preconstruct · emmatown · May 6, 2026 · May 5, 2026 · May 5, 2026
diff --git a/.changeset/tall-comics-kick.md b/.changeset/tall-comics-kick.md
@@ -0,0 +1,5 @@
+---
+"@preconstruct/cli": patch
+---
+
+Fix UMD alias resolution for multi-entrypoint package subpaths
diff --git a/packages/cli/src/build/__tests__/build.ts b/packages/cli/src/build/__tests__/build.ts
@@ -428,6 +428,74 @@ test("monorepo umd with dep on other module", async () => {
   `);
 });
 
+test("monorepo umd with dep on multi-entrypoint module subpath", async () => {
+  let tmpPath = await testdir({
+    "package.json": JSON.stringify({
+      name: "monorepo-umd-subpath-alias",
+      main: "index.js",
+      workspaces: ["packages/*"],
+
+      preconstruct: {
+        packages: ["packages/*"],
+      },
+    }),
+    "packages/pkg-a/package.json": JSON.stringify({
+      name: "pkg-a",
+      main: "dist/pkg-a.cjs.js",
+      "umd:main": "dist/pkg-a.umd.min.js",
+
+      preconstruct: {
+        umdName: "pkgA",
+      },
+
+      dependencies: {
+        "pkg-b": "1.0.0",
+      },
+    }),
+    "packages/pkg-a/src/index.js": js`
+      import thing from "pkg-b/stuff";
+
+      export default thing;
+    `,
+    "packages/pkg-b/package.json": JSON.stringify({
+      name: "pkg-b",
+      main: "dist/pkg-b.cjs.js",
+      module: "dist/pkg-b.esm.js",
+
+      preconstruct: {
+        entrypoints: ["index.js", "stuff.js"],
+      },
+    }),
+    "packages/pkg-b/stuff/package.json": JSON.stringify({
+      main: "dist/pkg-b-stuff.cjs.js",
+      module: "dist/pkg-b-stuff.esm.js",
+    }),
+    "packages/pkg-b/src/index.js": js`
+      export default "wrong entrypoint";
+    `,
+    "packages/pkg-b/src/stuff.js": js`
+      export default "right entrypoint";
+    `,
+    "node_modules/pkg-b": {
+      kind: "symlink",
+      path: "packages/pkg-b",
+    },
+  });
+
+  await build(tmpPath);
+
+  expect(
+    await getFiles(path.join(tmpPath, "packages", "pkg-a"), ["dist/*umd*"])
+  ).toMatchInlineSnapshot(`
+    ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ dist/pkg-a.umd.min.js ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
+    !function(e,t){"object"==typeof exports&&"undefined"!=typeof module?module.exports=t():"function"==typeof define&&define.amd?define(t):(e="undefined"!=typeof globalThis?globalThis:e||self).pkgA=t()}(this,(function(){"use strict";return"right entrypoint"}));
+    //# sourceMappingURL=pkg-a.umd.min.js.map
+
+    ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ dist/pkg-a.umd.min.js.map ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
+    {"version":3,"file":"pkg-a.umd.min.js","sources":["../../pkg-b/src/stuff.js"],"sourcesContent":["export default \\"right entrypoint\\";"],"names":[],"mappings":"2OAAe"}
+  `);
+});
+
 test("monorepo single package", async () => {
   let tmpPath = f.copy("monorepo-single-package");
   await initBasic(tmpPath);

diff --git a/packages/cli/src/build/rollup.ts b/packages/cli/src/build/rollup.ts
@@ -228,18 +228,26 @@ export let getRollupConfig = (
 
 function getAliases(
   project: Project
-): {
-  [key: string]: string;
-} {
-  let aliases: { [key: string]: string } = {};
+): Array<{ find: RegExp; replacement: string }> {
+  let aliases: Array<{ find: RegExp; replacement: string }> = [];
   project.packages.forEach((pkg) => {
+    // pkg.entrypoints are already expanded from the configured entrypoint globs
+    // and sorted in Package.create() for deterministic package.json#exports
+    // emission, so we preserve that same stable order here.
     pkg.entrypoints.forEach((entrypoint) => {
-      aliases[entrypoint.name] = entrypoint.source;
+      aliases.push({
+        find: new RegExp(`^${escapeForRegex(entrypoint.name)}$`),
+        replacement: entrypoint.source,
+      });
     });
   });
   return aliases;
 }
 
+function escapeForRegex(value: string) {
+  return value.replace(/[|\\{}()[\]^$+*?.-]/g, "\\$&");
+}
+
 const cjsDynamicImportPlugin: Plugin = {
   name: "cjs render dynamic import",
   renderDynamicImport({ format }) {

diff --git a/packages/cli/test-utils/index.ts b/packages/cli/test-utils/index.ts
@@ -347,19 +347,31 @@ async function getSymlinkType(targetPath: string): Promise<"dir" | "file"> {
 
 export async function testdir(dir: Fixture) {
   const temp = realFs.realpathSync.native(f.temp());
+  const fileEntries: Array<[string, string]> = [];
+  const symlinkEntries: Array<[string, { kind: "symlink"; path: string }]> = [];
+
+  for (const [filename, output] of Object.entries(dir)) {
+    if (typeof output === "string") {
+      fileEntries.push([filename, output]);
+    } else {
+      symlinkEntries.push([filename, output]);
+    }
+  }
+
+  await Promise.all(
+    fileEntries.map(async ([filename, output]) => {
+      await fs.outputFile(path.join(temp, filename), output);
+    })
+  );
+
   await Promise.all(
-    Object.keys(dir).map(async (filename) => {
-      const output = dir[filename];
+    symlinkEntries.map(async ([filename, output]) => {
       const fullPath = path.join(temp, filename);
-      if (typeof output === "string") {
-        await fs.outputFile(fullPath, output);
-      } else {
-        const dir = path.dirname(fullPath);
-        await fs.ensureDir(dir);
-        const targetPath = path.resolve(temp, output.path);
-        const symlinkType = await getSymlinkType(targetPath);
-        await fs.symlink(targetPath, fullPath, symlinkType);
-      }
+      const dir = path.dirname(fullPath);
+      await fs.ensureDir(dir);
+      const targetPath = path.resolve(temp, output.path);
+      const symlinkType = await getSymlinkType(targetPath);
+      await fs.symlink(targetPath, fullPath, symlinkType);
     })
   );
   return temp;