Intermittent invalid-keys error from siteverify endpoint (reCAPTCHA v2 invisible)

Hello folks!

Hope this message finds you well :)

I'm working on a production application that handles around 400k daily requests, and we are currently facing an intermittent issue with reCAPTCHA validation. Occasionally, when validating tokens using the /siteverify endpoint, we receive the following response:

{
    "success": false,
    "error-codes": ["invalid-keys"]
}

Context

• We are using reCAPTCHA v2 Invisible

• Tokens are generated on the client and validated server-side using /siteverify

• The application runs under a single domain

• The integration has been stable and unchanged for some time

What we have verified

• The site key and secret key are correct and consistent

• Both keys belong to the same reCAPTCHA configuration

• We are not rotating keys or switching environments dynamically

• The same keys are used for all requests

• The error is intermittent — the majority of validations succeed

Additional observations

• The error only appears in the backend response from /siteverify

• We could not find invalid-keys documented as a possible error code in the official documentation:
  https://developers.google.com/recaptcha/docs/verify

• Because of this, it is unclear what exact condition triggers this error

Questions

• What does the invalid-keys error specifically mean in the context of siteverify?

• Under what conditions can it occur intermittently?

• Has anyone else experienced this behavior?

Any guidance or insight would be greatly appreciated.

Thanks in advance!