These methods are available to you:

page.driver.browser.switch_to.alert.accept
page.driver.browser.switch_to.alert.dismiss
page.driver.browser.switch_to.alert.text # the confirmation text

Rails Long Term Support

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Read more Show archive.org snapshot

Spreewald Show archive.org snapshot gives you steps like these:

When I confirm the browser dialog
When I cancel the browser dialog

Also see Type text into Javascript prompt dialogs in Capybara/Selenium.