User-Agent Parser and Decoder

What a user-agent string is

Every HTTP request carries a User-Agent header: a short string identifying the browser and platform, like Mozilla/5.0 (iPhone; CPU iPhone OS 17_5…) Safari/604.1. Servers and analytics tools parse it to report browsers, operating systems and device classes, and to filter out crawler traffic.

The format is famously messy for historical reasons: nearly every browser claims to be Mozilla, Chrome-based browsers also claim Safari, and Edge claims all three. Parsers work by checking the most specific markers first, which is what this tool does. Modern Chromium browsers additionally send structured Client Hints (Sec-CH-UA headers) that are much cleaner; good analytics prefers those and falls back to the UA string.

Bots and crawlers

Well-behaved bots identify themselves: Googlebot, Bingbot, GPTBot (OpenAI's crawler), ClaudeBot, PerplexityBot, plus uptime monitors and SEO crawlers. Automation tools (HeadlessChrome, curl, python-requests) are detectable too. Analytics that doesn't filter these inflates every number you care about, which is why FocusStat drops known bots at ingestion and reports them separately.

Parsing user agents in Python and JavaScript

If you need this in code rather than in a browser tab, do not write regexes yourself. Both ecosystems have a maintained parser built on a shared, regularly updated list of browser and device patterns.

Python

pip install ua-parser

from ua_parser import parse

ua = parse("Mozilla/5.0 (iPhone; CPU iPhone OS 17_5 like Mac OS X) "
           "AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.5 Mobile/15E148 Safari/604.1")

print(ua.user_agent.family, ua.user_agent.major)  # Mobile Safari 17
print(ua.os.family, ua.os.major)                  # iOS 17
print(ua.device.family)                           # iPhone

JavaScript and Node

npm install ua-parser-js

import { UAParser } from "ua-parser-js";

const { browser, os, device } = UAParser(req.headers["user-agent"]);
console.log(browser.name, browser.version); // Mobile Safari 17.5
console.log(os.name, os.version);           // iOS 17.5
console.log(device.type);                   // mobile

Neither library flags bots, which is the half that matters for analytics. Keep a separate list of crawler markers (Googlebot, GPTBot, HeadlessChrome, python-requests and friends) and check it first, exactly as the tool above does.