. The result is compared against the test case's expected_labels entry, if any, to decide pass or fail.