How to add evaluators to an existing experiment (Python only)

Evaluation of existing experiments is currently only supported in the Python SDK.

After running an experiment, you may want to add new evaluation metrics without re-running your application. This is useful when you’ve added new evaluators or want to apply different scoring criteria to existing results. Instead of re-executing your target function on all examples, you can evaluate the existing experiment traces directly.

To add evaluators to an existing experiment, pass the experiment name or ID to evaluate() / aevaluate() instead of a target function. The evaluators will run on the cached traces from the original experiment, accessing the inputs, outputs, and any intermediate steps that were logged.

Example

from langsmith import evaluate

def always_half(inputs: dict, outputs: dict) -> float:
    return 0.5

experiment_name = "my-experiment:abc"  # Replace with an actual experiment name or ID

evaluate(experiment_name, evaluators=[always_half])

Related topics

• Retry failed examples in experiments
• Run an evaluation
• Run an evaluation asynchronously

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