Cortana in Windows 10. Image source: Microsoft.

When it rains, it pours. (NASDAQ:AMZN) was the first tech giant to really open up its virtual assistant Alexa to third-party developers, releasing software development kits (SDKs) last year in an effort to turn Alexa into a platform. The move was notable because while Amazon was among the last major tech players to launch a virtual assistant, it was the first to open it up to third-party developers. That's largely why Alexa has become so powerful, as third-party developers are often the innovative lifeblood of any platform. Apple (NASDAQ:AAPL) is well aware of this, but it only opened up Siri to third-party developers earlier this year...half a decade after I asked it to. Alphabet's (NASDAQ:GOOG) (NASDAQ:GOOGL) Google just recently opened up Google Assistant with Actions on Google.

Now, software giant Microsoft (NASDAQ:MSFT) has just announced that it is following suit, and it will open up Cortana to third-party developers with an SDK of its own.

Everybody's doing it

In an official blog post, Microsoft details the release of its Cortana Skills Kit and Cortana Devices SDK. Cortana Skills Kit is designed for third-party developers, while the Cortana Devices SDK is for original equipment manufacturers (OEMs) and original design manufacturers (ODMs) to incorporate Cortana into their products. The company also notes that there are now 145 million Cortana users across all platforms, although it didn't indicate how much those people actually use the virtual assistant.

This is also very much a coup targeting Alexa's developer base, as Microsoft is making it easy for developers to repurpose code from Alexa skills to bring those capabilities to Cortana.

The Cortana Devices SDK is in a small private preview for the time being, with broader availability planned for 2017. One of the first hardware partners to use the Cortana Devices SDK will be Harman Kardon, and the two companies are teasing an upcoming Cortana-powered speaker that will challenge Amazon's popular Echo, Google's new Home, and whatever it is that Apple is working on.

It begins

We're about to see a sort of renaissance in virtual assistants. Due largely to limited functionality (read: no third-party developers), Siri remained mostly a novelty for its first couple of years of existence, and arguably still fits that classification. But as it tends to do, competition promotes action, and all four of these major virtual assistants (there are some others out there but they've made little impact thus far) are on the verge of becoming true voice-driven computing platforms built on artificial intelligence (AI) and machine learning.

Like other platforms, consumers will be attracted to whichever virtual assistant is most capable, which is a function of how much developer support each has. Amazon's Alexa is the clear leader thus far, hitting 3,000 skills in September. Initially, developers will likely spread themselves across all platforms as a hedge, but once the leaders emerge, they'll inevitably focus their development efforts there.

Underneath the virtual assistants, which primarily serve as just user interfaces, the real battle will boil down to the underlying AI that each company is developing. And that could be the next big thing in tech.