Bad value X for attribute “src” on element “script”: Illegal character in query. “{” is not allowed.

A { in the query string of a <script> element's src is not a valid URL character, and it almost always comes from a template placeholder that was never replaced.

Curly braces can appear in a URL only when they're percent-encoded as %7B and %7D. In a script URL they usually belong to a cache-busting parameter such as ?v=${build.version} or ?v={{version}}. The template engine or the build step should have swapped the placeholder for a version number or a timestamp, but the raw text reached the page instead. A misspelled variable name, or a template file the build step doesn't process, leaves it behind.

Browsers still load the script, sending the placeholder text as part of the query. The cache buster stops doing its job, though: the URL stays the same after every deploy, so browsers and CDNs can keep serving an old copy of the file.

Fix the template or the build configuration so the placeholder renders as a real value. If the braces are meant to be literal, percent-encode them.

Invalid example

<script src="/js/app.js?v=${build.version}"></script>

Valid example

<script src="/js/app.js?v=2.4.1"></script>

Learn more:

MDN Reference

MDN: The script element

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MDN Reference

MDN: Percent-encoding

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WHATWG Spec

URL Living Standard

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