promptfoo--promptfoo/site/docs/configuration/expected-outputs/similar.md

sidebar_position, description

sidebar_position	description
55	Calculate semantic similarity scores between actual and expected outputs using advanced embedding models and multiple distance metrics

Similarity (embeddings)

The similar assertion checks if an embedding of the LLM's output is semantically similar to the expected value, using a configurable similarity or distance metric with a threshold.

By default, embeddings are computed via OpenAI's text-embedding-3-large model.

Example:

assert:
  - type: similar
    value: 'The expected output'
    threshold: 0.8

If you provide an array of values, the test will pass if it is similar to at least one of them:

assert:
  - type: similar
    value:
      - The expected output
      - Expected output
      - file://my_expected_output.txt
    threshold: 0.8

The negated not-similar assertion is the logical inverse: with an array of values it passes only when the output is dissimilar to every value (and fails as soon as it is too similar to any one of them). This is the natural way to assert that an output does not resemble any item in a list of forbidden or canned answers.

Similarity Metrics

You can specify which metric to use by including it in the assertion type. The default is similar (cosine similarity).

Cosine Similarity (default)

Measures the cosine of the angle between two vectors. Range: -1 to 1 (higher is more similar), though text embeddings typically produce values between 0 and 1.

assert:
  # Default - uses cosine similarity
  - type: similar
    value: 'The expected output'
    threshold: 0.8

  # Explicit cosine
  - type: similar:cosine
    value: 'The expected output'
    threshold: 0.8

When to use: Best for semantic similarity where you care about the direction of the embedding vector, not its magnitude. This is the industry standard for embeddings.

Dot Product

Computes the dot product of two vectors. Range: unbounded, but typically 0 to 1 for normalized embeddings (higher is more similar).

assert:
  - type: similar:dot
    value: 'The expected output'
    threshold: 0.8

When to use: Useful when you want to match the metric used in your production vector database (many use dot product for performance). For normalized embeddings, dot product is nearly equivalent to cosine similarity.

Euclidean Distance

Computes the straight-line distance between two vectors. Range: 0 to ∞ (lower is more similar).

assert:
  - type: similar:euclidean
    value: 'The expected output'
    threshold: 0.5 # Maximum distance threshold

When to use: When you care about both the angle and magnitude differences between vectors. Note that the threshold represents the maximum distance (not minimum similarity), so lower values are stricter.

Important: For euclidean distance, the threshold semantics are inverted - it represents the maximum acceptable distance rather than minimum similarity.

Overriding the provider

By default similar will use OpenAI. To specify the model that creates the embeddings, do one of the following:

1. Use test.options or defaultTest.options to override the provider across the entire test suite. For example:

   defaultTest:
     options:
       provider:
         embedding:
           id: azureopenai:embedding:text-embedding-ada-002
           config:
             apiHost: xxx.openai.azure.com
   tests:
     assert:
       - type: similar
         value: Hello world
   

2. Set assertion.provider on a per-assertion basis. For example:

   tests:
     assert:
       - type: similar
         value: Hello world
         provider: huggingface:sentence-similarity:sentence-transformers/all-MiniLM-L6-v2