@mastra/core/dist/docs/references/reference-evals-textual-difference.md

Mastra is a framework for building AI-powered applications and agents with a modern TypeScript stack.

# Textual difference scorer

The `createTextualDifferenceScorer()` function uses sequence matching to measure the textual differences between two strings. It provides detailed information about changes, including the number of operations needed to transform one text into another.

## Parameters

The `createTextualDifferenceScorer()` function doesn't take any options.

This function returns an instance of the MastraScorer class. See the [MastraScorer reference](https://mastra.ai/reference/evals/mastra-scorer) for details on the `.run()` method and its input/output.

## `.run()` returns

**runId** (`string`): The id of the run (optional).

**analyzeStepResult** (`object`): Object with difference metrics: { confidence: number, changes: number, lengthDiff: number }

**score** (`number`): Similarity ratio (0-1) where 1 indicates identical texts.

`.run()` returns a result in the following shape:

```typescript
{
  runId: string,
  analyzeStepResult: {
    confidence: number,
    ratio: number,
    changes: number,
    lengthDiff: number
  },
  score: number
}
```

## Scoring details

The scorer calculates several measures:

- **Similarity Ratio**: Based on sequence matching between texts (0-1)
- **Changes**: Count of non-matching operations needed
- **Length Difference**: Normalized difference in text lengths
- **Confidence**: Inversely proportional to length difference

### Scoring Process

1. Analyzes textual differences:

   - Performs sequence matching between input and output
   - Counts the number of change operations required
   - Measures length differences

2. Calculates metrics:

   - Computes similarity ratio
   - Determines confidence score
   - Combines into weighted score

Final score: `(similarity_ratio * confidence) * scale`

### Score interpretation

A textual difference score between 0 and 1:

- **1.0**: Identical texts – no differences detected.
- **0.7–0.9**: Minor differences – few changes needed.
- **0.4–0.6**: Moderate differences – noticeable changes required.
- **0.1–0.3**: Major differences – extensive changes needed.
- **0.0**: Completely different texts.

## Example

Measure textual differences between expected and actual agent outputs:

```typescript
import { runEvals } from '@mastra/core/evals'
import { createTextualDifferenceScorer } from '@mastra/evals/scorers/prebuilt'
import { myAgent } from './agent'

const scorer = createTextualDifferenceScorer()

const result = await runEvals({
  data: [
    {
      input: 'Summarize the concept of recursion',
      groundTruth:
        'Recursion is when a function calls itself to solve a problem by breaking it into smaller subproblems.',
    },
    {
      input: 'What is the capital of France?',
      groundTruth: 'The capital of France is Paris.',
    },
  ],
  scorers: [scorer],
  target: myAgent,
  onItemComplete: ({ scorerResults }) => {
    console.log({
      score: scorerResults[scorer.id].score,
      groundTruth: scorerResults[scorer.id].groundTruth,
    })
  },
})

console.log(result.scores)
```

For more details on `runEvals`, see the [runEvals reference](https://mastra.ai/reference/evals/run-evals).

To add this scorer to an agent, see the [Scorers overview](https://mastra.ai/docs/evals/overview) guide.

## Related

- [Content Similarity Scorer](https://mastra.ai/reference/evals/content-similarity)
- [Completeness Scorer](https://mastra.ai/reference/evals/completeness)
- [Keyword Coverage Scorer](https://mastra.ai/reference/evals/keyword-coverage)