Support templates in our string evaluators

[maraisr]

Much like our web prompt tooling, we should allow the models cli to support templated variables in string evaluators.

Before

Running test case 1/1...
  ✗ FAILED
    Model Response: Goodbye! Take care and see you next time! 🌍👋
    ✗ string evaluator (score: 0.00)
      Expected to contain: '{{expected}}' ⬅️⬅️⬅️⬅️
    ✓ similarity check (score: 0.25)
      LLM evaluation matched choice: '2'

After

Running test case 1/1...
  ✗ FAILED
    Model Response: Hello there! How can I assist you today? 😊
    ✓ string evaluator (score: 1.00)
      Expected to contain: 'hello' ⬅️⬅️⬅️⬅️
    ✗ similarity check (score: 0.00)
      LLM evaluation matched choice: '1'

[Copilot]

Pull Request Overview

Support templated variables in string evaluators for the CLI, allowing test cases to inject dynamic values into string checks.

• Introduces string fields in example prompts and switches evaluator contains to use {{string}}
• Updates runStringEvaluator signature to accept a testCase map and applies templateString in each comparison
• Adjusts tests to pass an empty testCase into runStringEvaluator

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
examples/sample_prompt.yml	Added string keys under testData and updated evaluator contains
cmd/eval/eval_test.go	Changed runStringEvaluator calls to include the new testCase param
cmd/eval/eval.go	Refactored runStringEvaluator to template all string criteria

Comments suppressed due to low confidence (2)

examples/sample_prompt.yml:9

• [nitpick] The key string in testData is very generic and may be confused with the evaluator’s string block. Consider renaming it to something more descriptive like templateVar or placeholder.

    string: hello

cmd/eval/eval_test.go:132

• There are no tests covering the new templating functionality or error paths when a placeholder is missing. Consider adding tests that include {{string}} substitutions and missing-key scenarios to validate templateString behavior.

result, err := handler.runStringEvaluator("test", tt.evaluator, map[string]interface{}{}, tt.response)

[maraisr]

Yeah I do agree with Copilot in that this looks a little gnarly. But with the way that Go wants to handle errors we kinda need this everywhere.

That is unless, we create the helper method here — that should there be an error, we just default back to the provided string. But open to suggestions/opinions.