One of the best things to come out of Amazon’s unsuccessful smartphone experiment, the Fire, was Alexa, the company's responsive, voice-activated, assistant. While Amazon quickly snuffed out its Fire smartphone, it soon found a new home for Alexa in an omnidirectional WiFi speaker called the Echo, which became popular among early adopters and ushered in a new era in the connected-home experience.

And she’s back on a smartphone. But this time it’s an iPhone enabled by an app called Lexi, a $5 download from the Apple app store produced by a company called Bluetoo Ventures that taps into Amazon's virtual assistant. I tried it on an iPhone 6 Plus and quickly found that Apple's own Siri assistant has nothing to worry about.

To understand why, you have to understand what makes Alexa an appealing digital entity in the home. With the Echo speaker serving as her voice and ears, the soft-voiced Alexa responds quickly to your commands after you utter “Alexa,” the default wake word. (If Alexa doesn't work for you, you can change the wake word to Amazon or Echo in Settings.) “Alexa, what’s the weekend forecast?” “Alexa, how did the New York Mets do?” "Alexa, shuffle Chill Music playlist." “Alexa, dim the lights 30 percent.” In many cases, depending on the complexity of the request and the quality of your Internet connection, the answer comes back in about a second. When stumped, Alexa politely asks you to repeat your question or admits defeat.

Alexa has since moved into two other Amazon devices: a more portable, battery-powered speaker called the Amazon Tap, and a hockey-puck sized disc called the Echo Dot that can bring her interactive magic to auxiliary, hopefully better-sounding, audio equipment. But while those Alexa extensions work well, the Lexi iPhone app falls short.

Here’s why.

Alexa speaks freely—and forgets she's got a screen. Alexa was limited and easily confused by imprecise language in her Echo debut, at least in comparison to the voice-activated assistants on smartphones. However, Alexa has become more capable over time, as Amazon has added more apps and services from itself and its partners.

For instance, a short while after Echo's launch, Amazon added stock and sports updates, the schedules for regional transportation services, flower ordering, and cost estimates for inter-city air travel. You can control lights and appliances from Philips, Insteon, Nest, and other makers, with some fiddling that is most conveniently done using Amazon's own Alexa smartphone app (a free download for both iPhone and Android devices, not to be confused with Lexi).

Lexi can handle many of these tasks, but she still acts like a talking speaker. So if you ask her how a particular stock is doing, she’ll tell you—and everyone within earshot—but you won’t see the easier-to-interpret numbers, tables, or graphs that any halfway decent stock-market app would show you. That's a waste of an iPhone display.

And, surprisingly Lexi can't play music from Amazon Prime, Spotify, or other streaming services. When you ask her to play a song, Alexa says “Amazon Music is not supported for this device.” That feels like a remarkable failure—especially since Siri can launch the Amazon Music app if you ask her to.

Voice dialing—something a flip phone from 10 years ago could do—is also missing from Lexi's Can-Do list. However, you can set up reminders if you have a calendar linked to your Amazon account.

Lexi isn't very mobile. When you install Lexi on an iPhone, it will appear as an Alexa device on your Amazon account. On my account, it appeared as Michael's Lexi. But as with my other Alexa devices, I had to add my address or zip code in the account settings so that it could provide such information as weather and local news. And that’s a problem.

Lexi apparently has extremely limited access to iPhone’s map and GPS data. For instance, if you ask her “Where is the nearest McDonald’s?” she’ll suggest one that’s close to the zip code or address you entered in settings, and she won’t be able to launch a navigation app to help you find it. 

If you ask the app where you are, Alexa will tell you how far you've traveled from home, in miles, but not your location—not even the name of the town you're in. That’s a serious shortcoming on a mobile device, especially one that already has Siri to help you find, order from, and direct you to practically anything with very little effort on your part.

Bottom line. One has to question the wisdom of producing a severely limited voice-activated assistant for a device that already has quite a capable one built in. And to charge $5 for such a disappointment is an insult. Consumers would probably like a mobile app that combined all the capabilities of Alexa with the display, critical apps, and location-based data of its smartphone host. But the Lexi iPhone app falls far short of that mark. 

Correction: This article has been updated to clarify that the Lexi app was not produced by Amazon, but by a third party using Amazon's Voice Service SDK, or software development kit.