KaiOS.dev

A closer look at the TCL Go Flip 4 5G and KaiOS 4.0

Mon, 02 Mar 2026 00:00:00 +0800

The TCL Go Flip 4 5G (commercial reference T440W-EATBUS1-V for T-Mobile) is the first device to ship with KaiOS 4.0. Given that CVE-2023-33294 and CVE-2023-27108 were discovered on the original Alcatel Go Flip 4 running KaiOS 3.0, it was worth checking whether these issues carried forward to new hardware.

The short answer: KaiOS 4.0 is architecturally indistinguishable from KaiOS 3.0. Many of the known vulnerabilities have been partially addressed, but the surface area remains broad.

Announcing kaios-types: KaiOS TypeScript Declarations

Mon, 24 Nov 2025 00:00:00 +0800

Building for KaiOS using modern tools can be challenging. Most KaiOS devices run KaiOS 2.5 which is based on Firefox 48, initially released nearly a decade ago in 2016. However, KaiOS includes several modern features like Promisesand CSS grid that don’t perfectly align with it’s base Gecko engine.

Building KaiOS apps often requires a transpiler like Babel, but to make things even more complicated, KaiOS uses a number of non-standard APIs or APIs based on outdated specifications. That’s where kaios-types can help!

Deep Dive on Cloud Phone User Agents

Tue, 03 Jun 2025 00:00:00 +0800

Deep Dive on Cloud Phone User Agents

Why it matters? User agents are the most common way to identify devices on the web.

A similar deep dive was completed for KaiOS user agents.

Key Takeaways

Cloud Phone user agents follow a consistent pattern including several components:

User Agent Components

Cloud Phone identifies all Cloud Phone devices
Puffin identifies the remote-browser architecture shared with the Puffin browser
Nokia 225 4G is the device model name. Generic is used by the Cloud Phone Simulator or pre-production devices
Chrome/128.0.6613.170 identifies the Chromium version used by Cloud Phone servers

For compatibility, Chromium adds strings like KHTML, like Gecko and Safari.

An In-Depth Look At KaiOS Smart Touch

Sun, 26 Jan 2025 00:00:00 +0800

KaiOS Smart Touch TCL Prototype

KaiOS Smart Touch almost had a chance to compete with budget Android Go smartphones. That is, if it had not been killed before hitting the market.

KaiOS Smart Touch

KaiOS Smart Touch (ST) was a mobile operating system based on Firefox OS designed for budget touchscreen smartphones. ST dates back to 2020 as a fork of KaiOS with a somewhat unconventional user interface, a catalog of web apps from the KaiStore and JioStore, and a focus on affordability. It was initially developed for Reliance Jio following the massive success of the original JioPhone. However, Google’s multi-billion dollar investment in Jio ensured the telco would pivot to Android, killing KaiOS Smart Touch before it got off the ground.

What's coming in KaiOS 4.0?

Sun, 08 Dec 2024 00:00:00 +0800

May 2025 Update: the TCL Flip 4 launched with KaiOS 4.0 matching many of the predictions below. It’s available in the United States and is the first KaiOS phone to support 5G.

At CES 2024, KaiOS discussed 5G smart feature phones “slated for an early 2025 rollout” in partnership with Qualcomm.

KaiOS 4.0

KaiOS 4.0 is the next major version of KaiOS, expected to be released in 2025. Hints have been discovered in user agent logs and online forums. KaiOS 4.0 made it’s first appearance on the KaiOS Developer Portal in April 2024. Here what we know so far:

The Definitive Guide to Publishing KaiOS Apps on the JioStore

Sun, 10 Nov 2024 00:00:00 +0800

Publishing Apps on the JioStore

With the majority of KaiOS users located in India, publishing on the JioStore is a must to maximizing your app’s reach.

At a Glance

KaiOS has two app stores: the KaiStore (Global) and JioStore (India)
The JioStore is only available on the Reliance JioPhone
~90% of all KaiOS users are located in India
Less than 10% of KaiOS apps are publishing on the JioStore
Publishing on the JioStore is much more difficult than the KaiStore
Jio refers to its version of KaiOS as “JioOSLite”
Games are not published on the JioStore but on a separate platform, JioGames
Jio prefers apps integrated with the JioAds SDK (although payment is not supported for developers outside of India)

The JioStore Basics

The JioStore has a web portal to publish apps. Like the KaiStore, the JioStore accepts two types of apps: packaged and hosted. This is a significant improvement over the previous process of submitting builds as ZIP files via WhatsApp.

What APIs are supported on CloudMosa's Cloud Phone?

Fri, 09 Aug 2024 00:00:00 +0800

How Cloud Phone Works

Cloud Phone is a remote browser with two key components: a thin client application that renders a proprietary vector graphics language on device, and a sandboxed server running Chrome to translate websites comprised of HTML, CSS, and JavaScript into a vector graphics language. Cloud Phone is based on the Puffin web browser and uses the proprietary Puffin Vector Graphics Language. This greatly reduces the hardware requirements and bandwidth for running the thin client, allowing it to work effectively on devices like budget feature phones.

A Developer's Introduction to CloudMosa's Cloud Phone

Sun, 16 Jun 2024 00:00:00 +0800

A Developer’s Introduction to CloudMosa’s Cloud Phone

At a Glance

Cloud Phone is a Chrome-based remote browser from CloudMosa, Inc for budget feature phones to seamlessly interact with media-rich web apps (called “widgets”) like YouTube Shorts and TikTok. Cloud Phone is based on Puffin, a remote browser similar to Opera Mini that renders websites on cloud servers, sending markup language representations to draw on device.

