One validator for the everyday formats: paste JSON, XML, CSV, EDN, a URL or a regular expression and instantly see whether it's valid — with the reason when it isn't.
Syntactic validity: for JSON/XML/CSV/EDN that the document parses, for URLs the structure, for regex that the pattern compiles.
On the server, with the same parsers real applications use — so the verdict matches what your code would say.
Usually inconsistent column counts between rows or an unclosed quoted field.