Amazon is bringing out its first fitness tracker, a wristband called the Halo, which will eavesdrop on its owner to assess if they are happy or sad based on their tone of voice.

A pair of always-on microphones will listen to users, trying to assess their mood and stress levels. After a training period, the band periodically checks in and listens to your speech, categorising it into a number of emotional buckets – such as hopeful, elated or hesitant – and offer an overall view of peak positive and negative periods.

Meanwhile, a set of AI tools built into the accompanying service, which will cost $3.99 a month in the US (it is not currently available in the UK), will attempt to gauge information such as weight and body fat percentage by asking users to take undressed selfies with the app.

The company says these features allow the Halo system to do more with less. The Halo Band itself, which costs $99 and resembles a wristwatch turned inside out, has a relatively simple array of sensors compared with its competitors. An accelerometer, temperature sensor and heart rate monitor allows it to do simple tracking of exertion levels and basic health. But without a GPS system, for instance, it can’t accurately track movement, while advanced features like the Apple Watch’s EKG are also absent.

Instead, Amazon hopes that it will make up with its AI tools what it lacks in pure sensors. “Halo analyses the intensity and duration of your movement, not just steps,” the company says, citing guidance from the American Heart Association.

“Despite the rise in digital health services and devices over the last decade, we have not seen a corresponding improvement in population health in the US. We are using Amazon’s deep expertise in artificial intelligence and machine learning to offer customers a new way to discover, adopt, and maintain personalised wellness habits,” said Dr Maulik Majmudar, Amazon Halo’s principal medical officer.

For many, those AI tools may prove a step too far. “Amazon is building a surveillance empire, a privately owned panopticon,” said Evan Greer, deputy director of Fight for the Future. “That’s their entire business model. They harvest enormous amounts of data about the most intimate details of our lives and use it to fuel their profit machine and strengthen their monopolistic grip.

“A wearable device that scans your body and polices your tone of voice is very on-brand for a company that seems hellbent on bringing dystopian science fiction to life,” she added.

The always-on microphones, for instance, aren’t connected to Amazon’s Alexa service, as one might expect. Instead, they power Halo’s Tone system, which provides “tone of voice analysis”. Those recordings are never uploaded to the cloud, Amazon says, nor will any humans ever listen to them: instead, they are analysed on a user’s device, and then deleted.

Another new feature, Body, comes with similar promises. The feature asks users to take whole-body selfies using the app, which are analysed in the cloud to uncover metrics such as body fat percentage, before being deleted and stored locally.

“Health is inherently personal, and taking photographic scans of your body is about as private as it gets. That’s why Amazon Halo and Body were built with privacy in mind,” the company says.

Rounding out the feature set is a sleep-tracking service, which “uses motion, heart rate, and temperature to measure time asleep and time awake”.

The Halo launches on Friday in the US, where customers can request early access at a reduced rate of $64.99, including six months of the Halo membership service.