Quick take. Cloud Phone performs well on budget hardware, incentivizes 4G bandwidth utilization, and requires minimal web app refactoring. It’s well-positioned to expand rapidly among the 200M+ feature phones sold annually.

KaiOS System Properties

Thu, 28 Mar 2024 00:00:00 +0800

KaiOS Architecture (Source: scottiestech.info)

Reading and writing system properties on KaiOS

KaiOS is a partially open source mobile Linux distribution that, under the hood, makes significant use of Android components like the Hardware Abstraction Layer (HAL). Gonk is KaiOS’ platform denomination for a combination of the Linux kernel and Android HAL, borrowed from it’s predecessor Firefox OS.

Like Android, KaiOS has system properties that provide a convenient way to share information like configurations, system-wide. These are stored in read-only property files (.prop extension) like bootimg/ramdisk/default.prop and system/build.prop. System properties are then managed by property_service to populate its internal in-memory database at start-up.

Swiss Bike Courier Turned KaiOS Developer

Wed, 20 Mar 2024 00:00:00 +0800

About John-David Deubl

John-David Deubl, who goes by the aliases perry and strukturart on GitHub, is a graphic designer, bike messenger, and an early KaiOS developer. John-David began programming for smart feature phones in 2020 out of a personal interest to combat his smartphone addiction. All of John-David’s apps are open source available under the MIT License.

How did you get started with KaiOS?

I am a bicycle messenger, programmer, and designer. I came to KaiOS in 2020 because, after years of using my smartphone, I noticed an addiction that was limiting me. With the limited possibilities of a feature phone, I am more independent again. Since some system apps in KaiOS are not fully optimized for use with a feature phone, I started writing and publishing some apps myself.

WebAssembly (WASM) on KaiOS

Tue, 05 Mar 2024 00:00:00 +0800

WebAssembly and asm.js on KaiOS

What is WebAssembly (WASM)? WASM allows developers to execute assembly-like low-level code written in languages like C/C++ or Rust via speecifically-compiled binaries within a web browser (in a sandboxed environment).

KaiOS 2.5 doesn’t support WASM. However, KaiOS supports asm.js, a highly optimized subset of JavaScript used as a low-level language compilation target.

KaiOS 3.0 supports WASM. Fortunately, KaiOS 3.0, which is based on Gecko 84, does support WebAssembly natively.

The Complete List of KaiOS Phones (2024)

Sat, 02 Mar 2024 00:00:00 +0800

There are over 100 KaiOS phones sold worldwide, and many more when you consider SKUs and network variants of the same model name (i.e. there are 20+ models of the JioPhone). More models may be observed from user agents.

Disclosure: Some links below are affiliate links. This means that, at no cost to you, I may earn affiliate commission if you click the link and finalize a purchase, which supports this blog financially.

Nutritional Scientist in Bangladesh builds KaiOS apps

Wed, 28 Feb 2024 00:00:00 +0800

About Hazrat Ali

Hazrat Ali completed his master’s degree in Nutritional Science in 2021 at the Islamic University in Kushtia, Bangladesh. Durng his studies, he started coding KaiOS apps as a hobby after stumbling on a Facebook Ad for a KaiOS device recently launched in Bangladesh. Fast forward to 2024, Hazrat has published almost two dozen KaiOS applications on the KaiStore including popular apps like kiNET TV and Radio Waves.

How did you get started with KaiOS?

In 2013, while still in college, I began my journey with web development. At that time, I experimented with creating websites for mobile devices using tools like Wapka and XtGem, which sparked my interest in the field. Fast forward to 2020, when I came across a Facebook promotion about the KaiOS device launch in Bangladesh. I was intrigued by the fact that KaiOS apps were built with HTML5, a technology I was already familiar with.

Developing with Flutter on KaiOS

Mon, 26 Feb 2024 00:00:00 +0800

Developing with Flutter on KaiOS

Why it matters? Flutter is a leading, open source, multi-platform development framework by Google to build apps for iOS and Android, as well as web browsers and embedded devices.

In a nutshell. Framework selection is critical for KaiOS development, since devices have limited storage and resources. Runtimes like Flutter notorious for large bundle sizes are not recommended for KaiOS.

Practical Optimizations

Developers have been experimenting with Flutter on KaiOS since at least 2021. If you’re determined to build a KaiOS application using Flutter, or you’d like to optimize an existing Flutter project for KaiOS, use the following tips.

Deep Dive on KaiOS User Agents

Fri, 23 Feb 2024 00:00:00 +0800

Deep Dive on KaiOS User Agents

Why it matters. User agents are one of the most common ways to identify devices on the web.

Key Takeaways

User agents are a mess. Some are missing spaces, others missing model numbers, and others still include two OS identifiers (Android and KAIOS). They can also be easily spoofed, but they’re still a useful mechanism to adapt your application at runtime or server-side.

KaiOS
- Every KaiOS device includes the Mobile identifier
- All KaiOS devices include the identifier KAIOS/ followed by the OS version (i.e. 2.5.3.2 or 3.2)
- KaiOS 2.5 devices are based on the Gecko engine and Firefox/48.0; KaiOS 3.0 is based on Firefox/84.0
- Some (but not all) KaiOS 2.5 devices with 256mb RAM have the K-Lite identifier
- Many devices include the yet-unknown CAEN identifier
JioPhone
- Only JioPhone models have the Android identifier
- All JioPhone models except the latest JioPhone Prima 4G (F491H) include the LYF identifier
- JioPhone user agents include the full Build Identifier, i.e. LYF-LF2403N-001-02-23-170319

User Agents

Here’s a list of User-Agent header values from recent requests to PodLP.

