User-Agent Explained: How It Relates to Browser, OS & Device in 1 Minute

Anyone who has worked on websites, cross-border e-commerce, data analysis, or risk control, or has encountered anti-scraping, must have heard of the term User-Agent.

Many people know it is important, but when it comes to explaining clearly the relationship between User-Agent, browsers, operating systems, and devices, it can be somewhat unclear.

Next, the author will specifically explain what User-Agent really is, its relationship with browsers, operating systems, and devices, and why sometimes it is accurate and sometimes not.

1. What is a User-Agent? Explained in one sentence

User-Agent (abbreviated UA) is essentially: a string that the browser sends to the server when visiting a website, actively telling the server "who I am."

This string usually contains the following information:

•  Browser type (Chrome, Safari, Firefox, etc.)

•  Browser version

•  Operating system (Windows, macOS, Android, iOS)

•  Device type (PC, mobile, tablet)

•  Rendering engine (WebKit, Blink, etc.)

In other words, the reason websites can tell which browser, operating system, and device you are using is largely thanks to the User-Agent.

2. User-Agent Parsing and the Relationship with Browsers, OS, and Devices

Many people tend to confuse these three, but the logic is actually simple.

1️⃣ Browser is the "sender"

Browsers (Chrome, Edge, Safari) are responsible for generating and sending the User-Agent.

Different browsers have different UA structures, which forms the basis for identifying browser types via UA.

2️⃣ Operating System is "background information"

The operating system does not directly send the UA, but its information is included in the UA, for example:

•  Windows NT 10.0

•  Mac OS X 10_15_7

•  Android 13

•  iPhone OS 17_0

Therefore, after parsing the User-Agent, websites can know which operating system you are running.

3️⃣ Device type is embedded in UA details

UA does not explicitly say "I am a mobile" or "I am a PC"; it hints through keywords:

•  Mobile, Android → most likely a mobile phone

•  iPhone, iPad → iOS device

•  No Mobile keyword → usually a PC

This is also the basis for many websites to implement mobile adaptation and PC/mobile redirection.

3. How User-Agent Parsing Works

User-Agent parsing is essentially: breaking down a UA string into structured information such as "browser / OS / device / engine."

Common parsing methods include:

•  Regular expression parsing

•  UA libraries (e.g., ua-parser)

•  Third-party UA detection APIs

⚠️ Note: UA can be modified and spoofed. This is why relying solely on UA for device detection is no longer sufficient in risk control and anti-fraud scenarios.

4. Limitations of UA Detection: Why it is not “absolutely reliable”

In practice, User-Agent has several obvious issues:

•  Browsers allow manual UA modification

•  Emulators and automation tools can easily spoof UA

•  Different devices may use the same UA

•  The same device may have UA changes after browser updates

Therefore, many systems now check: 👉 "Is your UA really trustworthy?"

5. Browser Fingerprinting: More Reliable than UA

Browser fingerprinting does not rely solely on UA, but combines multiple dimensions, such as:

•  Canvas fingerprint

•  WebGL fingerprint

•  Font list

•  Time zone, language

•  Screen resolution

•  Hardware concurrency

•  UA information itself

These combined form a relatively stable "browser fingerprint."

Even if the User-Agent is modified, inconsistent fingerprint features make it easy to detect.

6. What the ToDetect Fingerprint Tool Can Do

If you want to visually see your UA and browser fingerprint information, you can use

the ToDetect Fingerprint Tool. Common uses include:

•  View current User-Agent details

•  Test UA parsing and recognition

•  Check browser fingerprint uniqueness

•  Analyze fingerprint leakage risks

•  Verify anti-fingerprinting or environment isolation

For developers, operators, cross-border e-commerce, ad campaigns, and anti-scraping research, it is a very practical tool.

7. User-Agent Parsing + Browser Fingerprinting is the Current Mainstream

In summary, common industry practices today include:

👉 UA is basic information, while fingerprinting is advanced verification.

Conclusion

Ultimately, User-Agent is not meant for “anti-fraud”; it is more like a self-introduction, actively telling platforms: “This is the browser, operating system, and device I am using.”

UA can be changed, spoofed, or copied in bulk, which is why more and more platforms are combining browser fingerprinting for more precise identification.

If you want to truly understand what information your environment exposes or verify UA parsing accuracy, you can use the ToDetect Fingerprint Tool for a complete check.