cypress-io/cypress · warning · Error

name should be a single string but got: ${from.query.name}

Error message

name should be a single string but got: ${from.query.name}

What it means

Thrown by the /redirect route handler in the Cypress app router. The route reads query.name and requires it to be a single string. Express/Vue Router parse repeated query params (?name=A&name=B) into an array, which fails the typeof === 'string' guard. This is a defensive validation on an internal redirect used by changeUrlToDebug in the server.

Source

Thrown at packages/app/src/router/router.ts:42

  /**
   * Redirect route useful for passing page params when routing to a particular page
   *
   * @param name route name
   * @param params url encoded JSON object of page component params
   *
   * @example
   * // redirects to the Debug page passing a parameter of `from` set to `notification`
   * "/redirect?name=Debug&params=%7B%22from%22%3A%22notification%22%7D"
   *
   * @see changeUrlToDebug in packages/server/lib/open_project.ts
   */
  routes.push({
    path: '/redirect',
    redirect: (from) => {
      if (from.query.name) {
        if (typeof from.query.name !== 'string') {
          throw new Error(`name should be a single string but got: ${from.query.name}`)
        }

        let params = {}

        if (from.query.params) {
          if (typeof from.query.params !== 'string') {
            throw new Error(`params should be a string but got: ${from.query.params}`)
          }

          try {
            params = JSON.parse(from.query.params)
          } catch {
            throw new Error(`params was not valid JSON: ${from.query.params}`)
          }
        }

        return {
          name: from.query.name,

View on GitHub (pinned to 0d85fdc912)

Solutions

1. If you construct the URL, ensure `name` appears exactly once: `/redirect?name=Debug`.
2. When serializing params from JS, use URLSearchParams with set (not append) for `name`.
3. Report a Cypress bug if you hit this from normal app navigation — the server should not be emitting duplicate name params.

Example fix

// before
const url = `/redirect?name=Debug&name=${secondary}`
// after
const params = new URLSearchParams()
params.set('name', 'Debug')
params.set('params', JSON.stringify({ from: secondary }))
const url = `/redirect?${params.toString()}`

function singleString (v: unknown): string | null {
  return typeof v === 'string' ? v : null
}
const name = singleString(from.query.name)
if (!name) throw new Error('name must be a single string')

function isSingleString (v: unknown): v is string {
  return typeof v === 'string'
}

When it happens

Trigger: A client (the server's open_project.ts changeUrlToDebug, or a hand-crafted URL) issues /redirect?name=Debug&name=Runs causing Express to surface name as ['Debug','Runs']. Also reproducible if a buggy caller appends name twice.

Common situations: Internal Cypress URL-construction bug that double-appends name, browser extension or proxy mangling query strings, or third-party code linking to /redirect with duplicate params.

Related errors

• params should be a string but got: ${from.query.params}
• params was not valid JSON: ${from.query.params}
• Missing event for ${event}

AI-assisted analysis of cypress-io/cypress@0d85fdc912 (2026-08-12). Data as JSON: /api/errors/05fd01c6619fdff3. Report an issue: GitHub.