UserModuleLoadError

The Cause

A rule, rule set, or plugin specifier resolved to a file, but the file itself is not a loadable kind. This is the same underlying check for all three module types, and it declines for one of the following reasons:

The file is a TypeScript declaration file

"<name>.d.ts" is a TypeScript declaration file (.d.ts) — declaration files are never loadable, regardless of contents.

A .d.ts file is never loadable, no matter what it contains — even if it happens to also export a value at runtime.

The file has an .mts or .cts extension

".mts" is not a loadable extension — .mts/.cts are not supported for rules, rule sets, or plugins; use .ts instead.

Use .ts instead — .mts/.cts are not part of the loadable set.

The specifier has a missing or unrecognised extension

"<name>" does not have a loadable extension — expected one of .ts, .js, .mjs, .cjs …

A local specifier must carry an explicit, loadable extension. There is no extension guessing, no index/directory resolution.

The extension’s casing does not match the file on disk

".TS" does not match the on-disk casing ".ts" for "<name>.ts" — extension casing is case-sensitive.

Extension matching is case-sensitive against the real on-disk casing, even on a case-insensitive filesystem.

The specifier does not resolve to a regular file

"<specifier>" resolves to "<name>", which is not a regular file.

The path exists but is a directory (or another non-file kind), not a loadable module.

A bare specifier is a Node.js builtin

"<specifier>" is a Node.js builtin module — builtins are not loadable user modules.

Builtin module ids (http, url, …) are never loadable as rules, rule sets, or plugins.

A bare specifier resolves to unbuilt TypeScript source

"<specifier>" ships unbuilt TypeScript source (<file>); publish built JavaScript.

An installed package must ship built JavaScript (.js/.mjs/.cjs). A package whose resolved entry point is .ts/.mts/.cts is declined — Thymian does not transpile node_modules dependencies.

The Solution

Reference a .ts, .js, .mjs, or .cjs file with an explicit extension — never .d.ts, .mts, or .cts. For an installed package, make sure it publishes built JavaScript rather than TypeScript source. See Loading Rules and Plugins for the full loading contract.