Subpath imports (#) fail on Windows when package.json path >= 260 chars (MAX_PATH)

Why this is a good first issue

Windows MAX_PATH limitation causes subpath imports to fail.

AI Summary

The issue involves subpath imports failing on Windows when the path exceeds the MAX_PATH limit. The problem is reproducible with a provided test case, but the solution may require understanding Windows file system limitations and Node.js module resolution. The scope is somewhat clear, but the fix might involve cross-cutting changes.

Issue Description

Version

v24.12.0

Platform

Microsoft Windows NT 10.0.26200.0 x64

Subsystem

No response

What steps will reproduce the bug?

I ran into this while running pnpm dev in a project with a deep node_modules path:

TypeError: Package import specifier "#module-sync-enabled" is not defined imported from
C:\Users\...\node_modules\.pnpm\@[email protected]_...\@react-router\dev\module-sync-enabled\index.mjs

Minimal repro — save as test.mjs and run with node test.mjs on Windows (with LongPathsEnabled):

import fs from "node:fs";
import { createRequire } from "node:module";
import path from "node:path";

// \\?\ prefix bypasses MAX_PATH limit for Win32 file APIs
const lp = (p) => "\\\\?\\" + p;
const ok = (fn) => {
	try { fn(); return "OK"; }
	catch (e) { console.error(e); return "FAIL"; }
};

const MAX = 259; // MAX_PATH (260) minus null terminator
const cwd = process.cwd();
const fixedLen = cwd.length + 2 + "package.json".length;
const results = {};

for (const target of [MAX, MAX + 1]) {
	const dir = path.join(cwd, "a".repeat(target - fixedLen));
	const dep = path.join(dir, "node_modules", "dep");

	fs.mkdirSync(lp(dep), { recursive: true });
	fs.writeFileSync(
		lp(path.join(dir, "package.json")),
		JSON.stringify({ imports: { "#foo": "./foo.mjs" } }),
	);
	fs.writeFileSync(lp(path.join(dir, "foo.mjs")), "export default 1\n");
	fs.writeFileSync(
		lp(path.join(dep, "package.json")),
		JSON.stringify({ name: "dep", exports: { ".": "./index.mjs" } }),
	);
	fs.writeFileSync(lp(path.join(dep, "index.mjs")), "export default 1\n");

	const { resolve } = createRequire(path.join(dir, "_.mjs"));
	results[`${target} chars`] = {
		"fs.access": ok(() => fs.accessSync(lp(path.join(dir, "package.json")))),
		'resolve("dep")': ok(() => resolve("dep")),
		'resolve("#foo")': ok(() => resolve("#foo")),
	};

	fs.rmSync(lp(dir), { recursive: true, force: true });
}

console.table(results);

How often does it reproduce? Is

GitHub Labels

modulewindows