KaiOS browser user agent
KaiOS is a lightweight operating system for smart feature phones, built on a fork of Firefox OS (Gecko). Its browser therefore presents a Gecko/Firefox-style mobile user agent, often with a KAIOS token. These devices are common in price-sensitive markets and have small screens and keypad navigation.
What this means
KaiOS targets smart feature phones: devices with physical keypads, small screens, and modest hardware that still run apps and a web browser. It descends from Firefox OS and uses the Gecko engine, so its browser user agent looks like a mobile Firefox/Gecko string and may include a KAIOS token plus a device identifier.
KaiOS has notable share in India and parts of Africa and Southeast Asia, where affordable connected feature phones are popular. Seeing KaiOS signals that audience.
Why it is not desktop Firefox
Because the engine is Gecko, a naive parser might bucket KaiOS under Firefox. The KAIOS token and mobile-device markers distinguish it. The hardware is far more constrained than a desktop, so heavy pages and large assets perform poorly.
Match on the KAIOS token where present, and treat these visits as low-bandwidth, small-screen, keypad-navigated sessions when deciding on layout and asset weight.
- Engine: Gecko (Firefox OS lineage)
- Distinguishing token: KAIOS plus device markers
- Small screen, keypad navigation, constrained hardware
Serving KaiOS audiences
Lightweight HTML, minimal JavaScript, and keypad-accessible navigation serve KaiOS users best. Pair the device signal with the Accept-Language header to choose the right localisation rather than guessing from the device.
Do not treat the constrained device or unfamiliar user agent as a reason to block; KaiOS visits are real people on real phones.
How it appears in analytics and logs
A mobile Gecko/Firefox-pattern user agent containing a KAIOS token is a human on a KaiOS feature phone — a small-screen, low-bandwidth device, typically in an emerging market. It is a genuine human visit, not a bot.
Diagnostic use case
Recognise KaiOS feature-phone visitors, who need lightweight, keypad-friendly pages, and avoid mistaking the Gecko-based string for desktop Firefox.
What WebmasterID can help detect
WebmasterID classifies KaiOS as a human mobile browser and surfaces the device family as context, so feature-phone audiences in emerging markets are visible in human analytics rather than lost among generic Firefox traffic.
Common mistakes
- Bucketing KaiOS under desktop Firefox because both use Gecko.
- Serving heavy, JavaScript-dependent pages to constrained feature phones.
- Assuming the KaiOS user agent indicates a bot rather than a real device.
Privacy and accuracy notes
KaiOS devices are recognised only from the user-agent token. WebmasterID treats the device class and any coarse market as context, never as exact location or a personal identifier.
Related pages
- Mobile user agents: phones, tablets, in-app
Mobile user agents carry platform descriptors like iPhone or Android and often a Mobile token, but tablets and in-app browsers complicate the picture. An in-app browser (inside a social or messaging app) usually adds its own token to the string. This page explains the patterns and their pitfalls.
- Firefox user agent pattern
Firefox's user agent is built around the Gecko engine token and a trailing Firefox product token. Compared with other browsers it has been relatively stable and predictable, which makes it straightforward to recognise. This page covers the pattern and how to tell Firefox apart from WebKit and Chromium clients.
- Android OS user agent tokens
Browsers on Android include a Linux; Android platform token with an OS version, and historically a device model identifier. With user-agent reduction, Chrome on Android now sends a generic device token instead of the real model, so model-based detection no longer works and Client Hints are needed for device detail.
- Bot vs human
Count feature-phone browsers as human visits, not automation.
Sources and verification notes
- KaiOS Technologies — developer portalGecko-based browser; KAIOS token in the mobile UA.
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.