From J2ME to KaiOS with Valeriy Skachko

Thu, 22 Feb 2024 00:00:00 +0800

About Valeriy Skachko

Valeriy Skachko is one of KaiOS’ top certified developers. Since 2018, he has published over 100 KaiOS apps, available on both the KaiStore and JioStore. Learn about his journey and lessons you can apply to your own software development.

How did you get started with KaiOS?

I started developing the first applications for mobile phones, at that time in Java ME (J2ME), in 2012, in addition to my teaching activities. I needed to give the students a new topic - and I decided to offer them the development of an application for mobile devices based on Java ME. After that, I became interested in developing applications; there were many projects on Android and iOS, but my favorites were still feature phones. It was very interesting to create applications that could work with minimal system requirements and a small screen. It was a challenge for me. Later, in 2018, I learned about KaiOS and immediately fell in love with this system. Since then, I have been mainly programming for KaiOS.

KaiOS App Publishing Guide

Mon, 19 Feb 2024 00:00:00 +0800

Everything you need to publish an app on KaiOS

Why it matters. Names, icons, and banners are how you stand out on a platform with limited discovery capabilities. Learn how to market and promote your application to grow your user base globally.

Dive deeper. Publishing requirements are different between the KaiStore and JioStore. This article focused on the KaiStore, but makes a few notes where design requirements differ on the JioStore.

Special Keys on KaiOS

Tue, 13 Feb 2024 00:00:00 +0800

Handle special keys (soft keys, volume, camera) on KaiOS

Why it matters. A great user experience on KaiOS starts with directional navigation and proper keyboard handling.

Dive deeper. Not all devices support the same hard keys, and while most have a T9 layout, some like the JioPhone 2 have full QWERTY keyboards.

Special Keys

Key	Key Code	Code	Description
SoftLeft	0	F13	Left Soft Key (LSK), typically performs action
SoftRight	0	F15	Right Soft Key (RSK), typically opens options menu
MicrophoneToggle	0	N/A	Launch the Voice Assistant (VA) via `voice-assistant` Web Activity
Enter	13	Home	Select the currently-focused element
Backspace	8	KeyB	Go to previous page
GoBack	0	N/A	Go to previous back (KaiOS Smart Touch only)
BrowserBack	0	N/A	Go to previous page (web browser only)
VolumeUp	182	AudioVolumeUp	Raise volume
VolumeDown	183	AudioVolumeDown	Lower volume
Call	0	N/A	Launch the call screen or accept current action
EndCall	95	Power	Cancel the current action or close app
Power	95	Power	Toggle screen on/ off; long-press to power down
Camera	0	N/A	Open Camera app
HeadsetHook	0	N/A	Open Music app
LaunchWebBrowser	0	F14	Launch Browser app
LaunchMail	0	N/A	Launch Email app
Message	0	KeyA	Launch SMS app
Notification	0	N/A	Open Notification panel
Symbol	224	F17	Toggle symbol input (i.e. #, *, -, +, @, etc)

Always use the KeyboardEvent#key property, as many keys lack a keyCode, and the code property changes based on keyboard layout and device configuration. Keep in mind that many keys are limited to certain devices, although every KaiOS device has d-pad navigation as well as LSK and RSK.

Complete Feature Detection on KaiOS

Wed, 31 Jan 2024 00:00:00 +0800

Feature Detection

What features can you detect on KaiOS?

Summary. KaiOS has two APIs, navigator.getFeature() and navigator.hasFeature, for detecting device and platform features. These APIs require the feature-detection permission.

Here is a simple TypeScript definition for the Feature Detection API on KaiOS 2.5:

1interface Navigator {
2 getFeature(name: string): Promise<any>;
3 hasFeature(name: string): Promise<boolean>;
4}

On KaiOS 3.0, the DeviceCapabilityManager API is loaded from http://shared.localhost/js/session/device_capability/device_capability.js and shortens getFeature to just get. The hasFeature API was removed in KaiOS 3.0. Here is an example for each KaiOS version:

KaiOS Open Source Resources

Sun, 28 Jan 2024 00:00:00 +0800

KaiOS Open Source Resources

Here is a compilation of KaiOS apps, tools, and resources to find inspiration and help you get started on your next endeavor. This list is by no means exhaustive, but attempts to cover a wide range of available open source projects related to KaiOS.

gecko-b2g - KaiOS’ MPL-licensed fork of Boot 2 Gecko (B2G), previously known as Firefox OS
api-daemon - Rust-based external API for KaiOS, esp. 3.0+

User Interface

KaiUI - React-based UI library (last code commit in April 2021)
Vue KaiUI - KaiUI written for Vue.js
Arrow key navigation
KaiUIng - CSS library
Onyx UI - Svelte component library
Mai UI - Preact component library

Apps

Wikipedia - Wikipedia KaiOS app
o.map - OpenStreetMap client available on the KaiStore
greg - Calendar app
feedolin - RSS feed reader and podcast app
Telekram - Svelte-based Telegram client
PinK - Pinterest app
KaiGram - KaiOS Telegram client
chooj - Matrix client
KaiAuth - Google Authenticator alternative
K-Music - Music player
Password Manager
YT Music - YouTube Music streaming
Discord4KaiOS - Discord client
Kaicord - Minimal discord client
kaid - React component library (inc. dialog, header, list, menu, and soft keys)
kaiak-wallet - Nano cryptocurrency wallet app, built using Svelte
kaidi - Kodi remote client
Foxcasts lite, mica - Podcast app
React notes app
Raincloud - SoundCloud streaming client
Books - eBooks (TXT) reader
Kaite - Twitter client
K-Pocket-Browser - Pocket bookmark client
Bing Wallpapers - Bing wallpaper of the day app
b2gfm - File manager
KaiStra - Strava client
FlipTV - IPTV streaming client
Passport - Digital passport wallet, with QR code scanner
CosmosFM - Cosmos.FM app
Caltrain schedule
PDF Reader
K-IPTV Tube - IPTV app
Tapestry - Wallpapers from Unsplash
Calculator app rewritten in Processing
Kaistodon - Mastodon app
Quotez - Daily quotes
Nihonoari - Inspired by the Android app with the same name
Kaix-mpp - XMPP client
Battery manager
Bitcoin Wallet
baka⑨player - gensokyo radio player
BankiTube - YouTube via Invidious
WifiManager - Connect to WiFi using QR codes
delta.map - OpenStreetMap client with weather, layers, and more

