Meaning Match

The Meaning Match metric evaluates whether an LLM’s output semantically matches a ground truth answer, regardless of phrasing or formatting. This metric is particularly useful for evaluating question-answering systems where the same answer can be expressed in different ways.

How to use the Meaning Match metric

The Meaning Match metric is available as an LLM-as-a-Judge metric in automation rules. You can use it to automatically evaluate traces in your project by creating a new rule.

Creating a rule with Meaning Match

1. Navigate to your project in Opik
2. Click on “Rules” in the sidebar
3. Click “Create new rule”
4. Select “LLM-as-judge” as the metric type
5. Choose “Meaning Match” from the prompt dropdown
6. Configure the variable mapping:
   • input: The original question or prompt
   • ground_truth: The expected correct answer
   • output: The LLM’s generated response
7. Select your preferred LLM model for evaluation
8. Configure sampling rate and filters as needed
9. Click “Create rule”

Understanding the scores

The Meaning Match metric returns a boolean score:

• true (1.0): The output conveys the same essential answer as the ground truth, even if worded differently
• false (0.0): The output contradicts, differs from, or fails to include the core answer in the ground truth

Each score includes a detailed reason explaining the judgment.

Evaluation Guidelines

The Meaning Match metric follows these rules when evaluating responses:

1. Focus on factual equivalence - Ignores style, grammar, or verbosity
2. Accept aliases and synonyms - “NYC” ≈ “New York City”; “Da Vinci” ≈ “Leonardo da Vinci”
3. Ignore formatting - Case, punctuation, and whitespace differences are acceptable
4. Allow extra context - Additional details are okay if they don’t contradict the main answer
5. Reject hedging - Uncertain or incomplete answers score as false
6. Treat numeric equivalents - “100” = “one hundred”
7. Reject multiple alternatives - If the output includes the correct answer with incorrect alternatives, it scores as false

Example evaluations

Meaning Match Prompt

Opik uses an LLM as a Judge to evaluate semantic equivalence. By default, the evaluation uses the model you select when creating the rule. The prompt template used for evaluation is:

You are an expert semantic equivalence judge. Your task is to decide whether the OUTPUT conveys the same essential answer as the GROUND_TRUTH, regardless of phrasing or formatting.
## What to judge
- TRUE if the OUTPUT expresses the same core fact/entity/value as the GROUND_TRUTH.
- FALSE if the OUTPUT contradicts, differs from, or fails to include the core fact/value in GROUND_TRUTH.
## Rules
1. Focus only on the factual equivalence of the core answer. Ignore style, grammar, or verbosity.
2. Accept aliases, synonyms, paraphrases, or equivalent expressions.
   Examples: "NYC" ≈ "New York City"; "Da Vinci" ≈ "Leonardo da Vinci".
3. Ignore case, punctuation, and formatting differences.
4. Extra contextual details are acceptable **only if they don't change or contradict** the main answer.
5. If the OUTPUT includes the correct answer along with additional unrelated or incorrect alternatives → FALSE.
6. Uncertain, hedged, or incomplete answers → FALSE.
7. Treat numeric and textual forms as equivalent (e.g., "100" = "one hundred").
8. Ignore whitespace, articles, and small typos that don't change meaning.
## Output Format
Your response **must** be a single JSON object in the following format:
{
  "score": true or false,
  "reason": ["short reason for the response"]
}
## Example
INPUT: "Who painted the Mona Lisa?"
GROUND_TRUTH: "Leonardo da Vinci"
OUTPUT: "It was painted by Leonardo da Vinci."
→ {"score": true, "reason": ["Output conveys the same factual answer as the ground truth."]}
OUTPUT: "Pablo Picasso"
→ {"score": false, "reason": ["Output names a different painter than the ground truth."]}
INPUT:
{{input}}
GROUND_TRUTH:
{{ground_truth}}
OUTPUT:
{{output}}

Use cases

The Meaning Match metric is ideal for:

• Question-answering systems - Evaluate if answers are semantically correct
• Information extraction - Verify extracted entities match expected values
• Knowledge base validation - Check if responses align with ground truth knowledge
• RAG systems - Assess if retrieved information correctly answers questions
• Multi-language systems - Compare answers across translations (when ground truth is translated)

Best practices

• Provide clear ground truth - The more specific the ground truth, the more accurate the evaluation
• Use with other metrics - Combine with other metrics like hallucination or answer relevance for comprehensive evaluation
• Monitor false positives/negatives - Review evaluation results periodically to ensure the metric works well for your use case
• Test with edge cases - Try the metric with ambiguous or borderline cases to understand its behavior