URL Encoder / Decoder

Reserved characters, and why there are two modes

A URL is not a flat string; it is a structured address in which certain characters are punctuation. The : separates scheme from the rest, / separates path segments, ? begins the query, & separates parameters, = separates a parameter name from its value, and # begins the fragment. Percent-encoding exists so that data can contain those same characters without being mistaken for punctuation. That is why there are two encoding functions rather than one: when you are encoding a single value, every reserved character in it is data and must be escaped; when you are encoding an entire URL, those same characters are the structure holding it together and escaping them destroys it. No tool can infer which case you are in from the string alone, which is why the choice is yours to make explicitly here.

Spotting double-encoding

Double-encoding happens when a value passes through two encoding steps without a decode between them. The tell is the percent sign itself getting escaped: a space becomes %20 on the first pass, and on the second the % becomes %25, leaving %2520. It usually appears when a framework encodes parameters automatically and application code encodes them again first. The parsed breakdown in this tool decodes each parameter value once, so if what you see still contains percent triples, the value arrived already encoded and the extra step is upstream of you.