Codeception namespaces not recognised

I'm using PhpStorm 2017.2.4 on Windows 10.

I've downloaded Codeception 2.3.6 manually and added its directory to the PHP Include Paths under "Languages & Frameworks". I've also added the exact file (codecept.phar) to "Path to Codeception executable" and added the "Default configuration file" (codeception.yml) to "Test Runner".

Despite all this and seeing it work temporarily, the namespaces are all errored, and the auto-complete doesn't work, e.g. "$this->ass" should suggest "assertSame", etc.

Pressing the "Run" icon runs the Codeception tests within PhpStorm and displays the results, so it knows it's there in part.

I've also tried doing "Synchronize" as sometimes that helps when there have been changes to a project, but it didn't make a difference.

What do I need to change to get it to recognise Codeception completely?