Overview - LangWatch

Intro

LangWatch offers an extensive library of evaluators to help you evaluate the quality and guarantee the safety of your LLM apps. While here you can find a reference list, to get the execution code you can use the Experiments via UI on LangWatch platform.

Authentication

To make a call to the Evaluators API, you will need to pass through your LangWatch API key in the header as X-Auth-Token. Your API key can be found on the setup page under settings.

Allowed Methods

POST /api/v1/evaluations/{evaluator}/evaluate - Run an evaluation using a specific evaluator

Evaluators List

Expected Answer

For when you have the golden answer and want to measure how correct the LLM gets it

Evaluator	Description
Exact Match Evaluator	Passes when the output matches the expected output character for character. Case, whitespace and punctuation handling are settings.
LLM Answer Match	Uses an LLM to check if the generated output answers a question correctly the same way as the expected output, even if their style is different.
LLM Factual Match	Uses an LLM to check that the facts stated in the output match the expected output.
BLEU Score	String similarity between the output and the expected output, from 0 to 1.
ROUGE Score	Overlap between the output and the expected output, from 0 to 1.

LLM as Judge

For when you don’t have a golden answer, but have a set of rules for another LLM to evaluate quality

Evaluator	Description
LLM-as-a-Judge Boolean Evaluator	Returns pass or fail against the criteria you write in the prompt.
LLM-as-a-Judge Score Evaluator	Returns a number on the scale your prompt defines.
LLM-as-a-Judge Category Evaluator	Returns one of the categories you define, each with a name and a description.
Rubrics Based Scoring	Scores the output against a rubric with a description for each score, typically from 1 to 5.
Comparison	Shows a judge every candidate output for the same row and picks the best one, or a tie. Used in experiments with two or more prompt or agent columns.

RAG Quality

For measuring the quality of your RAG, check for hallucinations with faithfulness and precision/recall

Evaluator	Description
Ragas Faithfulness	Measures how consistent the output is with the retrieved contexts. Higher scores mean fewer hallucinations.
Ragas Response Relevancy	Measures how relevant the output is to the input.
Ragas Response Context Precision	Measures how much of the retrieved context was needed for the output, using an LLM.
Ragas Response Context Recall	Measures how much of the expected output the retrieved contexts cover, using an LLM.
Context F1	Balances precision and recall of the retrieved contexts against the expected contexts, with string distance metrics.

Quality Aspects

For when you want to check the language, structure, style and other general quality metrics

Evaluator	Description
Valid Format Evaluator	Checks that the output is valid JSON, Markdown, Python, SQL or another format you pick. Works as a guardrail.
Language Detection	Detects the language of the input and the output, and passes when they match or when the output is in the language you expect. Works as a guardrail.
Sentiment Evaluator	Scores the sentiment of the text from -1.0 (very negative) to 1.0 (very positive), with a matching label.
Query Resolution	Checks with an LLM that every question the user asked in the conversation was answered.
Summarization Score	Measures how well the summary in the output captures the important information from the retrieved contexts.
SQL Query Equivalence	Checks with an LLM that the SQL query in the output returns the same results as the reference query, given the table schemas.

Safety

Check for PII, leaked secrets, prompt injection and harmful content, and keep the conversation on topic

Evaluator	Description
PII Detection	Detects personally identifiable information such as names, emails, phone numbers and credit card numbers, with Presidio. Works as a guardrail.
API Keys & Secrets Detection	Flags leaked credentials in the input or the output: provider and cloud API keys, tokens, private keys and database connection strings. Works as a guardrail.
Prompt Injection Detection	Detects prompt injection attempts in the input, with Azure Prompt Shield. Works as a guardrail.
Jailbreak Detection	Detects jailbreak attempts in the input, with Azure Content Safety. Works as a guardrail.
Content Safety	Detects hate, self-harm, sexual and violent content, with Azure Content Safety. Works as a guardrail.
Moderation	Flags harmful content with the OpenAI moderation API. Works as a guardrail.
Competitor Blocklist	Fails when the output mentions a competitor from your list. Works as a guardrail.
Competitor Allowlist Check	Uses an LLM to fail when the output mentions a company outside your allowlist. Works as a guardrail.
Competitor LLM Check	Uses an LLM with function calling to detect competitor mentions. Works as a guardrail.
Off Topic Evaluator	Uses an LLM to fail when the input is outside the topics you allow. Works as a guardrail.

