SeleniumHQ/selenium · error · WebDriverError

Unknown error: ${JSON.stringify(data)}

Error message

Unknown error: ${JSON.stringify(data)}

What it means

Thrown by throwDecodedError() in error.js when the response data does not satisfy isErrorResponse() (i.e., it lacks a string `error` field) but was routed to the error-throwing path. The raw data is JSON-stringified into the message. This typically indicates a non-W3C-compliant or malformed response from the remote end.

Source

Thrown at javascript/selenium-webdriver/lib/error.js:532

 * if the provided `data` is not a valid encoded error.
 *
 * @param {{error: string, message: string}} data The error data to decode.
 * @throws {WebDriverError} the decoded error.
 * @see https://w3c.github.io/webdriver/webdriver-spec.html#protocol
 */
function throwDecodedError(data) {
  if (isErrorResponse(data)) {
    let ctor = ERROR_CODE_TO_TYPE.get(data.error) || WebDriverError
    let err = new ctor(data.message)
    // TODO(jleyba): remove whichever case is excluded from the final W3C spec.
    if (typeof data.stacktrace === 'string') {
      err.remoteStacktrace = data.stacktrace
    } else if (typeof data.stackTrace === 'string') {
      err.remoteStacktrace = data.stackTrace
    }
    throw err
  }
  throw new WebDriverError('Unknown error: ' + JSON.stringify(data))
}

/**
 * Checks a legacy response from the Selenium 2.0 wire protocol for an error.
 * @param {*} responseObj the response object to check.
 * @return {*} responseObj the original response if it does not define an error.
 * @throws {WebDriverError} if the response object defines an error.
 */
function checkLegacyResponse(responseObj) {
  // Handle the legacy Selenium error response format.
  if (isObject(responseObj) && typeof responseObj.status === 'number' && responseObj.status !== 0) {
    const { status, value } = responseObj

    let ctor = LEGACY_ERROR_CODE_TO_TYPE.get(status) || WebDriverError

    if (!value || typeof value !== 'object') {
      throw new ctor(value + '')
    } else {

View on GitHub (pinned to aa36b38e69)

Solutions

1. Inspect the JSON-stringified data in the error message to see what the server actually returned.
2. Verify the remote endpoint URL is correct and points to a WebDriver-compatible server.
3. Check for intermediate proxies (nginx, corporate proxy) that may inject non-protocol responses.
4. Upgrade the remote server/Grid to a W3C-compliant version matching the client.
5. Enable debug logging to capture the raw HTTP response.

try {
  // ... execute WebDriver command ...
} catch (e) {
  if (e instanceof WebDriverError && e.message.startsWith('Unknown error:')) {
    console.error('Non-W3C response from server. Raw data:', e.message)
    // check server URL, proxy, server version
  }
  throw e
}

When it happens

Trigger: The remote WebDriver/Grid endpoint returns a response object without an `error` string field where one was expected. A proxy or middleware returns unexpected JSON. A server returns an HTML error page that was parsed into a non-error object. Version mismatch between client expectations and server response shape.

Common situations: Misbehaving or outdated Selenium Grid/server; intermediate reverse proxy returning its own error JSON; third-party cloud grid with a non-standard error envelope; server crash returning a non-protocol response.

Related errors

• ${message}
• {error}: {response['message']}
• no sessionId in returned payload
• Switch to desired window before changing its size
• Switch to desired window before getting its size

AI-assisted analysis of SeleniumHQ/selenium@aa36b38e69 (2026-08-14). Data as JSON: /api/errors/c97f08ae4dd44e6a. Report an issue: GitHub.