fix: let commonjs plugins fall back to core packages on the desktop

src/backend/plugins/host-modules.ts

@@ -38,6 +38,20 @@ export function isUserPluginFile(
   );
 }
 
+/** Whether a failed import from a plugin should try core's copy. */
+export function shouldUseHostModule(
+  error: unknown,
+  specifier: string,
+  parentURL: string | undefined,
+  pluginsDir: string,
+): boolean {
+  const code = (error as { code?: string })?.code;
+  // import() says ERR_MODULE_NOT_FOUND, a CommonJS require MODULE_NOT_FOUND.
+  if (code !== "ERR_MODULE_NOT_FOUND" && code !== "MODULE_NOT_FOUND")
+    return false;
+  return isBareSpecifier(specifier) && isUserPluginFile(parentURL, pluginsDir);
+}
+
 export function installHostModuleFallback(): void {
   if (installed) return;
   installed = true;
@@ -49,9 +63,12 @@ export function installHostModuleFallback(): void {
         return nextResolve(specifier, context);
       } catch (error) {
         if (
-          (error as { code?: string })?.code !== "ERR_MODULE_NOT_FOUND" ||
-          !isBareSpecifier(specifier) ||
-          !isUserPluginFile(context.parentURL, getPluginsDir())
+          !shouldUseHostModule(
+            error,
+            specifier,
+            context.parentURL,
+            getPluginsDir(),
+          )
         ) {
           throw error;
         }

src/backend/tests/plugins/host-modules.test.ts

@@ -7,6 +7,7 @@ import { afterEach, describe, expect, it } from "vitest";
 import {
   isBareSpecifier,
   isUserPluginFile,
+  shouldUseHostModule,
 } from "../../plugins/host-modules.js";
 
 describe("isBareSpecifier", () => {
@@ -46,6 +47,36 @@ describe("isUserPluginFile", () => {
   });
 });
 
+describe("shouldUseHostModule", () => {
+  const root = path.resolve(os.tmpdir(), "termix-data", "plugins");
+  const parent = pathToFileURL(
+    path.join(root, ".unpacked", "demo", "dist", "backend.js"),
+  ).href;
+
+  it("falls back for import and require misses from a plugin", () => {
+    for (const code of ["ERR_MODULE_NOT_FOUND", "MODULE_NOT_FOUND"]) {
+      expect(shouldUseHostModule({ code }, "axios", parent, root)).toBe(true);
+    }
+  });
+
+  it("leaves other errors and files alone", () => {
+    expect(
+      shouldUseHostModule({ code: "ERR_REQUIRE_ESM" }, "axios", parent, root),
+    ).toBe(false);
+    expect(
+      shouldUseHostModule({ code: "MODULE_NOT_FOUND" }, "./x.js", parent, root),
+    ).toBe(false);
+    expect(
+      shouldUseHostModule(
+        { code: "MODULE_NOT_FOUND" },
+        "axios",
+        undefined,
+        root,
+      ),
+    ).toBe(false);
+  });
+});
+
 describe("installHostModuleFallback", () => {
   let dataDir: string | null = null;