k6: [ERROR] Cannot load test script – threshold evaluation with missing tags

Answer

When a threshold expression references a tag that never appears on any metric sample, k6 treats this as a fatal validation error. The engine aborts during script loading with the message "threshold evaluation with missing tags" and does not start any VUs. No warning is emitted; the error stops execution.

Confirmed facts

• Threshold definitions are validated at load time (k6 ≥ v0.44). Every tag used in a threshold expression must be present in at least one metric sample; otherwise loading fails.
• The error is logged to stderr and the process exits with a non‑zero status code.
• There is no CLI flag or runtime option to bypass this validation or to force evaluation of a threshold that references a missing tag.
• Because the script never reaches the execution phase, there is no internal state to inspect after the fact; the only detectable signal is the error message or exit code.

Steps to resolve

1. Identify the tag used in the failing threshold (e.g., method, status, or a custom tag).
2. Verify that at least one request or custom metric in the script emits that tag. Add a dummy sample if needed, e.g.:

import http from 'k6/http';
import { Counter } from 'k6/metrics';

const myCounter = new Counter('my_counter');

export default function () {
  // Ensure the tag exists
  http.get('https://httpbin.org/get', { tags: { method: 'GET' } });
  myCounter.add(1, { method: 'GET' });
}

1. Correct any typo in the tag name within the threshold expression.
2. Re‑run the script: k6 run script.js. The error should disappear and the test will proceed.

Detection

To programmatically detect that a threshold was rejected because of a missing tag, capture stderr or the process exit code:

OUTPUT=$(k6 run script.js 2>&1)
if [[ $? -ne 0 && $OUTPUT == *"threshold evaluation with missing tags"* ]]; then
  echo "Threshold validation failed due to missing tag"
fi

If the script loads successfully, no such message appears and the exit code is zero.