Games

Pak-Man - PacMan-inspired game
Wordly - Wordly-inspired game
Frogr - Frogger-inspired game
KaiChess - Chess for KaiOS
Space Shuttle
Falling Numbers
Amateur Archaeology

System Apps

qLauncher
Luna launcher
casaroro-launcher
Metro launcher
bHacker Store - BananaHackers store app
KaiStone - KaiStore client without being tracked
netstats - Network statistics and data usage
AppBuster - Safely disable pre-installed apps
FastContact - Lightweight alternative to the stock Contacts app
FastLog - A better call log app
RCVD - Sync with Casio watches via Bluetooth Low Energy (BLE)
Kopher - Gopher client/ browser
Stock Apps - Pre-installed KaiOS 2.5.4 app collection

Samples

React sample
Vue sample
Angular sample
Phaser sample
Svelte starter kit
Navigation and KaiAds sample
Vanilla JS sample

Tools

appscmd - utility for installs KaiOS apps
KaiOS Web Suite - PC suite for managing KaiOS 2.5 devices
KaiWare - Desktop app for managing KaiOS devices, written in TypeScript
KaiOS Lib - Library for interacting with KaiOS APIs
SQLite Scraper
Wallace Toolbox - Universal toolbox for internal tweaking and rooting, based on Libwallace
OrigAMI - Origins of Application Marketless Installation
CrossTweak - Lightweight KaiOS tweaker with 10 functions
Project Kava - Java 2 Mobile Edition (J2ME) emulator for KaiOS

Capyloon - experimental web-based operating system based on Boot 2 Gecko (B2G)
Banana Hackers - Web community with extensive information
postmarketOS - A real Linux distribution for phones
KaiOS User Agents - Listing recent examples of KaiOS User-Agent values
KaiOS Vulnerabilities - documented Common Vulnerabilities and Exposures (CVEs) for KaiOS Tech

Conclusion

With so many open source apps, games, and tools, here is no shortage of prior art to base your next project on. If yours business needs help navigating the overwhelm of starting fresh on an unfamiliar platform, contact the author from the About page.

KaiOS App Ideas

Wed, 24 Jan 2024 00:00:00 +0800

KaiOS App Ideas

Apps. There are just under 1,500 KaiOS apps as of January 2024. Here are some ideas for apps that aren’t yet available on the KaiStore.

Utilities

Smart TV Remote

What is a T9 feature phone with Bluetooth and Wi-Fi, if not a smart TV remote to control your Android TV or Roku? Numerous remote apps make the Top 100 list in the Utilities category for smartphones. KaiOS provides access to raw TCP/ UDP sockets, as well as low-level Bluetooth APIs, so this should be possible.

Practical KaiOS Optimization Tips

Sat, 20 Jan 2024 00:00:00 +0800

Practical KaiOS Optimization Tips

Lessons. Here are a few tips I learned first hand developing PodLP for KaiOS smart feature phones.

Memoization

What’s Memoization? It’s basically a form of caching, eliminating repetitive calculation and repeated function calls by storing results in memory.

How to memoize? There are many JavaScript libraries like moize that can wrap functions for automatic memoization, or you can do it yourself.

PodLP Using Podcast Accent Color

Data Storage on KaiOS

Tue, 16 Jan 2024 00:00:00 +0800

Data Storage on KaiOS

KaiOS offers standard Web Storage APIs (localStorage and sessionStorage), indexedDB, document.cookie, and the Cache API as well as platform-specific APIs like Device Storage and Data Store. The ideal API for your use case depends on factors including data size, security, and simplicity. Keep in mind that KaiOS devices have limited storage available (i.e. the Blu Zoey Smart only has 512MB), and many users will not have an external micro SD card, so its best to store the minimum data necessary and assume most persistence is best-effort.

What's missing from KaiOS development?

Thu, 11 Jan 2024 00:00:00 +0800

What’s Missing?

KaiOS was founded in 2016, but 8 years later key tools are missing from the KaiOS developer experience (DevEx).

Why it matters? KaiOS is available in 100+ countries with 170M+ devices shipped in 2022. Yet there are fewer than 1,500 KaiOS apps, limited by DevEx.

By the numbers As of January 2024, the KaiStore has 1,440 KaiOS 2.5 apps but only 520 KaiOS 3.0 apps. 3 years after launching, notable 3.0 omissions include both Facebook and WhatsApp.

Google Analytics and Alternatives for KaiOS Apps

Fri, 05 Jan 2024 00:00:00 +0800

Setup Google Analytics (GAv4) or alternative web traffic analytics for KaiOS apps.

Google Analytics

Google Analytics Demographics - KaiOS App

Google Analytics is by far the most popular web and mobile traffic and events analytics platform. It’s free, easy to use, and customizable to accurately measure user behavior, demographics, technical performance and errors, and more. However, Google Analytics does not work out of the box with KaiOS.

