Amazon has expanded its payment program for developers who build popular voice apps for the digital assistant Alexa, opening it up to independent developers for the first time and adding more categories of “skills” eligible for rewards.

Amazon has been testing the program, which pays developers of skills with the highest customer engagement on a monthly basis in the U.S., U.K. and Germany, since May. Today’s news represents the public release of the program.

While the beta test was limited to skills in the Games, Trivia & Accessories category, six additional categories are now eligible for the payment program: Education & Reference, Food & Drink, Health & Fitness, Lifestyle, Music & Audio, and Productivity.

Amazon did not say how much top skill developers are paid under the program. According to an FAQ, payment is based on customer engagement, and is evaluated on a monthly basis, so a skill with a long-shelf life could pay out more than a skill that goes viral but doesn’t have the depth to keep people engaged.

In a blog post announcing the expansion of the program, Amazon also gave developers some tips at how to make their skills popular and engaging. Alexa is of course a voice-based program, so designing skills with voice use in mind makes for a good starting. Amazon recommends developers check out its voice design guide to help learn about “planning your script, keeping it brief and conversational, ensuring it catches the various ways that people may say things, simplifying the user flow, providing ways to get help, and more. The most engaging skills are easy to use.”

Rewards like these are meant to entice developers to use Amazon’s platform, as more companies jump into the competitive digital assistant market. Amazon has the early lead, and Alexa piled up more than 15,000 skills as of the end of June. But the online retail giant is facing plenty of formidable challenges from the likes of Google and Apple, as well as recent entries from global tech giants like Alibaba and Samsung.

Alexa’s proliferation can be partially attributed to Amazon’s decision to open the digital brain up to developers and device manufacturers in 2015. Alexa Voice Service lets manufacturers integrate Alexa into their products. The Alexa Skills Kit encourages third-party developers to build skills for Alexa. Developers who want to add to Alexa’s abilities can write code that works with Alexa in the cloud, letting the smart assistant do the heavy lifting of understanding and deciphering spoken commands.

Amazon also opened up what amounts to Alexa’s ears, her 7-Mic Voice Processing Technology, to third party hardware makers who want to build the digital brain into their devices. Amazon opened up Amazon Lex, the artificial intelligence technology that powers Alexa to developers.

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