This happens because there is a table of recognised capabilities and idle-timeout
is not one of them. It's put in place to filter out bad configuration and notify you if you messed something up. The work around is to use 'extra_capabilities' option, that doesn't get normalised and validated and won't throw this error. Unfortunately the documentation doesn't cover a lot of things like that and you have to learn them yourself (or here).
Windows8_IE10:
context:
class: 'FeatureContext'
extensions:
Behat\MinkExtension\Extension:
selenium2:
browser: internet explorer
wd_host: seodevelopment:@ondemand.saucelabs.com/wd/hub
capabilities: { "platform": "Windows 8", "version": "10" }
extra_capabilities: { "idle-timeout": 200 }
Extra capabilities are merged with the capabilities object and passed to the Selenium server when Mink starts a new session. The difference between the two is that extra capabilities object doesn't get normalised like the capabilities, so you can specify whatever you want there. When you run the Selenium server it logs when the session is started and what capabilities the browser driver receives.
21:54:51.659 INFO - Executing: [new session: Capabilities [{tags=[Ian-Bytcheks-MacBook-Pro.local, PHP 5.5.15], platform=ANY, browserVersion=9, ignoreZoomSetting=false, browserName=chrome, deviceType=tablet, name=Behat feature suite, browser=chrome, deviceOrientation=portrait, chromeOptions={args=[--test-type]}, version=21}]])
Note, this relates to Behat 3, earlier versions may differ. Find the tag in the Mink Selenium 2 Driver repo and look for the similar class that implements this logic.