Localization for KaiOS Apps

Tue, 07 Nov 2023 00:00:00 +0800

Localization (L10n) for KaiOS Apps

PodLP Localization in Arabic, English, and Hindi

Why it matters? KaiOS is available in over 100 countries, with top demographics in India, United States, Nigeria, Tanzania, and Uganda. Localization facilitates market entry, improves user experience, and increases retention.

How it Works

Localization isn’t an outcome, it’s a process of adapting your application to specific markets. It includes many elements like:

[Guest Post] Building a Crypto Wallet on KaiOS

Thu, 02 Nov 2023 00:00:00 +0800

[Guest Post] Building a Crypto Wallet on KaiOS

Sorted Wallet is the first cryptocurrency wallet created for KaiOS feature phone devices and published on the KaiStore.

Why it matters? Sorted Wallet provides the basic functionality for crypto assets holders and enables non-smartphone owners to step into the world of cryptocurrency. It tackles the increasing wealth gap in emerging markets and provides support to underserved populations in their fight for financial inclusion.

By The Numbers

Sorted Wallet Installs

Running in the Background on KaiOS

Mon, 30 Oct 2023 00:00:00 +0800

Running in the Background on KaiOS

Despite KaiOS operating on resource-constrained hardware, apps can execute tasks in the background using several approaches.

Background Media
Activities & System Messages
Service Workers
Alarm API
Request Sync API

Background Media

The simplest way to do anything in the background is by using Audio Channels to continue audio playback while using another app. Using the content audio channel on an audio or video element, when the user presses Back/ EndCall, it will trigger your application to enter the background.

What's Next for the JioPhone?

Tue, 22 Aug 2023 00:00:00 +0800

What’s next for the JioPhone, the best-selling KaiOS phone globally?

The JioPhone

JioPhone Next (LS1542QWN), JioPhone 2 (F300B), and JioPhone (F90M)

Many Models. The JioPhone refers to one of nearly two dozen models.

Number One. The JioPhone once held the #1 spot for best selling feature phone globally.

JioPhone Lineup. The JioPhone, along with the more affordable ThreadX/Mocor-based Jio Bharat 4G, and more expensive Android-based JioPhone Next (Android Go rebranded as PragatiOS), form a lineup to move 250M+ 2G feature-phone subscribers up-market.

Certified Apps on KaiOS

Wed, 16 Aug 2023 00:00:00 +0800

What separates Certified and Privileged apps on KaiOS?

Summary

Privileged vs Certified. Packaged apps on KaiOS are bundled as ZIP files, containing all the HTML, CSS, and JS needed to run locally on device. There are two types of packaged apps: Privileged and Certified. The latter is the highest security level available, granting the app access to nearly all permissions and features afforded primarily to System apps.

Why go Certified? The most obvious reason is if you need a permission that is marked only for Certified apps. You might also find that writing a Certified app offers access to “hidden” features for deeper system integration and a better user experience.

Bluetooth Low Energy on KaiOS

Mon, 14 Aug 2023 00:00:00 +0800

Working with Bluetooth Low Energy (BLE) on KaiOS

Use Cases

Bluetooth Low Energy (BLE) is a recent version of Bluetooth designed for very low power consumption. Compared to Bluetooth Enhanced Data Rate (EDR), BLE is designed to transfer much smaller amounts of data between devices via an ephemeral connection, not a durable pairing. Common BLE applications include:

Healthcare devices - Thermometer, heart rate and glucose monitor, etc
Proximity sensing - Tile, AirTag, “Find my” device
Contact tracing - People near me
Remote sensing - Battery level, volume, etc

Accessing Bluetooth APIs

Permissions

The "bluetooth" permission is needed to access Bluetooth Low Energy APIs on KaiOS. Although the permission is available to both privileged and certified apps, many Bluetooth features are only available in certified apps.

Common KaiOS APIs and Interfaces

Sat, 15 Jul 2023 00:00:00 +0800

Learn how to use common KaiOS APIs and Interfaces

Wake Lock

KaiOS does not support the Wake Lock API. Instead, it offers it’s own API:

 1interface MozWakeLock {
 2 readonly topic: string;
 3
 4 /**
 5 * Release the wake lock.
 6 * @throw NS_ERROR_DOM_INVALID_STATE_ERR if already unlocked.
 7 */
 8 unlock(): void;
 9}
10
11// KaiOS 2.5
12navigator.requestWakeLock(aTopic: string): MozWakeLock;
13
14// KaiOS 3.0
15navigator.b2g.requestWakeLock(aTopic: string): MozWakeLock;

The KaiOS Wake Lock API will accept any value for aTopic, however, the following have actual utility:

Jio Bharat 4G V2 and KaiOS

Tue, 04 Jul 2023 00:00:00 +0800

Reliance Jio might be replacing it’s KaiOS flagship, the JioPhone, with the more budget-friendly Jio Bharat 4G V2.

Why it matters? Reliance Jio’s JioPhone represents the vast majority of KaiOS devices globally. The latest model, the JioPhone Value, was released in Fall 2021. Back in 2019, more than 70 million JioPhones had been sold. Two years later in 2021, KaiOS announced that it had surpassed 150 million KaiOS-enabled devices.

By my estimates, the JioPhone still makes up well over half of all active KaiOS devices today. At Rs 999 ($12 USD), the lower-end Jio Bharat 4G V2 might replace the JioPhone in Jio’s lineup, designed for the 250 million Indian consumers transitioning from 2G to 4G.

CVE-2023-33293: Installed App List from the KaiOS 3.0 Browser

Tue, 23 May 2023 00:00:00 +0800

CVE-2023-33293 was published today, outlining an information disclosure concern with api-daemon whereby websites can determine what apps and which versions are installed on KaiOS 3.0 and 3.1 devices.

