JEVAL
code · freepromptfoodeterministicmirrors promptfoo starts-with

Starts with

Starts with checks that the output begins with the expected string, ignoring case and surrounding whitespace. It mirrors promptfoo's `starts-with` and is free.

Run this example in the console runs in code · $0
how it works // frame by frame

What the evaluator does

Both strings are trimmed and lower-cased; the output must begin with the expected text.

frame 1/4 · the two inputs
output
Yes — the refund was issued on Monday.
expected
yes
1.trim and lower-case
2.compare the prefix
starts withscore 1
runs in code · $0 · ~0 ms · deterministic
when to use it

Reach for it when

  • Required openers: a greeting, a status word, a JSON opening brace.
  • Detecting preambles you do not want (invert the pass in your pipeline).

Not the right tool when

  • The opener may be paraphrased (use llm_judge with a rubric).

Watch out for

  • Markdown or code fences before the real content will fail the check; decide whether that is desired.
inputs and example

What to send

required fields
outputexpected
optional fields
none
in suites
PromptfooEverything
result shape

score 0–1 computed in code, label, passed at an evaluator-specific threshold, no probabilities.

POST /api/v1/evaluateopen in console 
{
  "evaluators": [
    "starts_with"
  ],
  "output": "Yes — the refund was issued on Monday.",
  "expected": "yes"
}
expected

Prefix present: score 1.

rolling it up

Aggregate with

One result per item is a fact; a dataset of them is a metric. These are the aggregations that fit this evaluator's output shape.

Pass rate
passed / total

Pass rate is the share of items whose evaluator result met its threshold.

related evaluators
regexcode · free
Output matches the regular expression in `pattern` (JavaScript syntax; `/…/flags` form allowed).
containscode · free
Output contains the expected string (case-insensitive).
exact_matchcode · free
Output equals expected after whitespace and case normalisation.
questions people ask
Is starts_with case-sensitive?
No. Both sides are lower-cased first.