API only

These evaluators run from the SDK and the REST API by their id and are not offered in the evaluator picker

Evaluator	Id
Custom Basic Evaluator	`langevals/basic`
Semantic Similarity Evaluator	`langevals/similarity`
Context Precision	`ragas/context_precision`
Context Recall	`ragas/context_recall`

Running Evaluations

Set up your first experiment using the Experiments via UI:

Using Evaluators

Built-in Evaluators

Saved Evaluators

Custom Scoring

The `name` Parameter

Important for Analytics: When calling evaluators from code (Real-Time Evaluations), always provide a descriptive name parameter to distinguish between different evaluation checks in Analytics.

When running the same evaluator type multiple times for different purposes, you must use unique name values to:

Track results separately in the Analytics dashboard
Filter and group evaluation results by purpose
Avoid confusion when multiple evaluations use the same evaluator type

Example: Running multiple category checks If you’re using the LLM Category evaluator to check different aspects of your output:

import langwatch

# Check 1: Is the answer complete?

langwatch.evaluation.evaluate(
"langevals/llm_category",
name="Answer Completeness Check", # Unique name for this check
data={"input": user_input, "output": response},
settings={"categories": [{"name": "complete"}, {"name": "incomplete"}]}
)

# Check 2: Is the tone appropriate?

langwatch.evaluation.evaluate(
"langevals/llm_category",
name="Tone Appropriateness Check", # Different name for this check
data={"input": user_input, "output": response},
settings={"categories": [{"name": "professional"}, {"name": "casual"}, {"name": "inappropriate"}]}
)

import { LangWatch } from "langwatch";

const langwatch = new LangWatch();

// Check 1: Is the answer complete?
await langwatch.evaluations.evaluate("langevals/llm_category", {
    name: "Answer Completeness Check",  // Unique name for this check
    data: { input: userInput, output: response },
    settings: { categories: [{ name: "complete" }, { name: "incomplete" }] }
});

// Check 2: Is the tone appropriate?
await langwatch.evaluations.evaluate("langevals/llm_category", {
    name: "Tone Appropriateness Check",  // Different name for this check
    data: { input: userInput, output: response },
    settings: { categories: [{ name: "professional" }, { name: "casual" }, { name: "inappropriate" }] }
});

Without unique names, all results would be grouped under the same auto-generated identifier (e.g., custom_eval_langevalsllm_category), making it impossible to analyze them separately.

Common Request Format

All evaluator endpoints follow a similar pattern:

POST /api/v1/evaluations/{evaluator_path}/evaluate

Each evaluator accepts specific input parameters and settings. Refer to the individual evaluator documentation pages for detailed request/response schemas and examples.

Response Format

Successful evaluations return an array of evaluation results with scores, details, and metadata specific to each evaluator type.

Last modified on October 12, 2026

​Intro

​Authentication

​Allowed Methods

​Evaluators List

​Expected Answer

​LLM as Judge

​RAG Quality

​Quality Aspects

​Safety

​API only

​Running Evaluations

​Using Evaluators

Built-in Evaluators

Saved Evaluators

Custom Scoring

​The name Parameter

​Common Request Format

​Response Format

Intro

Authentication

Allowed Methods

Evaluators List

Expected Answer

LLM as Judge

RAG Quality

Quality Aspects

Safety

API only

Running Evaluations

Using Evaluators

The `name` Parameter

Common Request Format

Response Format