Photo of app list with versions exposed on Alcatel Go Flip 4 (KaiOS 3.0) browser

CVE-2023-33294: KaiOS 3.0 Root Command Line in Browser via TCT Web Server

Mon, 22 May 2023 00:00:00 +0800

CVE-2023-33294 was published today, outlining a vulnerability in KaiOS 3.0 (but not KaiOS 3.1) whereby the /system/bin/tctweb_server binary exposes a local HTTP server on http://127.0.0.1:2929 that executes arbitrary commands as root.

Photo of build.prop file displayed on Alcatel Go Flip 4 (KaiOS 3.0) browser

Why it matters? This is among the most dangerous KaiOS vulnerability discovered to date. KaiOS 3.0 users in the US should be extremely careful browsing websites or installing third-party applications.

CVE-2023-27108: Call Logs in the Browser

Tue, 02 May 2023 00:00:00 +0800

CVE-2023-27108 was published today, outlining a vulnerability in KaiOS 3.0 & 3.1 exposing call logs without user interaction to apps and websites.

Photo of call logs exposed in Nokia 2780 Flip browser (Credit: @sosumi1984)

Why it matters? KaiOS 3.0 & 3.1 users in the US may be inadvertently exposing their call log records simply by visiting a web page.

KaiOS' Next Frontier: Africa [Updated February 2024]

Sun, 30 Apr 2023 00:00:00 +0800

KaiOS is positioned to grow significantly across Africa.

Why it matters? Market trends, as well as recent partnership and investments, suggests the smart feature phone operating system has an opportunity to capture hundreds of millions of first time internet users across the continent.

In KaiOS’ February Developer Update, it was announced that, “KaiOS is refocused on growth.”

KaiOS is expanding in developing markets, specifically in Africa (launching in 10+ new African markets)
KaiOS launched a device financing program in Tanzania with the TZ1
- Similar programs are expected soon in South Africa, Nigeria, Zambia, DRC and Rwanda
The first KaiOS smart feature phone was launched in Japan in Q1 2024
At CES 2024, KaiOS discussed 5G smart feature phones “slated for an early 2025 rollout” in partnership with Qualcomm

Growth Opportunities

Global telecommunication companies continue to migrate users from 2G to 4G
4G feature phones are often used as companion/ secondary devices
Feature phones cost less to make compared to smartphones
KaiOS offers a higher Average Revenue Per User (ARPU) over 4G Real-Time Operating System (RTOS)
KaiOS’ partnership with Trustonic to enable subsidy locking and device financing
KaiOS partners with Konnect to expand financial inclusion and provide secure payment solutions among communities and individuals in Africa
Finnfund invested $3.4 million so that, “KaiOS can expand into new markets in Sub-Saharan Africa”

By the numbers

The Mobile Economy in Sub-Saharan Africa 2019. GSMA 2019

Security and Privacy on KaiOS

Wed, 19 Apr 2023 00:00:00 +0800

Security and Privacy considerations for KaiOS smart feature phones

KaiOS is based on Firefox OS, and as a result, many of the same technologies apply to Security & Privacy within KaiOS apps.

Content Security Policy (CSP)

The Content Security Policy (CSP) prevents attacks like Cross-Site Scripting (XSS) by limiting the origins that specific resources like scripts, objects, and stylesheets can be loaded from. KaiOS imposes default CSPs on Privileged & Certified packaged apps. It’s possible to override this using the csp manifest property, but the provided value can only be more strict than KaiOS’ default.

What KaiOS phone should I get?

Mon, 10 Apr 2023 00:00:00 +0800

What KaiOS phone should I get?

With many KaiOS phones available for purchase, it can be difficult picking the right one for your needs. This article summarizes some of the popular KaiOS devices on the market today, as well as who these devices might best serve.

KaiOS App Optimization Tips

Sun, 02 Apr 2023 00:00:00 +0800

Optimizing apps for KaiOS smart feature phones

Modern app developers have become accustomed to iOS and Android smartphones with specifications comparable to laptop or desktop computers: multi-touch high-resolution displays, gigabytes of memory, hundreds of gigabytes of storage, and cutting-edge 5G and WiFi 6 networking. In contrast, KaiOS smart feature phones have either 256mb or 512mb of RAM, storage between 256mb - 8gb, small ~2" 240x320 screens, and 3G, 4G and Wi-Fi ABG. These hardware restrictions make it crucial to optimize and measure KaiOS web app performance to run well on low-end hardware.

Complete KaiOS Permission Guide

Mon, 20 Mar 2023 00:00:00 +0800

Complete Guide to App Permissions on KaiOS

There are more than 100 permissions available on KaiOS! Below is the complete list of permissions on KaiOS 2.5 and KaiOS 3.0 defined in PermissionsTable.jsm to help with developing your next application.

Hosted & Privileged Apps

Geolocation

 1geolocation: {
 2 app: PROMPT_ACTION,
 3 privileged: PROMPT_ACTION,
 4 certified: PROMPT_ACTION
 5},
 6"geolocation-noprompt": {
 7 app: DENY_ACTION,
 8 privileged: DENY_ACTION,
 9 certified: ALLOW_ACTION,
10 substitute: ["geolocation"]
11}

Similar to web apps, KaiOS apps can request access to the user’s Global Positioning System (GPS) coordinates using the Geolocation API. However, this API is not available unless your app requests the geolocation permission. Calling navigator.geolocation.getCurrentPosition at runtime will trigger a permission request dialog that the user can then accept or reject. For Certified apps there is also a substitute permission, geolocation-noprompt, which as the name implies does not trigger a permission request dialog.

Complete manifest.webapp Guide

Sat, 11 Mar 2023 00:00:00 +0800

Complete Guide to manifest.webapp Properties

The complete guide to every manifest.webapp property on KaiOS! This includes many undocumented but useful properties (like userAgentInfo) as well as properties only intended for system apps. The manifest.webapp file is a JSON-formatted document that contains important information about your app, including version, name, description, icon, locales, and more.

Most importantly, the app manifest contains the list of APIs, permissions, and dependencies that your app needs. It’s analogous to AndroidManifest.xml for Android apps or Info.plist for iOS apps.

Audio and Volume Control in KaiOS

Thu, 09 Mar 2023 00:00:00 +0800

Controlling volume and audio channels on KaiOS

Getting Started

All KaiOS phones come with a built-in speaker, microphone, and headphone jack. Additionally, most KaiOS phones come with Bluetooth and support for A2DP. Yes, that means you can connect a KaiOS phones to Apple AirPods!

Similar to Android, KaiOS enables app developers to build rich multimedia experience with the Audio Channels, Volume Manager, and Speaker Manager APIs. These collectively allow apps to control volume, known when headphones are unplugged, and force audio output through built-in speakers. KaiOS supports many common Media Formats including:

The Download on KaiOS Downloads

Sat, 25 Feb 2023 00:00:00 +0800

How to stream and store files on KaiOS

Background

Although KaiOS is based on Firefox, unlike modern smartphones or desktops, KaiOS phones are quite resource constrained. All KaiOS-powered devices have either 256MB or 512MB RAM for the entire operating system. That memory gets allocated for core system services like telephony, audio, and graphics. Only a fraction of that memory is available to your app.

KaiOS Memory Low Warning

Dos and Don'ts for KaiOS

Sat, 18 Feb 2023 00:00:00 +0800

Dos and dont’s of developing KaiOS apps

☑️ Dos

Develop small applications
- Minify all resources (JS, CSS, HTML)
- Right-size and compress multimedia assets
Have a caching strategy
Test on an actual device
Support offline usage (when possible)
Use global CDNs for edge caching
Only access secure resources (i.e. HTTPS)

Develop small applications

The maximum package size that the KaiStore accepts is 20MB, with an average of 2-4MB. Compare that to iOS, where iOS apps average 110 - 427MB. Smaller applications use less memory and this will perform better on KaiOS devices which only have 256MB or 512MB of RAM.

FAQ

Fri, 17 Feb 2023 00:00:00 +0800

KaiOS FAQs

Basics

What hardware specifications do KaiOS phones have?

240x320 ~2.4-3.2" displays
256mb, 512mb, or 1GB RAM
3G, 4G, or 5G
ARM CPUs
- Qualcomm, Spreadtrum & Mediatek
- 1.0GHz - 2.0GHz
- Dual-core, Quad-core, or Octa-core
T9 or QWERTY keyboards
D-Pad & Soft-Key (LSK & RSK) navigation
WiFI & Bluetooth (optional)

Is Instagram, Tik-Tok, Spotify, Uber, [insert app] available on KaiOS?

No. KaiOS has a number of popular apps, including:

YouTube
Google Assistant
Google Search
Google Maps
Microsoft Outlook

As of January 2026, most other popular social network, music streaming, chat, or shopping apps are not available. The following are not officially available on KaiOS:

Web Activities on KaiOS

Wed, 15 Feb 2023 00:00:00 +0800

Sharing, selecting, and viewing data cross-app with Web Activities and deep links

Web Activities

Example User Flow of configure Activity

Like App Intents on iOS and Intents on Android, Web Activities are the way to share, pick, and view across apps on KaiOS. The interfaces are very similar across platforms, consisting of two components: a name (or action in Android) and an arbitrary data payload. Again similar to Android’s Intents, Web Activities are registered on KaiOS within an app’s static manifest.

Developing for KaiOS on Mac

Sun, 12 Feb 2023 00:00:00 +0800

Developing KaiOS apps on Mac (updated for Ventura 13.2)

Getting Started

Developing and testing KaiOS apps on a real device using Mac is easy, and a great way to validate your apps on actual hardware. For setup, all you need is adb, WebIDE (a debug tool that used to come pre-packaged with Firefox), and a debug-enabled KaiOS device like the Nokia 8110 4G.

Waterfox Classic

According to the KaiOS ENV Setup guide, Firefox 59 is recommended for KaiOS development. However, as of Mac Ventura 13.2, Firefox 59 has a fatal crash on startup. Since Firefox 59 stopped working on Mac Ventura, the best choice for KaiOS development on Mac is Waterfox Classic. Waterfox Classic is a legacy web browser for older systems and with WebIDE, XPCOM and XUL extensions, and numerous unpatched security advisories. Perfect for KaiOS development, but not recommended as your default web browser.

KaiOS App Monetization

Sat, 11 Feb 2023 00:00:00 +0800

Monetizing apps with on KaiOS with KaiAds and JioAds.

KaiAds

KaiAd Examples

By far, KaiAds are the easiest way to monetize KaiOS apps. KaiOS provides a publisher SDK that works for both apps and websites. There are two types of KaiAds: responsive banners and full-page interstitials.

KaiAds can be preloaded in the background with a refresh rate of ~30 seconds, then displayed when the opportunity is right like during page transitions or when the user finishes a level in a game. To get started, you can register for an account at https://publishers.kaiads.com. Once complete, you’ll get a Publisher ID to use with the SDK.

Advanced KaiOS Development: Push Notifications

Thu, 09 Feb 2023 00:00:00 +0800

Sending Notifications with the Web Push Protocol on KaiOS.

The Web Push Protocol

Push Notifications on KaiOS

Push Notifications on KaiOS work more or less the same as they do in a web browser like Firefox, with some caveats. They are a great way to provide utility and drive engagement. For instance:

Chat app with notifications for new messages
Email app with notifications for new emails
Podcast app with notifications for new episodes
Sports app with notifications for game scores
Bank app with notifications for transactionals

Push Notifications require two components: a client with a registered ServiceWorker to receive notifications, and a web server to store subscriptions and send notifications. This article focuses on KaiOS-specific client-side interactions, as there are many great back-end libraries like web-push. There are also alternatives such as WebSockets and Server-Sent Events (SSE), although these are limited to interactions while the app is active in the foreground, while Web Push allows notifications to be received even if the app isn’t open.

The KaiOS Ecosystem: Apps & App Stores

Sat, 04 Feb 2023 00:00:00 +0800

The KaiOS ecosystem, packaged and hosted apps, and the KaiStore and JioStore.

KaiOS App Basics

KaiOS Apps (Source: 2020’s most popular KaiOS apps)

Unlike Android and iOS, “native” KaiOS apps are just modified web apps. That means no new programming languages to learn or complicated build processes, just standard HTML, CSS, and JavaScript. That means you can also write in TypeScript or any JavaScript version or language the compiles or transpiles into ECMAScript 2015 (ES6) and runs on Firefox 48.

Advanced KaiOS Development - CORS, TCP Sockets, and Let's Encrypt SSL Certificates

Thu, 02 Feb 2023 00:00:00 +0800

Learn more about disabling CORS, Navigator.mozTCPSocket, and issues with Let’s Encrypt and Sectigo certificates on KaiOS

`XMLHttpRequest` without CORS

There are several ways that dynamic web apps communicate with a back-end server. XMLHttpRequest (XHR) is the original method, used heavily in AJAX programming. XHR allows web apps to make a variety of HTTP requests like GET, PUT, POST, and DELETE, both synchronously (now deprecated) and asynchronously.

KaiOS fully supports XMLHttpRequest, including a few non-standard flags that can be provided to it’s constructor.

History of KaiOS

Tue, 24 Jan 2023 00:00:00 +0800

Learn about KaiOS, the world’s third most popular mobile operating system.

KaiOS’ Origins

KaiOS Logo

KaiOS is the successor to the now-defunct Firefox OS and Boot2Gecko (B2G). Firefox OS was first demoed publicly in 2012 and discontinued four years later in 2016. It sought to provide a standards-based and open web alternative to Android and iOS by offering APIs that progressively bridged the gap between traditional web apps and native mobile apps, without requiring developers rewrite their existing apps using new technologies.

Privacy Policy

Sun, 01 Jan 2023 00:00:00 +0800

Effective: November 26th, 2025

Last Byte LLC (“The Company”, “we”, “us”, “kaios.dev”) is committed to protecting your privacy when you visit our website, KaiOS.dev. You can access KaiOS.dev and browse our site without disclosing any personal data. KaiOS.dev does not collect personal information about individuals, such as names or e-mail addresses unless it is knowingly provided by the individual and he or she has agreed to the terms under which KaiOS.dev will use it.

KaiOS.dev

A closer look at the TCL Go Flip 4 5G and KaiOS 4.0

Announcing kaios-types: KaiOS TypeScript Declarations

Deep Dive on Cloud Phone User Agents

Deep Dive on Cloud Phone User Agents

Key Takeaways

An In-Depth Look At KaiOS Smart Touch

KaiOS Smart Touch

What's coming in KaiOS 4.0?

KaiOS 4.0

The Definitive Guide to Publishing KaiOS Apps on the JioStore

Publishing Apps on the JioStore

At a Glance

The JioStore Basics

What APIs are supported on CloudMosa's Cloud Phone?

How Cloud Phone Works

A Developer's Introduction to CloudMosa's Cloud Phone

A Developer’s Introduction to CloudMosa’s Cloud Phone

At a Glance

KaiOS System Properties

Reading and writing system properties on KaiOS

Swiss Bike Courier Turned KaiOS Developer

About John-David Deubl

How did you get started with KaiOS?

WebAssembly (WASM) on KaiOS

WebAssembly and asm.js on KaiOS

The Complete List of KaiOS Phones (2024)

Nutritional Scientist in Bangladesh builds KaiOS apps

About Hazrat Ali

How did you get started with KaiOS?

Developing with Flutter on KaiOS

Developing with Flutter on KaiOS

Practical Optimizations

Deep Dive on KaiOS User Agents

Deep Dive on KaiOS User Agents

Key Takeaways

User Agents

From J2ME to KaiOS with Valeriy Skachko

About Valeriy Skachko

How did you get started with KaiOS?

KaiOS App Publishing Guide

Special Keys on KaiOS

Special Keys

Complete Feature Detection on KaiOS

Feature Detection

KaiOS Open Source Resources

KaiOS Open Source Resources

User Interface

Apps

Games

System Apps

Samples

Tools

Related

Conclusion

KaiOS App Ideas

KaiOS App Ideas

Utilities

Smart TV Remote

Practical KaiOS Optimization Tips

Practical KaiOS Optimization Tips

Memoization

Data Storage on KaiOS

Data Storage on KaiOS

What's missing from KaiOS development?

What’s Missing?

Google Analytics and Alternatives for KaiOS Apps

Google Analytics

Localization for KaiOS Apps

Localization (L10n) for KaiOS Apps

How it Works

[Guest Post] Building a Crypto Wallet on KaiOS

[Guest Post] Building a Crypto Wallet on KaiOS

By The Numbers

Running in the Background on KaiOS

Running in the Background on KaiOS

Background Media

What's Next for the JioPhone?

The JioPhone

Certified Apps on KaiOS

`XMLHttpRequest` without CORS